In class2, you are calling Console.WriteLine(c1.inf1());.
So class1.inf1 should return a string as you are trying to output it to the console.
However, class1.inf1() recursively calls itself with no exit and does not return a string.
So I think this may be what you are trying to accomplish:
protected internal string inf1()
{
return "\n......inf1() \n";
}
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solved Getting Infinite Loop Issue. Process Terminated due to StackOverflowException?