In C# what category does the colon ” : ” fall into, and what does it really mean?

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Colons are used in a dozen fundamentally different places (that I can think of, with the help of everyone in the comments):

  • Separating a class name from its base class / interface implementations in class definitions

    public class Foo : Bar { }

  • Specifying a generic type constraint on a generic class or method

    public class Foo<T> where T : Bar { }

public void Foo<T>() where T : Bar { }

  • Indicating how to call another constructor on the current class or a base class’s constructor prior to the current constructor

    public Foo() : base() { }

public Foo(int bar) : this() { }

  • Specifying the global namespace (as C. Lang points out, this is the namespace alias qualifier)

    global::System.Console

  • Specifying attribute targets

    [assembly: AssemblyVersion("1.0.0.0")]

  • Specifying parameter names

    Console.WriteLine(value: "Foo");

  • As part of a ternary expression

    var result = foo ? bar : baz;

  • As part of a case or goto label

    switch(foo) { case bar: break; }

goto Bar;
Foo: return true;
Bar: return false;

  • Since C# 6, for formatting in interpolated strings

    Console.WriteLine($"{DateTime.Now:yyyyMMdd}");

  • Since C# 7, in tuple element names

    var foo = (bar: "a", baz: "b");
Console.WriteLine(foo.bar);

In all these cases, the colon is not used as an operator or a keyword (with the exception of ::). It falls into the category of simple syntactic symbols, like [] or {}. They are just there to let the compiler know exactly what the other symbols around them mean.

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