public static bool IsAssignableFrom(TypeInfo toTypeInfo, TypeInfo fromTypeInfo, FoundationTypes foundationTypes)
{
if (toTypeInfo == null)
throw new NullReferenceException();
if (fromTypeInfo == null)
return false; // It would be more appropriate to throw ArgumentNullException here, but returning "false" is the desktop-compat behavior.
if (fromTypeInfo.Equals(toTypeInfo))
return true;
if (toTypeInfo.IsGenericTypeDefinition)
{
// Asking whether something can cast to a generic type definition is arguably meaningless. The desktop CLR Reflection layer converts all
// generic type definitions to generic type instantiations closed over the formal generic type parameters. The .NET Native framework
// keeps the two separate. Fortunately, under either interpretation, returning "false" unless the two types are identical is still a
// defensible behavior. To avoid having the rest of the code deal with the differing interpretations, we'll short-circuit this now.
return false;
}
if (fromTypeInfo.IsGenericTypeDefinition)
{
// The desktop CLR Reflection layer converts all generic type definitions to generic type instantiations closed over the formal
// generic type parameters. The .NET Native framework keeps the two separate. For the purpose of IsAssignableFrom(),
// it makes sense to unify the two for the sake of backward compat. We'll just make the transform here so that the rest of code
// doesn't need to know about this quirk.
fromTypeInfo = fromTypeInfo.GetGenericTypeDefinition().MakeGenericType(fromTypeInfo.GenericTypeParameters).GetTypeInfo();
}
if (fromTypeInfo.CanCastTo(toTypeInfo, foundationTypes))
return true;
Type toType = toTypeInfo.AsType();
Type fromType = fromTypeInfo.AsType();
// Desktop compat: IsAssignableFrom() considers T as assignable to Nullable<T> (but does not check if T is a generic parameter.)
if (!fromType.IsGenericParameter)
{
Type nullableUnderlyingType = Nullable.GetUnderlyingType(toType);
if (nullableUnderlyingType != null && nullableUnderlyingType.Equals(fromType))
return true;
}
return false;
}