You can use implicitly typed local variables whenever you want the compiler to determine the type of a local variable. You must use implicitly typed local variables to store anonymous types, which are often used in query expressions. The following examples illustrate both optional and required uses of implicitly typed local variables in queries.
Implicitly typed local variables are declared by using the var contextual keyword. For more information, see Implicitly Typed Local Variables and Implicitly Typed Arrays.
Examples
The following example shows a common scenario in which the var keyword is required: a query expression that produces a sequence of anonymous types. In this scenario, both the query variable and the iteration variable in the foreach statement must be implicitly typed by using var because you do not have access to a type name for the anonymous type. For more information about anonymous types, see Anonymous Types.
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideLINQ/CS/csRef30LangFeatures_2.cs32
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
namespace Immutable
{
//how to implement an immutable class
//<snippet30>
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// constructor to initialize its properties.
class Contact
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Public constructor.
public Contact(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
}
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// static method and private constructor to initialize its properties.
public class Contact2
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Private constructor.
private Contact2(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
// Public factory method.
public static Contact2 CreateContact(string name, string address)
{
return new Contact2(name, address);
}
}
public class Program
{
static void Main()
{
// Some simple data sources.
string[] names = {"Terry Adams","Fadi Fakhouri", "Hanying Feng",
"Cesar Garcia", "Debra Garcia"};
string[] addresses = {"123 Main St.", "345 Cypress Ave.", "678 1st Ave",
"12 108th St.", "89 E. 42nd St."};
// Simple query to demonstrate object creation in select clause.
// Create Contact objects by using a constructor.
var query1 = from i in Enumerable.Range(0, 5)
select new Contact(names[i], addresses[i]);
// List elements cannot be modified by client code.
var list = query1.ToList();
foreach (var contact in list)
{
Console.WriteLine("{0}, {1}", contact.Name, contact.Address);
}
// Create Contact2 objects by using a static factory method.
var query2 = from i in Enumerable.Range(0, 5)
select Contact2.CreateContact(names[i], addresses[i]);
// Console output is identical to query1.
var list2 = query2.ToList();
// List elements cannot be modified by client code.
// CS0272:
// list2[0].Name = "Eugene Zabokritski";
// Keep the console open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
/* Output:
Terry Adams, 123 Main St.
Fadi Fakhouri, 345 Cypress Ave.
Hanying Feng, 678 1st Ave
Cesar Garcia, 12 108th St.
Debra Garcia, 89 E. 42nd St.
*/
//</snippet30>
}
namespace csrefLINQExamples
{
//Return Subsets of Element Properties
class AnonymousTypes : StudentClass
{
static void Main(string[] args)
{
QueryByScore();
}
//<snippet31>
private static void QueryByScore()
{
// Create the query. var is required because
// the query produces a sequence of anonymous types.
var queryHighScores =
from student in students
where student.ExamScores[0] > 95
select new { student.FirstName, student.LastName };
// Execute the query.
foreach (var obj in queryHighScores)
{
// The anonymous type's properties were not named. Therefore
// they have the same names as the Student properties.
Console.WriteLine(obj.FirstName + ", " + obj.LastName);
}
}
/* Output:
Adams, Terry
Fakhouri, Fadi
Garcia, Cesar
Omelchenko, Svetlana
Zabokritski, Eugene
*/
//</snippet31>
}
// How to use implicitly typed locals
class ImplicitTyping : StudentClass
{
static void Main(string[] args)
{
QueryNames('S');
}
//<snippet32>
private static void QueryNames(char firstLetter)
{
// Create the query. Use of var is required because
// the query produces a sequence of anonymous types:
// System.Collections.Generic.IEnumerable<????>.
var studentQuery =
from student in students
where student.FirstName[0] == firstLetter
select new { student.FirstName, student.LastName };
// Execute the query and display the results.
foreach (var anonType in studentQuery)
{
Console.WriteLine("First = {0}, Last = {1}", anonType.FirstName, anonType.LastName);
}
}
//</snippet32>
static void QueryMethod(List<Student> students)
{
//how to use implicitly typed locals #2
//<snippet33>
// Variable queryId could be declared by using
// System.Collections.Generic.IEnumerable<string>
// instead of var.
var queryId =
from student in students
where student.Id > 111
select student.LastName;
// Variable str could be declared by using var instead of string.
