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

Question

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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