The value is invalid according to its datatype 'clientcontracttype'

I see this question is pretty old and I don't know if you have found a solution by now, but just in case, this is what I have found will resolve:

1) In the Solution Explorer, under the Service References folder, right-click the service reference name with the issue and select 'Configure Service Reference'.

2) The Service Reference Settings window will appear. Uncheck the box labeled 'Reuse types in referenced assemblies' and click the OK button.

3) Rebuild the project.

After rebuilding, the warning should disappear.

I found this question looking for the same error on a web service project.

In my case this error happened when I forgot to add the [ServiceContract] attribute on the IServiceBase interface.

As soon as I added it in the error went away.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Web;
using System.Text;

namespace MyService
{
    [ServiceContract]
    public interface IServiceBase
    {
        [OperationContract]
        IEnumerable<ListItem> GetListItems();

        [OperationContract]
        void SaveListItems(IEnumerable<ListItem> listItems);
    }

The same error occurs when you're missing a reference [in the project with the .config] to the actual project/library containing the interface/service contract...

In my case I changed the Service Contract by unwittingly removing an interface which was key to the Service Contract.

Weeks later I found a broken client with an out of date service reference. Since the removal of the interface was in error I put it back in.

Another possible resolution would be to have rebuild the service reference to match the new service contract (in my case would have broken the project build since the referenced interface had been removed).

Thanks to OrangeKing89 for pointing me in the right direction. I knew there was the potential of the Service Contract being changed.