Yan Cui
I help clients go faster for less using serverless technologies.
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On the Type class there are two very useful methods which allows you to determine the inheritance relationship of two arbitrary types at runtime – IsSubclassOf method and IsAssignableFrom method.
IsSubclassOf
The MSDN documentation for the IsSubclassOf method states:
Determines whether the class represented by the current Type derives from the class represented by the specified Type.
1: public class ClassA { }
2: public class ClassB : ClassA { }
3: public class ClassC : ClassB { }
4: ...
5: Console.WriteLine(typeof(ClassB).IsSubclassOf(typeof(ClassA))); // TRUE
6: Console.WriteLine(typeof(ClassB).IsSubclassOf(typeof(ClassB))); // FALSE
7: Console.WriteLine(typeof(ClassB).IsSubclassOf(typeof(ClassC))); // FALSE
8: Console.WriteLine(typeof(ClassC).IsSubclassOf(typeof(ClassA))); // TRUE
9: Console.WriteLine(typeof(ClassC).IsSubclassOf(typeof(ClassB))); // TRUE
Pretty useful, eh? However, it doesn’t work when it comes to interfaces:
The IsSubclassOf method cannot be used to determine whether an interface derives from another interface, or whether a class implements an interface.
1: public interface ID { }
2: public interface IE : ID { }
3:
4: public class ClassD : ID { }
5:
6: Console.WriteLine(typeof(ClassD).IsSubclassOf(typeof(ID))); // FALSE
7: Console.WriteLine(typeof(IE).IsSubclassOf(typeof(ID)); // FALSE
Ok, roger that, but what about generics? Let’s see..
1: public class ClassF<T> { }
2: public class ClassG<T> : ClassF<T> { }
3: public class ClassH : ClassF<int> { }
4:
5: // no type constraint, no good
6: Console.WriteLine(typeof(ClassG<>).IsSubclassOf(typeof(ClassF<>))); // FALSE
7:
8: // no covariance support here
9: Console.WriteLine(typeof(string).IsSubclassOf(typeof(object))); // TRUE
10: Console.WriteLine(typeof(ClassG<string>).IsSubclassOf(typeof(ClassF<object>))); // FALSE
11:
12: // type constraint has to match
13: Console.WriteLine(typeof(ClassG<int>).IsSubclassOf(typeof(ClassF<int>))); // TRUE
14: Console.WriteLine(typeof(ClassH).IsSubclassOf(typeof(ClassF<int>))); // TRUE
15: Console.WriteLine(typeof(ClassH).IsSubclassOf(typeof(ClassF<long>))); // FALSE
IsAssignableFrom
The MSDN documentation for the IsAssignableFrom method states:
Determines whether an instance of the current Type can be assigned from an instance of the specified Type.
The IsAssignableFrom method basically works the same way as the is operator and does a simple assignment compatibility test to see if a variable of type A can be assigned with a variable of type B. Unlike the IsSubclassOf method, it also works for interfaces and remember, if typeof(B).IsSubclassOf(typeof(A)) is true then typeof(A).IsAssignableFrom(typeof(B)) is also true.
1: Console.WriteLine(typeof(IA).IsAssignableFrom(typeof(IB))); // TRUE
2: Console.WriteLine(typeof(IA).IsAssignableFrom(typeof(IA))); // TRUE
3: Console.WriteLine(typeof(IB).IsAssignableFrom(typeof(IA))); // FALSE
4: Console.WriteLine(typeof(IA).IsAssignableFrom(typeof(ClassA))); // TRUE
5: Console.WriteLine(typeof(ClassA).IsAssignableFrom(typeof(ClassB))); // TRUE
Again, let’s consider the generics case:
1: // no type constraint, still no good
2: Console.WriteLine(typeof(ClassF<>).IsAssignableFrom(typeof(ClassG<>))); // FALSE
3:
4: // note: whilst you can't specify generic variance on the class definition you can do it on the
5: // interface, e.g. IEnumerable<out T>, so we're able to do the following test
6: Console.WriteLine(typeof(object).IsAssignableFrom(typeof(string))); // TRUE
7: Console.WriteLine(typeof(IEnumerable<object>).IsAssignableFrom(typeof(IEnumerable<string>))); // TRUE
Performance Overheads
In general, doing reflection is an expensive business and you should always be mindful of the potential performance hit you get if you have to do lots of reflection in your code.
I can’t think of too many places where you’d need to repeatedly test if one type is subclass of/can be assigned from another type but nonetheless, out of pure curiosity, I decided to give it a test and see what sort of performance overhead these two methods carry and here are the results:
Test: execute 100000 times
| Method call | Return value | Avg time taken (milliseconds) over 3 tries |
| typeof(ClassB).IsSubclassOf(ClassA) | TRUE | 6 |
| typeof(ClassB).IsSubclassOf(typeof(ClassH)) | FALSE | 16 |
| typeof(ClassA).IsSubclassOf(typeof(IA)) | FALSE | 11 |
| typeof(IA).IsAssignableFrom(typeof(IB)) | TRUE | 6 |
| typeof(ClassB).IsAssignableFrom(typeof(ClassH)) | FALSE | 6 |
| typeof(IA).IsAssignableFrom(typeof(ClassH)) | FALSE | 6 |
Notice that with the IsSubclassOf method there’s noticeable difference in average run time between cases that returns true and those that return false. Whilst the IsAssignableFrom method consistently runs at 6 milliseconds.
However, given the small amount of time taken to execute each check 100000 times the morale of this story is really “Don’t lose any sleep over it”!
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