I'm trying write small program using c# interface concept for the area of circle & square.While Giving specific condition if (args[0] == "S") there is an error IndexOutOfRangeException:
if (args[0]=="S")
    fig = new Square();
if (args[0]=="C")
    fig = new Circle();
                        
That will happen if args is empty. You can't ask for the first element of an empty array, because there isn't one. You should check the length first:
if (args.Length == 0)
{
    // Maybe exit? Is it valid not to specify any arguments?
}
// Either use an "else" here, or if you've quit in the "if" block
// then you don't need to because you know that there's at least
// one argument by now
                                
                            
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