Blocking Duplicate Quotes in Dynamics 365 Sales Professional / Enterprise (C#)

namespace JJG.MyPlugins
{
   using System;
   using Microsoft.Xrm.Sdk;
   using Microsoft.Xrm.Sdk.Query;

   /// <summary>
   /// Blocks the Create or Update action for a Quote, if it's detected that another Draft Quote exists linked to the same Opportunity.
   /// </summary>
   public class BlockDuplicateQuoteForOpportunity : IPlugin
   {
      public void Execute(IServiceProvider serviceProvider)
      {
         IPluginExecutionContext context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
         ITracingService tracer = (ITracingService)serviceProvider.GetService(typeof(ITracingService));

         // Check for EntityType and Message supported by your Plug-In
         if (!(context.MessageName == "Create" || context.MessageName == "Update") || context.PrimaryEntityName != "quote")
         {
            throw new InvalidPluginExecutionException($"Plug-In {this.GetType()} is not supported for message {context.MessageName} of {context.PrimaryEntityName}");
         }

         tracer.Trace($"Starting execution of {nameof(BlockDuplicateQuoteForOpportunity)}");

         // Get the newly create Quote (Create) or the Post Image for the Quote (Update), and the Opportunity lookup value
         Entity quote = (Entity)context.InputParameters["Target"];
         EntityReference opportunity = quote.GetAttributeValue<EntityReference>("opportunityid");

         if (opportunity == null)
         {
            tracer.Trace($"No Opportunity is present for Quote ID {quote.Id}. Cancelling plug-in execution");
            return;
         }

         // Attempt to retrieve other Draft Quotes that are linked to the same Opportunity ID we have here
         tracer.Trace($"Attempting to retrieve other Quote rows linked to Opportunity ID {opportunity.Id}...");

         IOrganizationServiceFactory serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
         IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

         QueryExpression qe = new QueryExpression()
         {
            EntityName = "quote",
            ColumnSet = new ColumnSet("quoteid"),
            Criteria =
            {
               Conditions =
               {
                  new ConditionExpression("opportunityid", ConditionOperator.Equal, opportunity.Id),
                  new ConditionExpression("statecode", ConditionOperator.Equal, 0),
                  new ConditionExpression("quoteid", ConditionOperator.NotEqual, quote.Id),
               },
            },
         };

         EntityCollection quotes = service.RetrieveMultiple(qe);
         tracer.Trace($"Got {quotes.Entities.Count} Quotes!");

         // If one or more exist, then we throw an error to block the Create / Association
         if (quotes.Entities.Count >= 1)
         {
            tracer.Trace($"Multiple Draft Quotes exist for Opportunity ID {opportunity.Id}. Throwing error to cancel operation...");
            throw new InvalidPluginExecutionException("Draft Quote(s) already exist for the selected Opportunity. Only a single Draft Quote is allowed. Please edit or delete the other Draft Quote(s) before proceeding.");
         }
         else
         {
            tracer.Trace($"No other Draft Quotes are linked to Opportunity ID {opportunity.Id}. No action required. Cancelling plug-in execution");
            return;
         }
      }
   }
}