When we implement interface then we have to implement all the methods in it. Hence while implementing IMathOperationDemo interface you have to implement all the methods, but you are missing the implementation of
string getMinimum(decimal Alpha);
string getMaximum (decimal Beta);
in your class Normale.
got it..
You can refer the following link for more help:
http://www.csharp-station.com/Tutorials/Lesson13.aspx[
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