General structure of a C# program

You build C# programs from these core building blocks: namespaces organize your types, types (classes, structs, interfaces, enums, and delegates) define behavior and data, and statements and expressions perform work at run time. The way you structure the entry point depends on which application style you choose.

#Choosing your application style

When you create a C# program, make two independent choices about how to structure it:

• File-based or project-based?
  • A file-based app runs from a single .cs file with no project file.
  • A project-based app uses a .csproj file and can span multiple source files.
• Top-level statements or Main method?
  • Top-level statements let you write executable code directly at the top of a file.
  • A Main method wraps the entry point in an explicit static method.

Both project-based apps and file-based apps support either entry-point style.

#File-based apps vs. project-based apps

Starting with C# 14 and .NET 10, file-based apps let you run a program contained in a single *.cs file without a project file. Store the following code in a file named hello-world.cs and run it with dotnet run hello-world.cs or dotnet hello-world.cs:

sample · csharpdocs/csharp/fundamentals/program-structure/snippets/file-based-program/hello-world.cs

#!/usr/bin/env dotnet
Console.WriteLine("Hello, World!");

File-based apps support all C# syntax and can use preprocessor directives to configure the build system. Use file-based apps for small command-line utilities, prototypes, and experiments. A file-based app consists of a single file in a directory:

my-app/
└── hello-world.cs

Project-based apps use a .csproj file and the .NET CLI commands dotnet new, dotnet build, and dotnet run workflow. Choose project-based apps when your program spans multiple files or needs fine-grained build configuration. A project-based app includes a project file alongside one or more source files:

my-app/
├── my-app.csproj
├── Program.cs
├── Models/
│   └── Person.cs
└── Services/
    └── GreetingService.cs

If your file-based app grows, you can easily convert it to a project-based app. Run dotnet project convert to generate a project file from your existing source file.

If you know your app needs multiple source files from the start, begin with a project-based app. You avoid the conversion step and can organize your code into separate files right away.

#Top-level statements vs. Main method

By using top-level statements, you can write executable code directly in one file without wrapping it in a class and Main method. This style is the default when you create a new console app with dotnet new console. The following example shows a modern C# program that uses top-level statements:

sample · csharpdocs/csharp/fundamentals/program-structure/snippets/toplevel-structure/Program.cs

Console.WriteLine("Hello, World!");

namespace YourNamespace
{
    class YourClass
    {
    }

    struct YourStruct
    {
    }

    interface IYourInterface
    {
    }

    delegate int YourDelegate();

    enum YourEnum
    {
    }
}

Only one file in a project can have top-level statements, and the entry point is the first line of program text in that file. As you build larger programs, you include more program elements.

You can also define an explicit static Main method as the program's entry point:

sample · csharpdocs/csharp/fundamentals/program-structure/snippets/structure/Program.cs

// A skeleton of a C# program using an explicit Main method
namespace YourNamespace;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello, World!");
    }
}

Both entry-point styles work with file-based and project-based apps. Both styles support the same features.

#Building and running C# programs

C# is a compiled language. For project-based apps, use the dotnet build command to compile source files into a binary package. Use dotnet run to build and run in one step. The dotnet CLI, included in the .NET SDK, provides many tools to create, build, and manage C# projects.

For file-based apps, dotnet run hello-world.cs compiles and runs the single file directly - no project file required.

#Expressions and statements

If you followed the Get started tutorials, you already wrote expressions and statements. Every line of code you typed was one or the other (or both). Now let's define those terms.

Expressions and statements are the fundamental building blocks of a C# program. An expression produces a value. A statement performs an action and typically ends in a semicolon.

The following are expressions:

• 42 (literal value)
• x + y (arithmetic operation)
• Math.Max(a, b) (method call that produces a value)
• condition ? trueValue : falseValue (conditional expression)
• new Person("John") (object creation)

A statement performs an action. Statements control program flow, declare variables, or invoke operations. The following are statements:

• int x; (declaration statement)
• int x = 42; (declaration statement with initialization)
• Console.WriteLine("Hello"); (method call statement)
• if (condition) { /* code */ } (conditional statement)
• return result; (return statement)

Statements often contain expressions, and expressions can nest inside other expressions. For example, the following declaration statement assigns maxResult to the result of an addition expression. That addition expression adds the results of two method call expressions:

var maxResult = Math.Max(a, b) + Math.Max(c, d);

For detailed information about statements, see Statements. For information about expression-bodied members, see Expression body definitions.

#Related content

• Classes
• Structs
• Namespaces
• Interfaces
• Enums
• Delegates
• Basic concepts in the C# language specification