IsDerived

public static bool IsDerived<T>(this Type type)
{
	Type baseType = typeof(T);

	if(baseType.FullName == type.FullName)
	{
		return true;
	}

	if(type.IsClass)
	{
		return baseType.IsClass
			? type.IsSubclassOf(baseType)
			: baseType.IsInterface 
				? IsImplemented(type, baseType) 
				: false;
	}
	else if(type.IsInterface && baseType.IsInterface)
	{
		return IsImplemented(type, baseType);
	}
	return false;
}

public static bool IsImplemented(Type type, Type baseType)
{
	Type[] faces = type.GetInterfaces();
	foreach(Type face in faces)
	{
		if(baseType.Name.Equals(face.Name))
		{
			return true;
		}
	}
	return false;
}

if(typeof(UserEntity).IsDerived<IExtend>())
{
	// do something..
}