How to pass a value from one Class file to another Class ???

Question

Hi,
Req: I want to pass the value from one class to another class file.
Example: ClassA.cs having one property like 'ProductId'. i require this value in my second class  -> ClassB.cs. Using this value  i am doing some calculation in my ClassB (May be ClassB update this property value too). Then again  i am returning to my ClassA. My object life-cycle is like ClassA --> ClassB --> ClassA
Please share some ideas to retain this prperty value.