Let's answer these in reverse.
Quote:
Why do I have to type the sentence which is at least 30 charachers?
To make you think about what you are asking, and to prompt you to give a reasonable amount of information. The better the info, the better your question. The better your question, the better an answer we can give!
Asking questions is a skill: it focuses your mind and what you want to know, and can often give you the answer without having to ask anyone else...
Quote:
What is the feature of desktop window class?
...
I have found out the class name.
But I don't know how to find out the feature.
So start with the official documentation:
Object Class (System): MSDN[
^] and you will find all you need to know.
If you want to know about a specific class (and there are no classes in C at all, it doesn't even have the concept of them) then start with google and type the name of the class followed by the language you want to use. For example "Form C#" will give you this:
Form C# - Google Search[
^] and the first hit in the list is the MSDN documentation for the Form class:
Form Class (System.Windows.Forms): MSDN[
^]