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.