foreach (string str in queryId)
{
Console.WriteLine($"Last name: {str}");
}
//</snippet33>
}
}
//how to init a dictionary with coll initializer
//<snippet34>
class StudentName
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
}
class CollInit
{
Dictionary<int, StudentName> students = new Dictionary<int, StudentName>()
{
{ 111, new StudentName { FirstName = "Sachin", LastName="Karnik", Id = 211 } },
{ 112, new StudentName { FirstName = "Dina", LastName="Salimzianova", Id = 317 } },
{ 113, new StudentName { FirstName = "Andy", LastName="Ruth", Id = 198 } }
};
}
//</snippet34>
class ObjInit
{
//How to: Initialize Objects without Calling a Constructor (C# Programming Guide)
//<snippet35>
public class Program
{
public static void Main()
{
// Declare a StudentName by using the constructor that has two parameters.
StudentName student1 = new StudentName("Craig", "Playstead");
// Make the same declaration by using an object initializer and sending
// arguments for the first and last names. The default constructor is
// invoked in processing this declaration, not the constructor that has
// two parameters.
StudentName student2 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
};
// Declare a StudentName by using an object initializer and sending
// an argument for only the Id property. No corresponding constructor is
// necessary. Only the default constructor is used to process object
// initializers.
StudentName student3 = new StudentName
{
Id = 183
};
// Declare a StudentName by using an object initializer and sending
// arguments for all three properties. No corresponding constructor is
// defined in the class.
StudentName student4 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
Id = 116
};
System.Console.WriteLine(student1.ToString());
System.Console.WriteLine(student2.ToString());
System.Console.WriteLine(student3.ToString());
System.Console.WriteLine(student4.ToString());
}
// Output:
// Craig 0
// Craig 0
// 183
// Craig 116
}
public class StudentName
{
// The default constructor has no parameters. The default constructor
// is invoked in the processing of object initializers.
// You can test this by changing the access modifier from public to
// private. The declarations in Main that use object initializers will
// fail.
public StudentName() { }
// The following constructor has parameters for two of the three
// properties.
public StudentName(string first, string last)
{
FirstName = first;
LastName = last;
}
// Properties.
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
public override string ToString()
{
return FirstName + " " + Id;
}
}
//</snippet35>
//How to: Initialize Objects without Calling a Constructor #2 (C# Programming Guide)
//<snippet36>
List<StudentName> students = new List<StudentName>()
{
new StudentName { FirstName = "Craig", LastName = "Playstead", Id = 116 },
new StudentName { FirstName = "Shu", LastName="Ito", Id = 112 },
new StudentName { FirstName = "Gretchen", LastName="Rivas", Id = 113 },
new StudentName { FirstName = "Rajesh", LastName="Rotti", Id = 114 }
};
//</snippet36>
}
//Object and collection initializers
//<snippet46>
// The following code consolidates examples from the topic.
class ObjInitializers
{
//<snippet39>
class Cat
{
// Automatically implemented properties.
public int Age { get; set; }
public string Name { get; set; }
}
//</snippet39>
static void Main()
{
//<snippet45>
Cat cat = new Cat { Age = 10, Name = "Fluffy" };
//</snippet45>
//<snippet41>
List<Cat> cats = new List<Cat>
{
new Cat(){ Name = "Sylvester", Age=8 },
new Cat(){ Name = "Whiskers", Age=2 },
new Cat(){ Name = "Sasha", Age=14 }
};
//</snippet41>
//<snippet42>
List<Cat> moreCats = new List<Cat>
{
new Cat(){ Name = "Furrytail", Age=5 },
new Cat(){ Name = "Peaches", Age=4 },
null
};
//</snippet42>
// Display results.
System.Console.WriteLine(cat.Name);
foreach (Cat c in cats)
System.Console.WriteLine(c.Name);
foreach (Cat c in moreCats)
if (c != null)
System.Console.WriteLine(c.Name);
else
System.Console.WriteLine("List element has null value.");
}
// Output:
//Fluffy
//Sylvester
//Whiskers
//Sasha
//Furrytail
//Peaches
//List element has null value.
