How to call a class1.cs in App_Code ..

Could you please elaborate "another place"? I am assuming you mean "other project".

App_code classes are public and can be used anywhere within web project.
If you want to use the class outside web project (in other project in your solution) then move the class into a separate class library project and reference it wherever you need the class (web project or the other project).

You have already had an answer to this when you posted it twenty minutes ago: how to call class1.cs class on page[^]

To reiterate Abhinav's response:

If you have created a class called class1 in you App_code folder, then in your page.cs all you have to do is declare an instance of class1 and use it as normal. Alternatively, as Abhinav said, you could declare static methods and properties and use those directly, just as you would with an other class.

Why is this giving you a problem?
Use the "Reply" feature below this answer to respond - do not create a new question.

Hey, are you going to learn OOP basics and C# language by asking a billion of questions? You should create class1 instance, say object c1, in another place, and call your property, say Property1, in a manner c1.Property1; .
If you now feel that you you don't know how to create a class instance, PLEASE, don't post this question here. Read some info, learn about OOP and C#, it's really worth it if you're going to be a programmer :). If you don't know the basics (and by your questions I assume you don't), you'll encounter a lot of questions, and posting them here in thousands will hardly help you