Updating Azure Table Entities using the C# API without overwriting value types

Question

I'm not sure if this is possible, but I'm trying to use the C# Azure Table API to update a property in table storage by creating a brand new entity and merging it:

// Create an object that only specifies the property to update
// (null properties are not updated)
var itemToUpdate = new TableEntity("PartitionKey", "RowKey");
itemToUpdate.DateLastAccessedUtc = DateTime.Now;
// can't do this!          
//itemToUpdate.DateCreatedUtc = null;

// Attach to the item, update it, and save the changes
_dataContext.AttachTo(_dataContext.TableName, itemToUpdate, "*");
_dataContext.UpdateObject(itemToUpdate);
_dataContext.SaveChanges();

Basically, I want to update the last accessed date without updating the created date, but since DateTimes cannot be null, I can't do it. Is it possible to do this without making two calls to the table? Since this will be called frequently, I don't want to retrieve the object before updating it if I can avoid it.

Answer 1

While it is true that a DateTime itself can't be null, if you use a Nullable<DateTime> (or DateTime? if you prefer) instead then you are able to set that date to null (and the Storage Client Library understands what to do with Nullable<> types. This may not make sense for the other places you're using this object though.

If using a nullable type doesn't make sense for that you could try this alternate idea (I'm not sure how sensible this is, but I think it will do what you want). Create a new class TableEntityJustLastAccessed which has the usual PartitionKey/RowKey/Timestamp properties plus just the DateLastAccessedUtc property that you want to update.

In your update code, rather than creating a TableEntity, create a TableEntityJustLastAccessed with the same PartitionKey/RowKey and save that. Because by default the storage client library merges changes rather than override the whole object, it should update just the property you care about.