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One method to find derived types of type AType is to enumerate all types defined in the assembly and compare their BaseType property to the type AType. This method is used in ILSpy to show derived types of a selected type. Implementation is in FindDerivedTypes method (DerivedTypesTreeNode.cs). To find types derived indirectly you have to iterate over BaseType property (using Resolve()) until the AType is reached or BaseType is equal to null.