Strange behaviour with Plugin on Update of activity - plugins

Question

I'm creating a plugin on creating a phonecall activity to update a field in the recipients contact entities. This plugin runs on Create and on Update of a phonecall activity.
On Create AND "new_targetfield" is null => Updates correctly
On Update AND "new_targetfield" not null => Updates correctly
On Update AND "new_targetfield" is null => Nothing happens
I tried running the Plugin Profiler but I keep getting an error:
<ErrorCode>-2147220970</ErrorCode>
<ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic
Here's a part of my code :
IPluginExecutionContext context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
IOrganizationServiceFactory serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);
if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is Entity)
{
Entity entity = context.InputParameters["Target"] as Entity;
if (entity.LogicalName != "phonecall")
{
return;
}
DateTime activitydate=entity.GetAttributeValue<DateTime>("actualstart");
if (activitydate ==null && context.MessageName =="Update")
{
activitydate=((Entity)context.PostEntityImages["PhoneCallPostImage"]).GetAttributeValue<DateTime>("actualstart");
}
if (activitydate != null)
{
// update recipients
EntityCollection Recipients = entity.GetAttributeValue<EntityCollection>("to");
if (Recipients == null && context.MessageName == "Update")
{
Recipients = ((Entity)context.PostEntityImages["PhoneCallPostImage"]).GetAttributeValue<EntityCollection>("to");
}
if (Recipients != null)
{
foreach (Entity recipient in Recipients.Entities)
{
EntityReference partyId = recipient.GetAttributeValue<EntityReference>("partyid");
if (partyId != null)
{
// get recipient id
if (partyId.LogicalName == "contact")
{
Guid contactid = partyId.Id;
// update the recipient Last Contacted with Phone call date
string fetch = #"<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false'>
<entity name='contact'>
<attribute name='contactid' />
<attribute name='new_targetfield' />
<filter type='and'>
<condition attribute='contactid' operator='eq' uitype='contact' value='"+contactid+#"' />
</filter>
</entity>
</fetch>";
EntityCollection result = service.RetrieveMultiple(new Microsoft.Xrm.Sdk.Query.FetchExpression(fetch));
if (result.Entities.Count > 0)
{
DateTime lasttouched = result.Entities[0].GetAttributeValue<DateTime>("new_targetfield");
if (lasttouched != null)
{
if (activitydate > lasttouched)
{
Entity contact = new Entity();
contact.LogicalName = "contact";
contact.Id = contactid;
contact.Attributes = new AttributeCollection();
contact.Attributes.Add("new_targetfield", activitydate);
service.Update(contact);
}
else
{
continue;
}
}
else
{
Entity contact = new Entity();
contact.LogicalName = "contact";
contact.Id = contactid;
contact.Attributes = new AttributeCollection();
contact.Attributes.Add("new_targetfield", activitydate);
service.Update(contact);
}
}
}
else
{
continue;
}
}
else
{
continue;
}
}
}

Answer 1

The problem was that a DateTime variable will never be null but equal to the MinValue of DateTime. That's why parts of the code weren't approached.