The class contains a solution to use Types signature of methods in WebHttpBinding in WCF.

WebHttpBehaviorExtension.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.ServiceModel;
using System.ServiceModel.Channels;
using System.ServiceModel.Description;
using System.ServiceModel.Dispatcher;
using System.Web;

namespace WcfService_Everything
{
    public class MyOperationBehavior : IOperationBehavior
    {
        public void AddBindingParameters(OperationDescription operationDescription, BindingParameterCollection bindingParameters)
        {
        }

        public void ApplyClientBehavior(OperationDescription operationDescription, ClientOperation clientOperation)
        {
        }

        public void ApplyDispatchBehavior(OperationDescription operationDescription, DispatchOperation dispatchOperation)
        {
            dispatchOperation.Invoker = new ValueCastInvoker(dispatchOperation.Invoker);
        }

        public void Validate(OperationDescription operationDescription)
        {
        }
    }

    public class MyWebHttpBehavior : WebHttpBehavior
    {
        public override void ApplyDispatchBehavior(ServiceEndpoint endpoint, EndpointDispatcher endpointDispatcher)
        {
            foreach (var operation in endpoint.Contract.Operations)
            {
                if (operation.Behaviors.Contains(typeof(MyOperationBehavior)))
                    continue;

                operation.Behaviors.Add(new MyOperationBehavior());
            }

            base.ApplyDispatchBehavior(endpoint, endpointDispatcher);
        }

        protected override IDispatchMessageFormatter GetRequestDispatchFormatter(OperationDescription operationDescription, ServiceEndpoint endpoint)
        {
            foreach (var item in operationDescription.Messages[0].Body.Parts)
            {
                item.Type = typeof(string);
            }

            return base.GetRequestDispatchFormatter(operationDescription, endpoint);
        }
    }

    public class ValueCastInvoker : IOperationInvoker
    {
        readonly IOperationInvoker _invoker;

        public ValueCastInvoker(IOperationInvoker invoker)
        {
            _invoker = invoker;
        }

        public ValueCastInvoker(IOperationInvoker invoker, Type type, Object value)
        {
            _invoker = invoker;
        }

        public object[] AllocateInputs()
        {
            return _invoker.AllocateInputs().ToArray();
        }

        private object[] CastCorrections(object[] inputs)
        {
            Guid obj;

            var value = inputs[0] as string;

            if (Guid.TryParse(value, out obj))
            {
                return new[] { (object)obj }.Concat(inputs.Skip(1)).ToArray();
            }

            return inputs.ToArray();
        }

        public object Invoke(object instance, object[] inputs, out object[] outputs)
        {
            return _invoker.Invoke(instance, CastCorrections(inputs), out outputs);
        }

        public IAsyncResult InvokeBegin(object instance, object[] inputs, AsyncCallback callback, object state)
        {
            return _invoker.InvokeBegin(instance, inputs, callback, state);
        }

        public object InvokeEnd(object instance, out object[] outputs, IAsyncResult result)
        {
            return _invoker.InvokeEnd(instance, out outputs, result);
        }

        public bool IsSynchronous
        {
            get { return _invoker.IsSynchronous; }
        }
    }

}