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Main() and command line arguments

The Main method is the entry point of a C# application. (Libraries and services do not require a Main method as an entry point.) When the application is started, the Main method is the first method that is invoked.

There can only be one entry point in a C# program. If you have more than one class that has a Main method, you must compile your program with the -main compiler option to specify which Main method to use as the entry point. For more information, see -main (C# Compiler Options).

class TestClass
{
    static void Main(string[] args)
    {
        // Display the number of command line arguments.
        Console.WriteLine(args.Length);
    }
}

Overview

  • The Main method is the entry point of an executable program; it is where the program control starts and ends.
  • Main is declared inside a class or struct. Main must be static and it need not be public. (In the earlier example, it receives the default access of private.) The enclosing class or struct is not required to be static.
  • Main can either have a void, int, or, starting with C# 7.1, Task, or Task<int> return type.
  • If and only if Main returns a Task or Task<int>, the declaration of Main may include the [async](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/async) modifier. Note that this specifically excludes an async void Main method.
  • The Main method can be declared with or without a string[] parameter that contains command-line arguments. When using Visual Studio to create Windows applications, you can add the parameter manually or else use the GetCommandLineArgs() method to obtain the command-line arguments. Parameters are read as zero-indexed command-line arguments. Unlike C and C++, the name of the program is not treated as the first command-line argument in the args array, but it is the first element of the GetCommandLineArgs() method.

The following is a list of valid Main signatures:

public static void Main() { }
public static int Main() { }
public static void Main(string[] args) { }
public static int Main(string[] args) { }
public static async Task Main() { }
public static async Task<int> Main() { }
public static async Task Main(string[] args) { }
public static async Task<int> Main(string[] args) { }

The preceding examples all use the public accessor modifier. That is typical, but not required.

The addition of async and Task, Task<int> return types simplifies program code when console applications need to start and await asynchronous operations in Main.