IService.cs
[OperationContract] [WebGet(UriTemplate = \"/IsValidUser?userid={userid}&password={password}\", RequestFormat = WebMessageFo
try this
[OperationContract] [WebGet(UriTemplate = "IsValidUser?userid={userid}&password={password}", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json)] string IsValidUser(string userid, string password);