Partial type definitions allow you to split the definition of a class, struct, interface, or record into multiple definitions. You can put these multiple definitions in different files within the same project. One type declaration contains only the signatures for partial members:
sample · csharpdocs/csharp/language-reference/keywords/snippets/PartialMembers.csDeclaringPart
using System.Text.RegularExpressions;
namespace PartialClassesAndMembers;
//<DeclaringPart>
partial class A
{
int num = 0;
void MethodA() { }
partial void MethodC();
}
//</DeclaringPart>
//<ImplementingPart>
partial class A
{
void MethodB() { }
partial void MethodC() { }
}
//</ImplementingPart>
//<OptionalPartialMethod>
partial class MyPartialClass
{
// Declaring definition
partial void OnSomethingHappened(string s);
}
// This part can be in a separate file.
partial class MyPartialClass
{
// Comment out this method and the program
// will still compile.
partial void OnSomethingHappened(string s) =>
Console.WriteLine($"Something happened: {s}");
}
//</OptionalPartialMethod>
// <SourceGeneratorPartial>
public partial class RegExSourceGenerator
{
[GeneratedRegex("cat|dog", RegexOptions.IgnoreCase, "en-US")]
private static partial Regex CatOrDogGeneratedRegex();
private static void EvaluateText(string text)
{
if (CatOrDogGeneratedRegex().IsMatch(text))
{
// Take action with matching text
}
}
}
// </SourceGeneratorPartial>
// <PartialMembersAll>
// Declaring declaration
public partial class PartialExamples
{
/// <summary>
/// Gets or sets the number of elements that the List can contain.
/// </summary>
public partial int Capacity { get; set; }
/// <summary>
/// Gets or sets the element at the specified index.
/// </summary>
/// <param name="index">The index</param>
/// <returns>The string stored at that index</returns>
public partial string this[int index] { get; set; }
public partial string? TryGetAt(int index);
}
public partial class PartialExamples
{
private List<string> _items = [
"one",
"two",
"three",
"four",
"five"
];
// Implementing declaration
/// <summary>
/// Gets or sets the number of elements that the List can contain.
/// </summary>
/// <remarks>
/// If the value is less than the current capacity, the list will shrink to the
/// new value. If the value is negative, the list isn't modified.
/// </remarks>
public partial int Capacity
{
get => _items.Count;
set
{
if ((value != _items.Count) && (value >= 0))
{
_items.Capacity = value;
}
}
}
public partial string this[int index]
{
get => _items[index];
set => _items[index] = value;
}
/// <summary>
/// Gets the element at the specified index.
/// </summary>
/// <param name="index">The index</param>
/// <returns>The string stored at that index, or null if out of bounds</returns>
public partial string? TryGetAt(int index)
{
if (index < _items.Count)
{
return _items[index];
}
return null;
}
}
// </PartialMembersAll>The other declaration contains the implementation of the partial members:
sample · csharpdocs/csharp/language-reference/keywords/snippets/PartialMembers.csImplementingPart
using System.Text.RegularExpressions;
namespace PartialClassesAndMembers;
//<DeclaringPart>
partial class A
{
int num = 0;
void MethodA() { }
partial void MethodC();
}
//</DeclaringPart>
//<ImplementingPart>
partial class A
{
void MethodB() { }
partial void MethodC() { }
}
//</ImplementingPart>
//<OptionalPartialMethod>
partial class MyPartialClass
{
// Declaring definition
partial void OnSomethingHappened(string s);
}
// This part can be in a separate file.
partial class MyPartialClass
{
// Comment out this method and the program
// will still compile.
partial void OnSomethingHappened(string s) =>
Console.WriteLine($"Something happened: {s}");
}
//</OptionalPartialMethod>
// <SourceGeneratorPartial>
public partial class RegExSourceGenerator
{
[GeneratedRegex("cat|dog", RegexOptions.IgnoreCase, "en-US")]
private static partial Regex CatOrDogGeneratedRegex();
private static void EvaluateText(string text)
{
if (CatOrDogGeneratedRegex().IsMatch(text))
{
// Take action with matching text
}
}
}
// </SourceGeneratorPartial>
// <PartialMembersAll>
// Declaring declaration
public partial class PartialExamples
{
/// <summary>
/// Gets or sets the number of elements that the List can contain.
/// </summary>
public partial int Capacity { get; set; }
/// <summary>
/// Gets or sets the element at the specified index.
/// </summary>
/// <param name="index">The index</param>
/// <returns>The string stored at that index</returns>
public partial string this[int index] { get; set; }
public partial string? TryGetAt(int index);
}
public partial class PartialExamples
{
private List<string> _items = [
"one",
"two",
"three",
"four",
"five"
];
// Implementing declaration
/// <summary>
/// Gets or sets the number of elements that the List can contain.
/// </summary>
/// <remarks>
/// If the value is less than the current capacity, the list will shrink to the
/// new value. If the value is negative, the list isn't modified.
/// </remarks>
public partial int Capacity
{
get => _items.Count;
set
{
if ((value != _items.Count) && (value >= 0))
{
_items.Capacity = value;
}
}
}
public partial string this[int index]
{
get => _items[index];
set => _items[index] = value;
}
/// <summary>
/// Gets the element at the specified index.
/// </summary>
/// <param name="index">The index</param>
/// <returns>The string stored at that index, or null if out of bounds</returns>
public partial string? TryGetAt(int index)
{
if (index < _items.Count)
{
return _items[index];
}
return null;
}
}
// </PartialMembersAll>The declarations for a partial type can appear in either the same or multiple files. Typically, the two declarations are in different files. You split a class, struct, or interface type when you're working with large projects, with automatically generated code such as that provided by the Windows Forms Designer, or Source generators like RegEx. A partial type can contain partial members.
ms.custom: "updateeachrelease"
The C# language reference documents the most recently released version of the C# language. It also contains initial documentation for features in public previews for the upcoming language release.
The documentation identifies any feature first introduced in the last three versions of the language or in current public previews.
Starting with C# 13, you can define partial properties and partial indexers. Starting with C# 14, you can define partial instance constructors and partial events. Before C# 13, only methods could be defined as partial members.
You can provide documentation comments on either the declaring declaration or the implementing declaration. When you apply documentation comments to both type declarations, the XML elements from each declaration are included in the output XML. For the rules on partial member declarations, see the article on partial members.
You can apply attributes to either declaration. The compiler combines all attributes from both declarations, including duplicates.
For more information, see Partial Classes and Methods.
C# language specification
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.