I have added "WCF Service Application" project type to my VS solution(4.0). Now, the default namespace that appears is "Service" and if I run the application (Pointing WCF service application as the startup project) it works fine.

Now I have changed the namespace to XXX.YYY.Service.PartnerPortal as under

namespace XXX.YYY.Service.PartnerPortal
{        
    public class Service1 : IService1
    {
        public string GetData(int value)
        {
            return string.Format("You entered: {0}", value);
        }        
    }
}

As well as the IService1 interface also

namespace XXX.YYY.Service.PartnerPortal
{    
    [ServiceContract]
    public interface IService1
    {
        [OperationContract]
        string GetData(int value);        
    }
}

Also I have changed the default namespace in the project properties

The app.config file is as under

<?xml version="1.0"?>
<configuration>
    <system.web>
        <compilation debug="true" targetFramework="4.0">
        </compilation>
        <pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID"/></system.web>
    <system.serviceModel>  

    <behaviors>
            <serviceBehaviors>
                <behavior>
                    <!-- To avoid disclosing metadata information, set the value below to false and remove the metadata endpoint above 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>
        <serviceHostingEnvironment multipleSiteBindingsEnabled="true"/>
    </system.serviceModel>
    <system.webServer>
        <modules runAllManagedModulesForAllRequests="true">
        </modules>
    </system.webServer>
</configuration>

And while trying to run, the error messageI am receiving is

Error: Cannot obtain Metadata from http://localhost:65192/Service1.svc If this is a Windows (R) Communication Foundation service to which you have access, please check that you have enabled metadata publishing at the specified address. For help enabling metadata publishing, please refer to the MSDN documentation at http://go.microsoft.com/fwlink/?LinkId=65455.WS-Metadata Exchange Error URI: http://localhost:65192/Service1.svc Metadata contains a reference that cannot be resolved: 'http://localhost:65192/Service1.svc'. The server did not provide a meaningful reply; this might be caused by a contract mismatch, a premature session shutdown or an internal server error.HTTP GET Error URI: http://localhost:65192/Service1.svc There was an error downloading 'http://localhost:65192/Service1.svc'. The request failed with the error message:-- The type 'Service.Service1', provided as the Service attribute value in the ServiceHost directive, or provided in the configuration element system.serviceModel/serviceHostingEnvironment/serviceActivations could not be found. body {font-family:"Verdana";font-weight:normal;font-size: .7em;color:black;} p {font-family:"Verdana";font-weight:normal;color:black;margin-top: -5px} b {font-family:"Verdana";font-weight:bold;color:black;margin-top: -5px} H1 { font-family:"Verdana";font-weight:normal;font-size:18pt;color:red } H2 { font-family:"Verdana";font-weight:normal;font-size:14pt;color:maroon } pre {font-family:"Lucida Console";font-size: .9em} .marker {font-weight: bold; color: black;text-decoration: none;} .version {color: gray;} .error {margin-bottom: 10px;} .expandable { text-decoration:underline; font-weight:bold; color:navy; cursor:hand; }

Server Error in '/' Application.

[InvalidOperationException: The type 'Service.Service1', provided as the Service attribute value in the ServiceHost directive, or provided in the configuration element system.serviceModel/serviceHostingEnvironment/serviceActivations could not be found.]   System.ServiceModel.Activation.ServiceHostFactory.CreateServiceHost(String constructorString, Uri[] baseAddresses) +51530   System.ServiceModel.HostingManager.CreateService(String normalizedVirtualPath) +1461   System.ServiceModel.HostingManager.ActivateService(String normalizedVirtualPath) +44   System.ServiceModel.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath) +651[ServiceActivationException: The service '/Service1.svc' cannot be activated due to an exception during compilation.  The exception message is: The type 'Service.Service1', provided as the Service attribute value in the ServiceHost directive, or provided in the configuration element system.serviceModel/serviceHostingEnvironment/serviceActivations could not be found..]   System.Runtime.AsyncResult.End(IAsyncResult result) +688590   System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) +190   System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(HttpApplication context, String routeServiceVirtualPath, Boolean flowContext, Boolean ensureWFService) +234   System.ServiceModel.Activation.HttpModule.ProcessRequest(Object sender, EventArgs e) +359   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +148   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

What wrong I am doing and how to fix it?

Open your svc file, and correct the namespace in the Service attribute value

<%@ ServiceHost Language="C#" Debug="true" Service="XXX.YYY.Service.PartnerPortal.Service1" .....