using System.Collections.Generic;

​

public abstract class AbstractClass {

    public AbstractClass(){

    }

}

​

// One of the possible types derived from AbstractClass

public class SomeClass : AbstractClass{

    public SomeClass(){

    }

}

​

// The class with the problematic method

public class YetSomeOtherObject {

    // The T has to be the type of the obj that is passed as parameter, as in, one of the subclasses of AbstractClass

    public void ProblematicMethod<T>(AbstractClass obj){

    }

}

​

public class Program

{

    public struct SomeStruct {

        // ... has other various members but asbtract this obj is the most important one

        // The obj becomes an instance of one of many subclasses that inherit from this abstract class

        public AbstractClass obj;

        public SomeStruct(AbstractClass obj){

            this.obj = obj;

        }

    }

    public static void Main()

    {

        YetSomeOtherObject objectWithImportantCall = new YetSomeOtherObject();

        List<SomeStruct> list = new List<SomeStruct>();

        list.Add(new SomeStruct(new SomeClass()));

        foreach(SomeStruct thing in list){

            // type has to be the type of thing.obj

            objectWithImportantCall.ProblematicMethod<type>(thing.obj);

        }

    }

}