There are a number of ways to handle this. The most direct way would be to wire up a method to the LinqDataSource_Inserting event and if "e" is null create a empty instance of that class and pass it along.

The fields in the database will have to allow null values. If not then you're gonna get "bla blah blah cannot be null" error :)Lately I have not used the LinqDataSource control for inserts and updates just "reads" because the paging is fast and easy.

When someone hits the button to do an insert or and update I just write code to handle it. It's not much code and problems like this are not really problems at all because you just wrap the logic in if then statements.

My guess would be, because it's value is "" your Linq configuration might be set to treat empty strings as NULL (or nullable column), therefore nothing needs setting.

Look at the column configuration in the model, as well as any parameters set on the datasource for the ConvertEmptyStringToNull attribute.

If this is the _only_ field in the insert/update list then there will be nothing to pass to the linqdatasource, and you can either handle it at the linq datasource events (NULL objects) instancing them by hand, or before that in something like an ItemInserting event on the control where you can add values to a dictionary:

(for ListView control,  but should be similar)

    protected void SomeListView_ItemInserting(object sender, ListViewInsertEventArgs e)
    {
      var somevalue = somecontrol.someproperty;
      e.Values.Add("somefield", somevalue);
    }
 

This will then feed through the linq datasource pipeline, instancing the object and necessary field. Or you can instance the object yourself:

    protected void SomeLinqDataSource_Inserting(object sender, LinqDataSourceInsertEventArgs e)
    {
      var newinstance = e.NewObject as Yourtypehere;
      if (newinstance == null)
        newinstance = new Yourtypehere();
        newinstance.someproperty = somecontrol.someproperty;
    }

Hope this gives a few move avenues for people to inivestigate...
Shea