}
//</snippet46>
class ObjCollInitializers
{
class Product
{
public string ProductName { get; set; }
public decimal UnitPrice { get; set; }
}
void MethodB()
{
List<Product> products = new List<Product>();
//<snippet40>
var productInfos =
from p in products
select new { p.ProductName, p.UnitPrice };
//</snippet40>
}
}
//Implicitly Typed Local Variables Conceptual Topic
class ImplicitlyTypedLocals
{
// compilation hack
class Customer
{
public string City { get; set; }
}
static List<Customer> customers = new List<Customer>();
static void Main()
{
//<snippet43>
// i is compiled as an int
var i = 5;
// s is compiled as a string
var s = "Hello";
// a is compiled as int[]
var a = new[] { 0, 1, 2 };
// expr is compiled as IEnumerable<Customer>
// or perhaps IQueryable<Customer>
var expr =
from c in customers
where c.City == "London"
select c;
// anon is compiled as an anonymous type
var anon = new { Name = "Terry", Age = 34 };
// list is compiled as List<int>
var list = new List<int>();
//</snippet43>
Console.WriteLine(s + i.ToString());
}
}
//<snippet44>
class ImplicitlyTypedLocals2
{
static void Main()
{
string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
// If a query produces a sequence of anonymous types,
// then use var in the foreach statement to access the properties.
var upperLowerWords =
from w in words
select new { Upper = w.ToUpper(), Lower = w.ToLower() };
// Execute the query
foreach (var ul in upperLowerWords)
{
Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
}
}
}
/* Outputs:
Uppercase: APPLE, Lowercase: apple
Uppercase: BLUEBERRY, Lowercase: blueberry
Uppercase: CHERRY, Lowercase: cherry
*/
//</snippet44>
// no snippets from this class yet.
//<snippet64>
class CustomJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product { Name="Tea", CategoryId = 001 },
new Product { Name="Mustard", CategoryId = 002 },
new Product { Name="Pickles", CategoryId = 002 },
new Product { Name="Carrots", CategoryId = 003 },
new Product { Name="Bok Choy", CategoryId = 003 },
new Product { Name="Peaches", CategoryId = 005 },
new Product { Name="Melons", CategoryId = 005 },
new Product { Name="Ice Cream", CategoryId = 007 },
new Product { Name="Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
CustomJoins app = new CustomJoins();
app.CrossJoin();
app.NonEquijoin();
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void CrossJoin()
{
var crossJoinQuery =
from c in categories
from p in products
select new { c.Id, p.Name };
Console.WriteLine("Cross Join Query:");
foreach (var v in crossJoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.Id, v.Name);
}
}
void NonEquijoin()
{
var nonEquijoinQuery =
from p in products
let catIds = from c in categories
select c.Id
where catIds.Contains(p.CategoryId) == true
select new { Product = p.Name, CategoryId = p.CategoryId };
Console.WriteLine("Non-equijoin query:");
foreach (var v in nonEquijoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.CategoryId, v.Product);
}
}
}
/* Output:
Cross Join Query:
1 Tea
1 Mustard
1 Pickles
1 Carrots
1 Bok Choy
1 Peaches
1 Melons
1 Ice Cream
1 Mackerel
2 Tea
2 Mustard
2 Pickles
2 Carrots
2 Bok Choy
2 Peaches
2 Melons
2 Ice Cream
2 Mackerel
3 Tea
3 Mustard
3 Pickles
3 Carrots
3 Bok Choy
3 Peaches
3 Melons
3 Ice Cream
3 Mackerel
Non-equijoin query:
1 Tea
2 Mustard
2 Pickles
3 Carrots
3 Bok Choy
Press any key to exit.
*/
//</snippet64>
//<snippet 66>
class Test2
{
// Func<int, bool> f;
bool M(Func<double, bool> func, int i)
{
int local = i;
Console.WriteLine(func(local));
Thread.Sleep(5300);
return func(local);
}
static void Main()
{
Test2 app = new Test2();
bool b = app.M(x => x % 2 == 0, DateTime.Now.Second);
//bool b2 = app.M(x => x +
Console.WriteLine(b.ToString());
Console.ReadKey();
}
}
//</snippet 66>
//reserving numbers 67-79 for anonymous methods
//<snippet80>
class MQ
{
// QueryMethod1 returns a query as its value.
