When should I use <> (or <,>)?

Question

At the moment I am struggling a bit with generic typeing.

While reading about it I sometimes encounter ISomeType<>.

E.g.:

Type generic = typeof(Dictionary<,>);

https://msdn.microsoft.com/en-us/library/system.type.makegenerictype%28v=vs.110%29.aspx

I can't really find any documentation on wat the empty <> means.

So: When should I use <> or <,>?

update:

As @dcastro stated, it is called open generics and more info can be found here: Generics -Open and closed constructed Types

update 2

As for the closure argument, this question is about the meaning of the syntax.

Answer 1

An assembly can define, within the same namespace, multiple types with the same name, as long as the number of type arguments (generic parameters) differs for each type. Try for example:

var strA = typeof(Action).ToString();     // "System.Action"
var strB = typeof(Action<>).ToString();   // "System.Action`1[T]"
var strC = typeof(Action<,>).ToString();  // "System.Action`2[T1,T2]"
var strD = typeof(Action<,,>).ToString(); // "System.Action`3[T1,T2,T3]"

In C#, one can only use empty <...>, i.e. avoid specifying what the type parameters are, within the typeof(...) keyword.

Another example: typeof(Dictionary<,>).ToString() gives "System.Collections.Generic.Dictionary`2[TKey,TValue]". As you can see, in .NET (the CLI) the number of generic parameters is given by appending a backtick followed by digit(s).

---

Examples of use:

static bool IsDictionary(object obj)
{
  if (obj == null)
    return false;

  var t = obj.GetType();
  return t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Dictionary<,>);
}

static object GetDictionary(object a, object b)
{
  var t = typeof(Dictionary<,>).MakeGenericType(a.GetType(), b.GetType());
  return Activator.CreateInstance(t);
}

But if possible, avoid reflection and use compile-time types, for example (not quite equivalent to the above):

static Dictionary<TA, TB> GetDictionary<TA, TB>(TA a, TB b)
{
  return new Dictionary<TA, TB>();
}