Update QueryExpression

I made a select to bring all the entities which contain the field "se_customdate" to null. Until then, no problem. However, I now need to assign the value of the "createdon" variable to the null fields, but I'm not getting any progress in the code.

1: I thought about putting a foreach, where every entity returned as null, will pass through the loop and will receive the value of createdon, it looks like this:

foreach(Entity etn in ec){  
               entity["se_customdate"] = "createdon";
            }

foreach points to error:

  

"foreach statement can not operate on variables of type 'microsoft.xrm.sdk.entitycollection' because 'microsoft.xrm.sdk.entitycollection' does not contain a public definition for 'GetEnumerator'.

Here is the code I used all along:

public class PreencheCampoDataHistorico : CodeActivity
{       
    private string _activityName = "Preencher Campo de Data Historico";
    protected override void Execute(CodeActivityContext executionContext)
    {
        ITracingService tracingService = executionContext.GetExtension<ITracingService>();

        if (tracingService == null)
        {
            throw new InvalidPluginExecutionException("Failed to retrieve tracing service.");
        }

        tracingService.Trace("Entered " + _activityName + ".Execute(), Activity Instance Id: {0}, Workflow Instance Id: {1}",
        executionContext.ActivityInstanceId,
        executionContext.WorkflowInstanceId);

        // Create the context  
        IWorkflowContext context = executionContext.GetExtension<IWorkflowContext>();

        if (context == null)
        {
            throw new InvalidPluginExecutionException("Failed to retrieve workflow context.");
        }

        tracingService.Trace(_activityName + ".Execute(), Correlation Id: {0}, Initiating User: {1}",
        context.CorrelationId,
        context.InitiatingUserId);

        IOrganizationServiceFactory serviceFactory = executionContext.GetExtension<IOrganizationServiceFactory>();
        IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

        try
        {                          
            Entity entity = new Entity("se_historicodelicenciamento");
            ColumnSet attributes = new ColumnSet(new string[] { "se_customdate","createdon" });

            QueryExpression query = new QueryExpression
            {
                EntityName = entity.LogicalName,
                ColumnSet = new ColumnSet (new string[] {"name","se_customdate","createdon"}),

                Criteria = 
                {
                    FilterOperator = LogicalOperator.And,
                    Conditions = 
                    {
                        new ConditionExpression
                        {
                            AttributeName = "se_customdatehistory",
                            Operator = ConditionOperator.Equal,
                            Values = {String.Empty}
                        }
                    }                       
            }              
        };  

            EntityCollection ec = service.RetrieveMultiple(query);

            foreach(Entity etn in ec){                                                                     
                entity["se_customdate"] = "createdon";
            }

I ask for ideas, new ways of approach / methods of resolution or even some light on how to proceed.