IEnumerable<string> QueryMethod1(ref int[] ints)
{
var intsToStrings = from i in ints
where i > 4
select i.ToString();
return intsToStrings;
}
// QueryMethod2 returns a query as the value of parameter returnQ.
void QueryMethod2(ref int[] ints, out IEnumerable<string> returnQ)
{
var intsToStrings = from i in ints
where i < 4
select i.ToString();
returnQ = intsToStrings;
}
static void Main()
{
MQ app = new MQ();
int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// QueryMethod1 returns a query as the value of the method.
var myQuery1 = app.QueryMethod1(ref nums);
// Query myQuery1 is executed in the following foreach loop.
Console.WriteLine("Results of executing myQuery1:");
// Rest the mouse pointer over myQuery1 to see its type.
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// You also can execute the query returned from QueryMethod1
// directly, without using myQuery1.
Console.WriteLine("\nResults of executing myQuery1 directly:");
// Rest the mouse pointer over the call to QueryMethod1 to see its
// return type.
foreach (string s in app.QueryMethod1(ref nums))
{
Console.WriteLine(s);
}
IEnumerable<string> myQuery2;
// QueryMethod2 returns a query as the value of its out parameter.
app.QueryMethod2(ref nums, out myQuery2);
// Execute the returned query.
Console.WriteLine("\nResults of executing myQuery2:");
foreach (string s in myQuery2)
{
Console.WriteLine(s);
}
// You can modify a query by using query composition. A saved query
// is nested inside a new query definition that revises the results
// of the first query.
myQuery1 = from item in myQuery1
orderby item descending
select item;
// Execute the modified query.
Console.WriteLine("\nResults of executing modified myQuery1:");
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet80>
//<snippet81>
class HowToOrderJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name="Condiments", Id = 002 },
new Category() { Name="Vegetables", Id = 003 },
new Category() { Name="Grains", Id = 004 },
new Category() { Name="Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
HowToOrderJoins app = new HowToOrderJoins();
app.OrderJoin1();
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void OrderJoin1()
{
var groupJoinQuery2 =
from category in categories
join prod in products on category.Id equals prod.CategoryId into prodGroup
orderby category.Name
select new
{
Category = category.Name,
Products = from prod2 in prodGroup
orderby prod2.Name
select prod2
};
foreach (var productGroup in groupJoinQuery2)
{
Console.WriteLine(productGroup.Category);
foreach (var prodItem in productGroup.Products)
{
Console.WriteLine(" {0,-10} {1}", prodItem.Name, prodItem.CategoryId);
}
}
}
/* Output:
Beverages
Cola 1
Tea 1
Condiments
Mustard 2
Pickles 2
Fruit
Melons 5
Peaches 5
Grains
Vegetables
Bok Choy 3
Carrots 3
*/
}
//</snippet81>
class NullValuesInQueries
{
#region Data
class Product
{
public string Name { get; set; }
public int? CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int? Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
new Category() { Name = "Grains", Id = 004 },
new Category() { Name = "Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
NullValuesInQueries app = new NullValuesInQueries();
Northwind db = new Northwind();
app.TestMethod(db);
}
void NullHandlingQuery()
{
//<snippet82>
var query1 =
from c in categories
where c != null
join p in products on c.Id equals
(p == null ? null : p.CategoryId)
select new { Category = c.Name, Name = p.Name };
//</snippet82>
}
#region compilation only
class Order
{
public int? OrderId = 1;
public int? EmployeeId = 1;
}
class Employee
{
public int EmployeeId = 1;
public string FirstName = string.Empty;
}
class Northwind
{
public Order[] Orders = null;
public Employee[] Employees = null;
}
#endregion
//<snippet83>
void TestMethod(Northwind db)
{
var query =
from o in db.Orders
join e in db.Employees
on o.EmployeeId equals (int?)e.EmployeeId
select new { o.OrderId, e.FirstName };
}
//</snippet83>
}
//<snippet 84 >
class DynamicPreds2
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category(){ Name = "Beverages", Id = 001 },
new Category(){ Name = "Condiments", Id = 002 },
new Category(){ Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
new Product{ Name = "Ice Cream", CategoryId = 007 },
new Product{ Name = "Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
DynamicPreds2 app = new DynamicPreds2();
var q = from p in app.products
select p;
// Identity select
Console.WriteLine("First iteration");
//foreach(var v in query)
// Console.WriteLine(v.Name);
// Mod 1
var q2 = from item in q
where item.Name[0] == 'M'
select item;
Console.WriteLine("2nd iteration");
foreach (var v in q2)
Console.WriteLine(v.Name);
//var q3 = from item in
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet 84>
} The following example uses the var keyword in a situation that is similar, but in which the use of var is optional. Because student.LastName is a string, execution of the query returns a sequence of strings. Therefore, the type of queryId could be declared as System.Collections.Generic.IEnumerable<string> instead of var. Keyword var is used for convenience. In the example, the iteration variable in the foreach statement is explicitly typed as a string, but it could instead be declared by using var. Because the type of the iteration variable is not an anonymous type, the use of var is an option, not a requirement. Remember, var itself is not a type, but an instruction to the compiler to infer and assign the type.
