The object reference is not set to an instance of an object

Hi,

Practically, I won't recommend the way you are writing the code. Let me correct a bit. First you are inheriting WebForm1 to your Test class. Then in WebForm1 you are trying to access method from class Test. This is like creating a chain between 2 classes. In class Test you better keep methods, functions or extensions that handles application wide business logic and you can call a central code in different forms. If you already can access both Label1 and TextBox1 in your WebForm1 then just to assign value of TextBox1 to Label1, you don't need to use second class, unless there is business logic that needs separation. 

In below code, I have created a function which you can call from WebForm1. When you get result, it gives you some additional text as well.

    class Test
    {
        public string SetText(string text)
        {
            //below code add some extra content with the parameter text and returns the result
            //or you can write any business logic code here
            return $"My content: {text}";
        }
    }

Regards,
Pasang