Reason 1:
In your web.config
<service name="A.B.C">
but your class is：
namespace A.B { ..... public class D : E {
Those names need to match! The name= attribute on the <service> tag in config must be exactly what your service class is called - fully qualified, including namespace - so in your case here, it should be:
<service name="A.B.D">
Reason 2:
Make sure Web.config have httpGetEnabled or httpsGenEnabled parameter in <serviceMetadata> tag.
<behaviors> <serviceBehaviors> <behavior> <!-- To avoid disclosing metadata information, set the values below to false before deployment --> <serviceMetadata httpGetEnabled="true"/> <!-- To receive exception details in faults for debugging purposes, set the value below to true. Set to false before deployment to avoid disclosing exception information --> <serviceDebug includeExceptionDetailInFaults="false"/> </behavior> </serviceBehaviors> </behaviors>
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原文：http://www.cnblogs.com/vincentDr/p/3968839.html