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideLINQ/CS/csRef30LangFeatures_2.cs33
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
namespace Immutable
{
//how to implement an immutable class
//<snippet30>
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// constructor to initialize its properties.
class Contact
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Public constructor.
public Contact(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
}
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// static method and private constructor to initialize its properties.
public class Contact2
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Private constructor.
private Contact2(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
// Public factory method.
public static Contact2 CreateContact(string name, string address)
{
return new Contact2(name, address);
}
}
public class Program
{
static void Main()
{
// Some simple data sources.
string[] names = {"Terry Adams","Fadi Fakhouri", "Hanying Feng",
"Cesar Garcia", "Debra Garcia"};
string[] addresses = {"123 Main St.", "345 Cypress Ave.", "678 1st Ave",
"12 108th St.", "89 E. 42nd St."};
// Simple query to demonstrate object creation in select clause.
// Create Contact objects by using a constructor.
var query1 = from i in Enumerable.Range(0, 5)
select new Contact(names[i], addresses[i]);
// List elements cannot be modified by client code.
var list = query1.ToList();
foreach (var contact in list)
{
Console.WriteLine("{0}, {1}", contact.Name, contact.Address);
}
// Create Contact2 objects by using a static factory method.
var query2 = from i in Enumerable.Range(0, 5)
select Contact2.CreateContact(names[i], addresses[i]);
// Console output is identical to query1.
var list2 = query2.ToList();
// List elements cannot be modified by client code.
// CS0272:
// list2[0].Name = "Eugene Zabokritski";
// Keep the console open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
/* Output:
Terry Adams, 123 Main St.
Fadi Fakhouri, 345 Cypress Ave.
Hanying Feng, 678 1st Ave
Cesar Garcia, 12 108th St.
Debra Garcia, 89 E. 42nd St.
*/
//</snippet30>
}
namespace csrefLINQExamples
{
//Return Subsets of Element Properties
class AnonymousTypes : StudentClass
{
static void Main(string[] args)
{
QueryByScore();
}
//<snippet31>
private static void QueryByScore()
{
// Create the query. var is required because
// the query produces a sequence of anonymous types.
var queryHighScores =
from student in students
where student.ExamScores[0] > 95
select new { student.FirstName, student.LastName };
// Execute the query.
foreach (var obj in queryHighScores)
{
// The anonymous type's properties were not named. Therefore
// they have the same names as the Student properties.
Console.WriteLine(obj.FirstName + ", " + obj.LastName);
}
}
/* Output:
Adams, Terry
Fakhouri, Fadi
Garcia, Cesar
Omelchenko, Svetlana
Zabokritski, Eugene
*/
//</snippet31>
}
// How to use implicitly typed locals
class ImplicitTyping : StudentClass
{
static void Main(string[] args)
{
QueryNames('S');
}
//<snippet32>
private static void QueryNames(char firstLetter)
{
// Create the query. Use of var is required because
// the query produces a sequence of anonymous types:
// System.Collections.Generic.IEnumerable<????>.
var studentQuery =
from student in students
where student.FirstName[0] == firstLetter
select new { student.FirstName, student.LastName };
// Execute the query and display the results.
foreach (var anonType in studentQuery)
{
Console.WriteLine("First = {0}, Last = {1}", anonType.FirstName, anonType.LastName);
}
}
//</snippet32>
static void QueryMethod(List<Student> students)
{
//how to use implicitly typed locals #2
//<snippet33>
// Variable queryId could be declared by using
// System.Collections.Generic.IEnumerable<string>
// instead of var.
var queryId =
from student in students
where student.Id > 111
select student.LastName;
// Variable str could be declared by using var instead of string.
