Tutorial :Best way to check if System.Type is a descendant of a given class



Question:

Consider the following code:

public class A   {  }      public class B : A   {  }      public class C : B   {  }      class D    {        public static bool IsDescendantOf(this System.Type thisType, System.Type thatType)        {            /// ???       }         void Main()      {          A cValue = new C();          C.GetType().IsDescendantOf(cValue.GetType());      }  }  

What is the best way to implement IsDescendantOf?


Solution:1

Type.IsSubclassOf() Determines whether the class represented by the current Type derives from the class represented by the specified Type.


Solution:2

You are probably looking for Type.IsAssignableFrom.


Solution:3

I realise this doesn't directly answer your question, but you might consider using this instead of the method in your example:

public static bool IsDescendantOf<T>(this object o)  {      if(o == null) throw new ArgumentNullException();      return typeof(T).IsSubclassOf(o.GetType());  }  

So you can use it like this:

C c = new C();  c.IsDescendantOf<A>();  

Also, to answer your question about the difference between Type.IsSubclassOf and Type.IsAssignableFrom - IsAssignableFrom is weaker in the sense that if you have two objects a and b such that this is valid:

a = b;  

Then typeof(A).IsAssignableFrom(b.GetType()) is true - so a could be a subclass of b, or an interface type.

In contrast, a.GetType().IsSubclassOf(typeof(B)) would only return true if a were a subclass of b. Given the name of your extension method, I'd say you should use IsSubclassOf instead of IsAssignable to;


Solution:4

I think you are looking for this Type.IsSubclassOf()

Edit:

I don't know your requirements but may be thats the best way:

bool isDescendant = cValue is C;  

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