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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?.

Alan
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    It's a character that is a part of expressions/statements, including the ternary operator and the case statement. What specifically are you asking about? – Kirk Woll Jun 10 '13 at 23:37
  • Possible duplicate of this? http://stackoverflow.com/questions/338398/thisfoo-syntax-in-c-sharp-constructors – Kyle Muir Jun 10 '13 at 23:38
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    Not duplicate. That was asking about extending constructors not the purpose of `:` – FabianCook Jun 10 '13 at 23:38
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    This question not very useful IMO. It's like asking "what is the purpose of the character 'a' in C#" and getting a list of "var", "class", etc – Blorgbeard Jun 11 '13 at 00:05
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    @Blorgbeard Attempting to denigrate this question as being like "_what is the purpose of the character 'a'_" is utterly absurd. As per the accepted answer, `:` has ***very specific semantics*** depending on a number of possible contexts. – Disillusioned May 09 '16 at 05:50
  • So what? So does `a`. – Blorgbeard May 09 '16 at 07:15

1 Answers1

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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.

Mathieu Guindon
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p.s.w.g
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    The generic type constraint answers the OP's interrogation about methods. `DoSomething(T foo) where T : IEnumerable` – Mathieu Guindon Jun 10 '13 at 23:51
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    In addition to calling the base constructor, you can use it to call another overload of the current class' constructor. eg: `public Foo() : this("value passed as argument") { }` – Simon Elms Jun 10 '13 at 23:53
  • @SimonTewsi / retailcoder Thanks I've included those alternate uses for completeness. They're sneaky little things--I forgot how often I used them. – p.s.w.g Jun 10 '13 at 23:59
  • So you might say that a colon is a way to qualify a type when the qualifier is too complex to fit in a single keyword, sort of how colons precede subtitles of books? – Nicholas Cousar Jun 20 '21 at 06:27