foreach (string str in queryId)
{
Console.WriteLine($"Last name: {str}");
}
//</snippet33>
}
}
//how to init a dictionary with coll initializer
//<snippet34>
class StudentName
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
}
class CollInit
{
Dictionary<int, StudentName> students = new Dictionary<int, StudentName>()
{
{ 111, new StudentName { FirstName = "Sachin", LastName="Karnik", Id = 211 } },
{ 112, new StudentName { FirstName = "Dina", LastName="Salimzianova", Id = 317 } },
{ 113, new StudentName { FirstName = "Andy", LastName="Ruth", Id = 198 } }
};
}
//</snippet34>
class ObjInit
{
//How to: Initialize Objects without Calling a Constructor (C# Programming Guide)
//<snippet35>
public class Program
{
public static void Main()
{
// Declare a StudentName by using the constructor that has two parameters.
StudentName student1 = new StudentName("Craig", "Playstead");
// Make the same declaration by using an object initializer and sending
// arguments for the first and last names. The default constructor is
// invoked in processing this declaration, not the constructor that has
// two parameters.
StudentName student2 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
};
// Declare a StudentName by using an object initializer and sending
// an argument for only the Id property. No corresponding constructor is
// necessary. Only the default constructor is used to process object
// initializers.
StudentName student3 = new StudentName
{
Id = 183
};
// Declare a StudentName by using an object initializer and sending
// arguments for all three properties. No corresponding constructor is
// defined in the class.
StudentName student4 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
Id = 116
};
System.Console.WriteLine(student1.ToString());
System.Console.WriteLine(student2.ToString());
System.Console.WriteLine(student3.ToString());
System.Console.WriteLine(student4.ToString());
}
// Output:
// Craig 0
// Craig 0
// 183
// Craig 116
}
public class StudentName
{
// The default constructor has no parameters. The default constructor
// is invoked in the processing of object initializers.
// You can test this by changing the access modifier from public to
// private. The declarations in Main that use object initializers will
// fail.
public StudentName() { }
// The following constructor has parameters for two of the three
// properties.
public StudentName(string first, string last)
{
FirstName = first;
LastName = last;
}
// Properties.
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
public override string ToString()
{
return FirstName + " " + Id;
}
}
//</snippet35>
//How to: Initialize Objects without Calling a Constructor #2 (C# Programming Guide)
//<snippet36>
List<StudentName> students = new List<StudentName>()
{
new StudentName { FirstName = "Craig", LastName = "Playstead", Id = 116 },
new StudentName { FirstName = "Shu", LastName="Ito", Id = 112 },
new StudentName { FirstName = "Gretchen", LastName="Rivas", Id = 113 },
new StudentName { FirstName = "Rajesh", LastName="Rotti", Id = 114 }
};
//</snippet36>
}
//Object and collection initializers
//<snippet46>
// The following code consolidates examples from the topic.
class ObjInitializers
{
//<snippet39>
class Cat
{
// Automatically implemented properties.
public int Age { get; set; }
public string Name { get; set; }
}
//</snippet39>
static void Main()
{
//<snippet45>
Cat cat = new Cat { Age = 10, Name = "Fluffy" };
//</snippet45>
//<snippet41>
List<Cat> cats = new List<Cat>
{
new Cat(){ Name = "Sylvester", Age=8 },
new Cat(){ Name = "Whiskers", Age=2 },
new Cat(){ Name = "Sasha", Age=14 }
};
//</snippet41>
//<snippet42>
List<Cat> moreCats = new List<Cat>
{
new Cat(){ Name = "Furrytail", Age=5 },
new Cat(){ Name = "Peaches", Age=4 },
null
};
//</snippet42>
// Display results.
System.Console.WriteLine(cat.Name);
foreach (Cat c in cats)
System.Console.WriteLine(c.Name);
foreach (Cat c in moreCats)
if (c != null)
System.Console.WriteLine(c.Name);
else
System.Console.WriteLine("List element has null value.");
}
// Output:
//Fluffy
//Sylvester
//Whiskers
//Sasha
//Furrytail
//Peaches
//List element has null value.
