Will IsConstructedGenericType always be the negation of IsGenericTypeDefinition, for a generic type?

Question

The documentation of the instance property Type.IsConstructedGenericType is unclear or misleading.

I tried the following code to find the actual behavior of this and related properties:

// create list of types to use later in a Dictionary<,>
var li = new List<Type>();

// two concrete types:
li.Add(typeof(int));
li.Add(typeof(string));

// the two type parameters from Dictionary<,>
li.Add(typeof(Dictionary<,>).GetGenericArguments()[0]);
li.Add(typeof(Dictionary<,>).GetGenericArguments()[1]);

// two unrelated type parameters
li.Add(typeof(Func<,,,>).GetGenericArguments()[1]);
li.Add(typeof(EventHandler<>).GetGenericArguments()[0]);

// run through all possibilities
foreach (var first in li)
{
    foreach (var second in li)
    {
        var t = typeof(Dictionary<,>).MakeGenericType(first, second);
        Console.WriteLine(t);
        Console.WriteLine(t.IsGenericTypeDefinition);
        Console.WriteLine(t.IsConstructedGenericType);
        Console.WriteLine(t.ContainsGenericParameters);
    }
}

The code runs through a Cartesian product consisting of 36 types t.

Results: For 32 types (all but the 4 combinations Dictionary<int, int>, Dictionary<int, string>, Dictionary<string, int>, Dictionary<string, string>), the value of ContainsGenericParameters was true.

For 35 types, IsGenericTypeDefinition was false while IsConstructedGenericType was true. For the last type, namely (unsurprisingly):

System.Collections.Generic.Dictionary`2[TKey,TValue]

the IsGenericTypeDefinition was true and IsConstructedGenericType was false.

Can I conclude that, for a generic type, the value of IsConstructedGenericType is always the opposite (negation) of IsGenericTypeDefinition?

(The documentation seems to claim that IsConstructedGenericType is instead the opposite of ContainsGenericParameters, but we clearly exhibited a lot of counterexamples to that.)

Answer 1

Yes this is correct. Assuming that the Type in question is a generic type, exactly one of IsGenericTypeDefinition or IsConstructedGenericType is true. We can easily from the reference source for RuntimeType (which is the concrete implementation of Type you get when you do GetType() or typeof) why this is the case:

public override bool IsConstructedGenericType 
{
    get { return IsGenericType && !IsGenericTypeDefinition; } 
}