Better way to write common function with parameter is different class

In my project, I have some functions like that:

func 1 (with 2 class: class1 and classs1_1):

List<class1> GetData1(class1_1 cls)
{
    List<class1> list = new List<class1>();
    cls = new cls();
    //do something for cls, like cls.GetSpecialCase();

    //fill data from cls to list, as using foreach to fill data to list

    return list;
}

func 2 (with 2 class: class2 and classs2_2):

List<class2> GetData2(class2_2 cls)
{
    List<class2> list = new List<class2>();
    cls = new cls();
    //do something for cls, like cls.GetSpecialCase();

    //fill data from cls to list, as using foreach to fill data to list

    return list;
}

How could I write a function like that (because I don't want to use a lot of functions as above):

common func (with T is class1, object cls is class1_1):

List<T> GetData(object cls)
{
    List<T> list = new List<T>();
    //how to recognize which class of cls
    if(cls == class1_1)     class1_1 other_cls = cls as class1_1;
    if(cls == class2_2)     class2_2 other_cls = cls as class2_2;
    //do something for other_cls 

    //fill data from other_cls to list
    return list;
}

I just want to pass parameter as object, and in the common function, I want to know which class I'm passing to so that I can treat to right class, and call right function.

Anyone helps me to solve this problem.

Thank you very much.