}
//</snippet46>
class ObjCollInitializers
{
class Product
{
public string ProductName { get; set; }
public decimal UnitPrice { get; set; }
}
void MethodB()
{
List<Product> products = new List<Product>();
//<snippet40>
var productInfos =
from p in products
select new { p.ProductName, p.UnitPrice };
//</snippet40>
}
}
//Implicitly Typed Local Variables Conceptual Topic
class ImplicitlyTypedLocals
{
// compilation hack
class Customer
{
public string City { get; set; }
}
static List<Customer> customers = new List<Customer>();
static void Main()
{
//<snippet43>
// i is compiled as an int
var i = 5;
// s is compiled as a string
var s = "Hello";
// a is compiled as int[]
var a = new[] { 0, 1, 2 };
// expr is compiled as IEnumerable<Customer>
// or perhaps IQueryable<Customer>
var expr =
from c in customers
where c.City == "London"
select c;
// anon is compiled as an anonymous type
var anon = new { Name = "Terry", Age = 34 };
// list is compiled as List<int>
var list = new List<int>();
//</snippet43>
Console.WriteLine(s + i.ToString());
}
}
//<snippet44>
class ImplicitlyTypedLocals2
{
static void Main()
{
string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
// If a query produces a sequence of anonymous types,
// then use var in the foreach statement to access the properties.
var upperLowerWords =
from w in words
select new { Upper = w.ToUpper(), Lower = w.ToLower() };
// Execute the query
foreach (var ul in upperLowerWords)
{
Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
}
}
}
/* Outputs:
Uppercase: APPLE, Lowercase: apple
Uppercase: BLUEBERRY, Lowercase: blueberry
Uppercase: CHERRY, Lowercase: cherry
*/
//</snippet44>
// no snippets from this class yet.
//<snippet64>
class CustomJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product { Name="Tea", CategoryId = 001 },
new Product { Name="Mustard", CategoryId = 002 },
new Product { Name="Pickles", CategoryId = 002 },
new Product { Name="Carrots", CategoryId = 003 },
new Product { Name="Bok Choy", CategoryId = 003 },
new Product { Name="Peaches", CategoryId = 005 },
new Product { Name="Melons", CategoryId = 005 },
new Product { Name="Ice Cream", CategoryId = 007 },
new Product { Name="Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
CustomJoins app = new CustomJoins();
app.CrossJoin();
app.NonEquijoin();
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void CrossJoin()
{
var crossJoinQuery =
from c in categories
from p in products
select new { c.Id, p.Name };
Console.WriteLine("Cross Join Query:");
foreach (var v in crossJoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.Id, v.Name);
}
}
void NonEquijoin()
{
var nonEquijoinQuery =
from p in products
let catIds = from c in categories
select c.Id
where catIds.Contains(p.CategoryId) == true
select new { Product = p.Name, CategoryId = p.CategoryId };
Console.WriteLine("Non-equijoin query:");
foreach (var v in nonEquijoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.CategoryId, v.Product);
}
}
}
/* Output:
Cross Join Query:
1 Tea
1 Mustard
1 Pickles
1 Carrots
1 Bok Choy
1 Peaches
1 Melons
1 Ice Cream
1 Mackerel
2 Tea
2 Mustard
2 Pickles
2 Carrots
2 Bok Choy
2 Peaches
2 Melons
2 Ice Cream
2 Mackerel
3 Tea
3 Mustard
3 Pickles
3 Carrots
3 Bok Choy
3 Peaches
3 Melons
3 Ice Cream
3 Mackerel
Non-equijoin query:
1 Tea
2 Mustard
2 Pickles
3 Carrots
3 Bok Choy
Press any key to exit.
*/
//</snippet64>
//<snippet 66>
class Test2
{
// Func<int, bool> f;
bool M(Func<double, bool> func, int i)
{
int local = i;
Console.WriteLine(func(local));
Thread.Sleep(5300);
return func(local);
}
static void Main()
{
Test2 app = new Test2();
bool b = app.M(x => x % 2 == 0, DateTime.Now.Second);
//bool b2 = app.M(x => x +
Console.WriteLine(b.ToString());
Console.ReadKey();
}
}
//</snippet 66>
//reserving numbers 67-79 for anonymous methods
//<snippet80>
class MQ
{
// QueryMethod1 returns a query as its value.
