WCF双工通讯，报无法调用错误

[睿智天成]

刚开始学习WCF，按照http://www.cnblogs.com/artech/archive/2007/03/02/661969.html#2381590 里面写一个双工通讯的示例，但是到最后程序能够运行但是无法显示结果，报错提示为：

下面是我WCF的配置文件：
<?xml version="1.0" encoding="utf-8" ?>
<configuration>

<system.web>
<compilation debug="true" />
</system.web>
<system.serviceModel>
<client>
<endpoint name="CalculatorService" address="http://localhost:8732/Services/CalculatorService" binding="wsDualHttpBinding"
contract="Services.ICalculator" />
</client>
<services>
<service name="CalculatorService">
<endpoint address="" binding="wsDualHttpBinding" contract="Services.ICalculator">
<identity>
<dns value="localhost" />
</identity>
</endpoint>
<endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
<host>
<baseAddresses>
<add baseAddress="http://localhost:8732/Services/CalculatorService" />
</baseAddresses>
</host>
</service>

</services>

<behaviors>
<serviceBehaviors>
<behavior>
<serviceDebug includeExceptionDetailInFaults="False" />
</behavior>
</serviceBehaviors>
</behaviors>

</system.serviceModel>

</configuration>