how to access classes or objects defined in APP_CODE folder ?

i defined some classes there but how to access those ?

Say you created a class called abc.cs and you have a static method within that class called method1(). This method1() returns a boolean value.

So in your code behind from within your project, you call it this way..

bool result = abc.method1(param);

In this example, you are sending the method some information via the variable "param" and the static method in that class is returning a boolean value and assigning that to the variable called "result".