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

Solution 1:

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.