What is the difference between a generic type and a generic type definition?

Question

I\'m Studying up on .net reflection and am having a hard time figuring out the difference.

From what I understand, List is a generic type defi

Answer 1

In your example List is a generic type definition. T is called a generic type parameter. When the type parameter is specified like in List or List or List then you have a generic type. You can see that by running some code like this...

public static void Main()
{
    var l = new List();
    PrintTypeInformation(l.GetType());
    PrintTypeInformation(l.GetType().GetGenericTypeDefinition());
}

public static void PrintTypeInformation(Type t)
{
    Console.WriteLine(t);
    Console.WriteLine(t.IsGenericType);
    Console.WriteLine(t.IsGenericTypeDefinition);    
}

Which will print

System.Collections.Generic.List`1[System.String] //The Generic Type.
True //This is a generic type.
False //But it isn't a generic type definition because the type parameter is specified
System.Collections.Generic.List`1[T] //The Generic Type definition.
True //This is a generic type too.                               
True //And it's also a generic type definition.

Another way to get the generic type definition directly is typeof(List<>) or typeof(Dictionary<,>).