IEnumerable<string> QueryMethod1(ref int[] ints)
{
var intsToStrings = from i in ints
where i > 4
select i.ToString();
return intsToStrings;
}
// QueryMethod2 returns a query as the value of parameter returnQ.
void QueryMethod2(ref int[] ints, out IEnumerable<string> returnQ)
{
var intsToStrings = from i in ints
where i < 4
select i.ToString();
returnQ = intsToStrings;
}
static void Main()
{
MQ app = new MQ();
int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// QueryMethod1 returns a query as the value of the method.
var myQuery1 = app.QueryMethod1(ref nums);
// Query myQuery1 is executed in the following foreach loop.
Console.WriteLine("Results of executing myQuery1:");
// Rest the mouse pointer over myQuery1 to see its type.
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// You also can execute the query returned from QueryMethod1
// directly, without using myQuery1.
Console.WriteLine("\nResults of executing myQuery1 directly:");
// Rest the mouse pointer over the call to QueryMethod1 to see its
// return type.
foreach (string s in app.QueryMethod1(ref nums))
{
Console.WriteLine(s);
}
IEnumerable<string> myQuery2;
// QueryMethod2 returns a query as the value of its out parameter.
app.QueryMethod2(ref nums, out myQuery2);
// Execute the returned query.
Console.WriteLine("\nResults of executing myQuery2:");
foreach (string s in myQuery2)
{
Console.WriteLine(s);
}
// You can modify a query by using query composition. A saved query
// is nested inside a new query definition that revises the results
// of the first query.
myQuery1 = from item in myQuery1
orderby item descending
select item;
// Execute the modified query.
Console.WriteLine("\nResults of executing modified myQuery1:");
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet80>
//<snippet81>
class HowToOrderJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name="Condiments", Id = 002 },
new Category() { Name="Vegetables", Id = 003 },
new Category() { Name="Grains", Id = 004 },
new Category() { Name="Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
HowToOrderJoins app = new HowToOrderJoins();
app.OrderJoin1();
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void OrderJoin1()
{
var groupJoinQuery2 =
from category in categories
join prod in products on category.Id equals prod.CategoryId into prodGroup
orderby category.Name
select new
{
Category = category.Name,
Products = from prod2 in prodGroup
orderby prod2.Name
select prod2
};
foreach (var productGroup in groupJoinQuery2)
{
Console.WriteLine(productGroup.Category);
foreach (var prodItem in productGroup.Products)
{
Console.WriteLine(" {0,-10} {1}", prodItem.Name, prodItem.CategoryId);
}
}
}
/* Output:
Beverages
Cola 1
Tea 1
Condiments
Mustard 2
Pickles 2
Fruit
Melons 5
Peaches 5
Grains
Vegetables
Bok Choy 3
Carrots 3
*/
}
//</snippet81>
class NullValuesInQueries
{
#region Data
class Product
{
public string Name { get; set; }
public int? CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int? Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
new Category() { Name = "Grains", Id = 004 },
new Category() { Name = "Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
NullValuesInQueries app = new NullValuesInQueries();
Northwind db = new Northwind();
app.TestMethod(db);
}
void NullHandlingQuery()
{
//<snippet82>
var query1 =
from c in categories
where c != null
join p in products on c.Id equals
(p == null ? null : p.CategoryId)
select new { Category = c.Name, Name = p.Name };
//</snippet82>
}
#region compilation only
class Order
{
public int? OrderId = 1;
public int? EmployeeId = 1;
}
class Employee
{
public int EmployeeId = 1;
public string FirstName = string.Empty;
}
class Northwind
{
public Order[] Orders = null;
public Employee[] Employees = null;
}
#endregion
//<snippet83>
void TestMethod(Northwind db)
{
var query =
from o in db.Orders
join e in db.Employees
on o.EmployeeId equals (int?)e.EmployeeId
select new { o.OrderId, e.FirstName };
}
//</snippet83>
}
//<snippet 84 >
class DynamicPreds2
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category(){ Name = "Beverages", Id = 001 },
new Category(){ Name = "Condiments", Id = 002 },
new Category(){ Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
new Product{ Name = "Ice Cream", CategoryId = 007 },
new Product{ Name = "Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
DynamicPreds2 app = new DynamicPreds2();
var q = from p in app.products
select p;
// Identity select
Console.WriteLine("First iteration");
//foreach(var v in query)
// Console.WriteLine(v.Name);
// Mod 1
var q2 = from item in q
where item.Name[0] == 'M'
select item;
Console.WriteLine("2nd iteration");
foreach (var v in q2)
Console.WriteLine(v.Name);
//var q3 = from item in
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet 84>
}