Question

I would like to set to variable result method from first class or second class which depend from type, what is wrong in my code??

    public ActionResult Contact()
    {
        ViewBag.Message = GetValue(new ClassOne(), "classOne");

        return View();
    }



    public string GetValue<T>(T customClass, string type)
    {
        if (type == "classOne") return customClass.ClassOneMethod();
        else customClass.ClassTwoMethod();
    }

    class ClassOne
    {
        public string ClassOneMethod()
        {
            return "ClassOneMethod";
        }
    }

    class ClassTwo
    {
        public string ClassTwoMethod()
        {
            return "ClassTwoMethod";
        }
    }

Answer 1

The problem is that the compiler knows nothing about type T at compile time and is unable to figure out that the object contained in customClass has a method called ClassOneMethod or ClassTwoMethod. You have several options, for example deriving from a common base class overriding a virtual method or implementing an interface.

Here some hints on how to do it using an interface. This is far from perfect but I tried to keep it quite close to your original code.

interface IHaveAMethod
{
    String GetString();
}

public ActionResult Contact()
{
    ViewBag.Message = GetValue(new ClassOne());

    return View();
}

public String GetValue<T>(T customClass) where T : IHaveAMethod
{
    return customClass.GetString();
}

class ClassOne : IHaveAMethod
{
    public String GetString()
    {
        return "ClassOneMethod";
    }
}

class ClassTwo : IHaveAMethod
{
    public String GetString()
    {
        return "ClassTwoMethod";
    }
}