Working on pre-operation plug-in to update "Modified By" field in MSCRM -- Need help fixing code - plugins

Question

I am trying to update the "Modified By" field based on a text field called "Prepared By", which contains the name of a user. I've created a pre-operation plug-in to do this and believe I am close to done. However, the "Modified By" field is still not successfully getting updated. I am relatively new to coding and CRM, and could use some help modifying the code and figuring out how I can get this to work.
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Query;
using System;
using System.Linq;
namespace TimClassLibrary1.Plugins
{
public class CreateUpdateContact : IPlugin
{
public void Execute(IServiceProvider serviceProvider)
{
var tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));
var context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
var factory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
var service = factory.CreateOrganizationService(context.UserId);
tracingService.Trace("Start plugin");
tracingService.Trace("Validate Target");
if (!context.InputParameters.Contains("Target") || !(context.InputParameters["Target"] is Entity))
return;
tracingService.Trace("Retrieve Target");
var target = (Entity)context.InputParameters["Target"];
String message = context.MessageName.ToLower();
SetCreatedByAndModifiedBy(tracingService, service, target, message);
}
private void SetCreatedByAndModifiedBy(ITracingService tracingService, IOrganizationService service, Entity target, string message)
{
tracingService.Trace("Start SetPriceList");
tracingService.Trace("Validate Message is Create or Update");
if (!message.Equals("create", StringComparison.OrdinalIgnoreCase) && !message.Equals("update", StringComparison.OrdinalIgnoreCase))
return;
tracingService.Trace("Retrieve Attributes");
var createdByReference = target.GetAttributeValue<EntityReference>("new_createdby");
var modifiedByReference = target.GetAttributeValue<EntityReference>("new_modifiedby");
tracingService.Trace("Retrieve And Set User for Created By");
RetrieveAndSetUser(tracingService, service, target, createdByReference, "createdby");
tracingService.Trace("Retrieve And Set User for Modified By");
RetrieveAndSetUser(tracingService, service, target, modifiedByReference, "modifiedby");
}
private void RetrieveAndSetUser(ITracingService tracingService, IOrganizationService service, Entity target, EntityReference reference, string targetAttribute)
{
tracingService.Trace("Validating Reference");
if (reference == null)
return;
tracingService.Trace("Retrieving and Validating User");
var user = RetrieveUserByName(service, reference.Name, new ColumnSet(false));
if (user == null)
return;
tracingService.Trace("Setting Target Attribute");
target[targetAttribute] = user.ToEntityReference();
}
private Entity RetrieveUserByName(IOrganizationService service, string name, ColumnSet columns)
{
var query = new QueryExpression
{
EntityName = "systemuser",
ColumnSet = columns,
Criteria = new FilterExpression
{
FilterOperator = LogicalOperator.And,
Conditions =
{
new ConditionExpression
{
AttributeName = "fullname",
Operator = ConditionOperator.Equal,
Values = { name }
}
}
}
};
var retrieveResponse = service.RetrieveMultiple(query);
if (retrieveResponse.Entities.Count == 1)
{
return retrieveResponse.Entities.FirstOrDefault();
}
else
{
return null;
}
}
}
}

Answer 1

If you do get use from method Retreiveusernyname then you have to use below code
target[“modifiedby”] = new EntityRefrence(user.logicalname,user.id);

Answer 2

I don't see anything obviously wrong with your update, however you are taking a complicated and unnecessary step with your RetrieveUserByName() method. You already have EntityReference objects from your new_createdby and new_modifiedby fields, you can simply assign those to the target:
if (message.Equals("create", StringComparison.OrdinalIgnoreCase))
{
target["createdby"] = target["new_createdby];
}
else if (message.Equals("update", StringComparison.OrdinalIgnoreCase))
{
target["modifiedby"] = target["new_modifiedby];
}
If new_createdby and new_modifiedby are not entity references, then that would explain why your existing code does not work, if they are, then use my approach.