using System;
using System.ServiceModel;
using System.Collections.ObjectModel;
using System.ServiceModel.Channels;
using System.ServiceModel.Description;
using System.ServiceModel.Dispatcher; 

namespace BehExt
{
    public class SessionBehavior : Attribute, IServiceBehavior
    {
        public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection bindingParameters)
        {
            //no-op
        }


        public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
        {           
                var sb = serviceDescription.Behaviors.Find<InstanceContextMode>();
                if (sb != null)
                    sb = InstanceContextMode.PerSession;            
        }

        public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
        {
            //no-op
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.ServiceModel;
using System.ServiceModel.Configuration;  

namespace BehExt
{
    public class MyExtElement : BehaviorExtensionElement  
    {
         protected override object CreateBehavior()  
         {
             return new SessionBehavior();  
         }  
    
         public override Type BehaviorType  
         {  
             get  
             {
                 return typeof(SessionBehavior);  
             }  
         }  
    }
}

<system.serviceModel>
    <extensions>
      <behaviorExtensions>
        <add name="MyExtElement" type="BehExt.MyExtElement, BehExt, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
      </behaviorExtensions>
    </extensions>
		<services>
			<service name="BehExt.Service1" behaviorConfiguration="BehExt.Service1Behavior">
				<!-- Service Endpoints -->
				<endpoint address="" binding="wsHttpBinding" contract="BehExt.IService1">
					<!-- 
              Upon deployment, the following identity element should be removed or replaced to reflect the 
              identity under which the deployed service runs.  If removed, WCF will infer an appropriate identity 
              automatically.
          -->
					<identity>
						<dns value="localhost"/>
					</identity>
				</endpoint>
				<endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
			</service>
		</services>
    
		<behaviors>
			<serviceBehaviors>
				<behavior name="BehExt.Service1Behavior">          
          <MyExtElement />
					<!-- 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>
		
	</system.serviceModel>