c# Generic class, but how?

I have a problem with which I can't get any further. Google brings me a lot of results, but I can't really get by.

I've tried different things with generic classes and reflections. Nothing worked. I have again minimized the following example so that only the most important things remain. I would be grateful for help.

roland

public class TestData1
{
    public long Id { get; set; } = 0;
    public string Firstname { get; set; }
    public string Name { get; set; }
    public string Fullname { get; set; }
    public string Beruf { get; set; }
    public DateTime Date { get; set; } = DateTime.Now;
}

public class TestData2
{
    public long Id { get; set; } = 0;
    public string Firstname { get; set; }
    public string Name { get; set; }
    public string Fullname { get; set; }
}



public class Doit
{

    public Doit( ) {

        // init two different classes with data
        TestData1 td1 = new TestData1 {Firstname="Roland", Name="Smith", Beruf="Developer"};
        TestData2 td2 = new TestData2 {Firstname="Petra", Name="Summer" };
            
        // call the function to do the same action with the diferent classes
        TestData1 tf1 = TestFunc<TestData1>(td1, "OtherParams" );
        TestData2 tf2 = TestFunc(td2, "OtherParams");

    }
    


    /// <summary>
    /// Example 
    /// All entries dosn't work. :(
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="prog"></param>
    /// <param name="textInserts"></param>
    /// <returns></returns>
    private T TestFunc<T>( T prog, string textInserts = null ) where T : class {
        string prgname = prog.GetType().Name;    // typeof(prog).Name;

        // Get two propertiers from the class. Ths properties aare in 
        //  all classes the same.
        var aa = T.Firstname;
        var bb = T.Name;

        // do somethin with the class and the properties
        T.Fullname = $"{aa} {bb}";

        // Do some others with the class and the properties.

        // return the class with the changed data
        return T;
    }
}