Are these tests for Nullable Type equivalent?

Question

Here is the condition I used to detect if we are dealing with a Nullable Type :

System.Nullable.GetUnderlyingType(itemType) != null

and here the code of my teammate :

itemType.IsGenericType && itemType.GetGenericTypeDefinition() == typeof(Nullable<>)

We actually didnt find a case where one will return true and the other false (or vice-versa) but are these 2 snippets strictly equivalent ?

Answer 1

From MSDN for Nullable.GetUnderlyingType Method:

The type argument of the nullableType parameter, if the nullableType parameter is a closed generic nullable type; otherwise, null.

So, yes it is safe to use the former version.

Decompiled from GetUnderlyingType:

public static Type GetUnderlyingType(Type nullableType)
{
  if (nullableType == null)
    throw new ArgumentNullException("nullableType");
  Type type = (Type) null;
  if (nullableType.IsGenericType && !nullableType.IsGenericTypeDefinition && nullableType.GetGenericTypeDefinition() == typeof (Nullable<>))
    type = nullableType.GetGenericArguments()[0];
  return type;
}

Answer 2

These 2 snippets are not fully equivalen.
Here is the test case that returns a different values for each snippets:

Type t = typeof(Nullable<>);

bool c1 = Nullable.GetUnderlyingType(t) != null; //false
bool c2 = t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>); //true

Thus the Nullable.GetUnderlyingType method more safe, because it's implementation already include this test case checking:

public static Type GetUnderlyingType(Type nullableType) {
    if (nullableType == null) 
        throw new ArgumentNullException("nullableType");
    Type type = null;
    if ((nullableType.IsGenericType && !nullableType.IsGenericTypeDefinition)
        && (nullableType.GetGenericTypeDefinition() == typeof(Nullable<>))) {
        type = nullableType.GetGenericArguments()[0];
    }
    return type;
}