Change notification with Orcas data model
Mon, Jul 9 2007 15:19
Jim Wooley recently brought my attention to some "peculiar" code in the LINQ to SQL generated properties. It rang alarm bells with me for a different reason.. that is the runtime behavior for LINQ to SQL classes will be different in VB than it is in C#.
The code in question is in the property sets:
<Column(Storage:="_ContactName", DbType:="NVarChar(30)")> _
Public Property ContactName() As String
Set(ByVal value As String)
If ((Me._ContactName = Value) _
= False) Then
Me._ContactName = Value
The If ((Me._ContactName = Value) = False) Then part is a little strange to say the least.
What they probably wanted was If Me._ContactName <> Value Then
The C# code is if ((this._ContactName != value))
But that is not the problem. VB treats equality of strings slightly different than C#. In VB if you use = or <>, null references are treated as being equal to String.Empty. That is If Nothing = "" Then is True.
So the question is what difference will this difference make ? Well if you are using Nothing to represent dbNull when saving the data, (assuming the field is nullable), then the result is the field is set. Implications of that include a plethora of concurrency issues.
Of course, data binding and standard text box controls have never been good in this regard anyway. But it would be preferable if both C# and Vb behaved the same. In this case using String.Equals would address that :
If String.Equals(Me._ContactName, Value) Then
The ADO.NET Entity model on the other hand at present doesn't even do a check of the existing value. Here's the same code for the ContactName property set, this time using ADO.NET Entity.
Me._ContactName = System.Data.Objects.DataClasses.StructuralObject.SetValidValue(value, true, 30)
So with the Entity model you need to check that yourself and watch for a lot of superfluous change notifications when really no change has occurred. I presume they'll change that otherwise it would result in a lot of un-necessary writes to the database, as well as be a real pain to use in any event driven designs.