WCF - How to fix error System.InvalidOperationException: The contract name 'GettingStartedLib.IService1' could not be found in the list of contracts implemented by the service 'CalculatorService'.

Error Message:

System.InvalidOperationException: The contract name 'GettingStartedLib.IService1' could not be found in the list of contracts implemented by the service 'CalculatorService'.

In order to fix it in App.Config replace

      <service name="GettingStartedLib.CalculatorService">
        <endpoint address="" binding="wsHttpBinding" contract="GettingStartedLib.IService1">
          <identity>

with      
<service name="GettingStartedLib.CalculatorService">
        <endpoint address="" binding="wsHttpBinding" contract="GettingStartedLib.ICalculator">
          <identity>