public void Calculate(object obj)
{
switch (obj.ToString())
{
case "Add":
objOperationModel.Add();
break;
case "Subtract":
objOperationModel.Subtract();
break;
case "Multiply":
objOperationModel.Multiply();
break;
case "Divide":
objOperationModel.Divide();
break;
}
PropertyChangedMethod("Result");
}