C# programs consist of one or more files. Each file contains zero or more namespaces. A namespace contains types such as classes, structs, interfaces, enumerations, and delegates, or other namespaces. The following example is the skeleton of a C# program that contains all of these elements.

using System;Console.WriteLine("Hello world!");namespace YourNamespace{    class YourClass    {    }    struct YourStruct    {    }    interface IYourInterface    {    }    delegate int YourDelegate();    enum YourEnum    {    }    namespace YourNestedNamespace    {        struct YourStruct        {        }    }}

The preceding example usestop-level statements for the program's entry point. Only one file can have top-level statements. The program's entry point is the first text line of program text in that file. In this case, it's theConsole.WriteLine("Hello world!");.You can also create a static method namedMain as the program's entry point, as shown in the following example:

// A skeleton of a C# programusing System;namespace YourNamespace{    class YourClass    {    }    struct YourStruct    {    }    interface IYourInterface    {    }    delegate int YourDelegate();    enum YourEnum    {    }    namespace YourNestedNamespace    {        struct YourStruct        {        }    }    class Program    {        static void Main(string[] args)        {            Console.WriteLine("Hello world!");        }    }}

In that case the program starts in the opening brace ofMain method, which isConsole.WriteLine("Hello world!");

Building and running C# programs

C# is acompiled language. In most C# programs, you use thedotnet build command to compile a group of source files into a binary package. Then, you use thedotnet run command to run the program. (You can simplify this process becausedotnet run compiles the program before running it if necessary.) These tools support a rich language of configuration options and command-line switches. Thedotnet command line interface (CLI), which is included in the .NET SDK, provides manytools to generate and modify C# files.

Beginning with C# 14 and .NET 10, you can createfile based programs, which simplifies building and running C# programs. You use thedotnet run command to run a program contained in a single*.cs file. For example, if the following snippet is stored in a file namedhello-world.cs, you can run it by typingdotnet run hello-world.cs:

#!/usr/local/share/dotnet/dotnet runConsole.WriteLine("Hello, World!");

The first line of the program contains the#! sequence for Unix shells. The location of thedotnet CLI can vary on different distributions. On any Unix system, if you set theexecute (+x) permission on a C# file, you can run the C# file from the command line:

./hello-world.cs

The source for these programs must be a single file, but otherwise all C# syntax is valid. You can use file based programs for small command-line utilities, prototypes, or other experiments. File based programs allowpreprocessor directives that configure the build system.

Expressions and statements

C# programs are built usingexpressions andstatements. Expressions produce a value, and statements perform an action:

Anexpression is a combination of values, variables, operators, and method calls that evaluate to a single value. Expressions produce a result and can be used wherever a value is expected. The following examples are expressions:

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

Astatement is a complete instruction that performs an action. Statements don't return values; instead, they control program flow, declare variables, or perform operations. The following examples are statements:

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

The key distinction: expressions evaluate to values, while statements perform actions. Some constructs, like method calls, can be both. For example,Math.Max(a, b) is an expression when used inint result = Math.Max(a, b);, but becomes an expression statement when written alone asMath.Max(a, b);.

For detailed information about statements, seeStatements. For information about expression-bodied members and other expression features, seeExpression-bodied members.

Related Sections

You learn about these program elements in thetypes section of the fundamentals guide:

• Classes
• Structs
• Namespaces
• Interfaces
• Enums
• Delegates

C# Language Specification

For more information, seeBasic concepts in theC# Language Specification. The language specification is the definitive source for C# syntax and usage.