Update using Pre and Post image into one field and get data from two fields?

 Code  inside Execute Functions in C#:

            // Extract the tracing service for use in debugging sandboxed plug-ins.

            ITracingService tracingService =

            (ITracingService)serviceProvider.GetService(typeof(ITracingService));

            // Obtain the execution context from the service provider.

            IPluginExecutionContext context =

            (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));

            // Obtain the organization service factory.

            IOrganizationServiceFactory serviceFactory =

            (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));

            // Obtain the organization service.    

            IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

            if (context.InputParameters.Contains("Target") &&

                          context.InputParameters["Target"] is Entity)

            {

                // Obtain the target entity from the input parameters.

                Entity entity = (Entity)context.InputParameters["Target"];

                // User is updating only email address in lead form so we will get only 

               

        // get the lead email from context.

                string email = entity["emailaddress1"].ToString();

                // get the current record guid from context

                Guid leadRecordGuid = entity.Id;

                // Define variables to store Preimage and Postimage

                string pretopic = string.Empty; string posttopic = string.Empty; 

                // in below leadimage has been added in plugin registration tool

                // get PreImage from Context

                if (context.PreEntityImages.Contains("LeadTopicImage") && context.PreEntityImages["LeadTopicImage"] is Entity)

                {

                    Entity preMessageImage = (Entity)context.PreEntityImages["LeadTopicImage"];

                    // get topic field value before database update perform

                    pretopic = (String)preMessageImage.Attributes["subject"];

                }

                // get PostImage from Context

                if (context.PostEntityImages.Contains("LeadTopicImage") &&

                       context.PostEntityImages["LeadTopicImage"] is Entity)

                {

                    Entity postMessageImage = (Entity)context.PostEntityImages["LeadTopicImage"];

                    // get topic field value after database update performed

                    posttopic = (String)postMessageImage.Attributes["subject"];

                }

                // update the old and new values of topic field in description field

                Entity leadObj = new Entity("lead", context.PrimaryEntityId);

                //service.Retrieve(context.PrimaryEntityName, leadRecordGuid, new ColumnSet("description"));

                leadObj["description"] =

                "Pre-Image of description- " + pretopic + "   " + "Post-Image of description-- " + posttopic;

                service.Update(leadObj);

            }