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General Structure of a C# Program

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

// A skeleton of a C# program
using 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 will start in the first line of Main method, which is Console.WriteLine("Hello world!");

Expressions and statements

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

An expression is a combination of values, variables, operators, and method calls that evaluates 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)

A statement 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 in int result = Math.Max(a, b);, but becomes an expression statement when written alone as Math.Max(a, b);.

For detailed information about statements, see Statements. For information about expression-bodied members and other expression features, see Expression-bodied members.

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

C# Language Specification

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