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

Question

I have been trying to get reference in the Microsoft Developer website about what the function of the : really is but I cant find it because it seems that it is neither a keyword or a operator so what is the function of the colon in C#? Also I have seen it being applied to a Method how does that function?.

Answer 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.