Type.IsSubclssOf and Type.IsAssignableFrom

Yan Cui

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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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