Question

I am using Azure table storage and have questions about how nulls and default values for entities work.

Specifically, I have a class that extends TableServiceEntity. The default constructor for this class is setting default values for various properties like so:

public class MyEntity: TableServiceEntry
{
    public MyEntity() : MyEntity("invalid", "invalid") {}
    public MyEntity(string f1, string f2) 
    {
        Field1 = f1;
        Field2 = f2;
    }

    public string Field1 { get; set; }
    public string Field2 { get; set; }
 }

I tested this class locally (on the emulator) by constructing the following entity:

MyEntity e = new MyEntity("hello", null);

I uploaded the entity and then retrieved it locally and the two fields were set to “hello” and null, respectively, as expected.

However, when I uploaded the same entity to the Azure cloud, what I received back was “hello” and “invalid”, respectively, for the two properties.

My code that saves the entity is below:

public class MyTable : TableServiceContext
{
    ...
    public void AddEntry(MyEntity e)
    {
        this.AddObject("MyTable", e);
        this.SaveChangesWithRetries(SaveChangesOptions.ReplaceOnUpdate);
    }
}

I was able to fix this by making the default constructor take no arguments, but now I feel like I have a fundamental misunderstanding of how table storage works. Is it true that when you specify defaults for properties of a TableServiceEntry, those become the defaults for each row in the table in the cloud but not in the emulator (i.e. cloud vs. SQL Express)? If so, why can’t I override those defaults with null in the cloud? Is there any documentation that explains how default constructors and nulls work in Azure table storage?

Answer 1

Yes, there is a difference between how table storage behaves in the emulator and in the cloud. The emulator implemented in SQL server, returns all columns defined for a table, even if not defined for a row, irrespective of the columns value (null / non-null). In the cloud, a property set to null is neither stored nor returned in the REST call.

A quick fix would be to check for null in property set, and only mutate the property if the value passed in, is not null.