Can't escape empty textbox

Question

I'm trying to track down the cause of an annoying interface bug in an app that was recently upgraded from VS2003 to VS2008 (the bug did not exist pre-migration). What happens is this :

1) User clicks in textbox containing a date.
2) User clears date
3) User tries to move to another field, but can't. No error messages appear - it's as if the validation failed.

Further info :

1) The textbox's Text property is bound to a dataview which uses a datatable as its source. The bound field is a nullable datetime field with no constraints or default.
2) The Validating event fires and the CancelEventArgs property is not set to Cancel. The Validated, LostFocus and Leave events all fire as well, going LostFocus > Leave > Validating
3) I can't see any code changes relating to the control or the datasource with a couple of exceptions. The first is that this :

Me.txtRangeEnd.DataBindings.Add(New System.Windows.Forms.Binding("Text", Me.dvClientNos, "RangeEnd"))

has now changed to this :

Me.txtRangeEnd.DataBindings.Add(New System.Windows.Forms.Binding("Text", Me.dvClientNos, "RangeEnd", True))

The second is that this :

Me.dcolRangeEnd.DataType = GetType(System.DateTime)

has now changed to this :

Me.dcolRangeEnd.DataType = GetType(Date)

There is also this, which has been in the code since day one :

AddHandler txtRangeEnd.DataBindings("Text").Format, AddressOf FormatBoxToDate

Private Sub FormatBoxToDate(ByVal sender As Object, ByVal e As ConvertEventArgs)
Try
    If Not e.Value Is DBNull.Value Then
            e.Value = Format(e.Value, "d")
        End If
    End Try
End Sub

Now, if I remove the ", True" from the adding of the databinding then I can exit the control with a blank value, but it then reverts to the original value. Removing the date formatting appears to make no difference to this (it just reverts to showing 06/01/2011 00:00:00 rather than the desired 06/01/2010). No other code refers to that textbox at all. I'm thinking something must have changed in validation of databound controls between VS2003 and VS2008, but it's just as likely I'm missing something mind-numbingly obvious.

Any ideas?

Answer 1

I have had this type of error before and I had to ensure that the underlying data source (in my case it was a dataset) was not set to read only and that the column allowed `null' values.

Once I had done this everything worked fine. It seemed like that the error that was being thrown in the Data Bindings was swallowed up somewhere and didn't propagate up.

Answer 2

The reason that you're seeing the observed behaviour is to do with how Windows Forms and it's Data Binding handles NULL database values.

The TL;DR reason:

See this Microsoft Connect suggestion: Provide better databinding support for nullable types

The long version:

What is essentially happening is that as you clear the Textbox (to an empty string) and subsequently tab away, the binding is converting your empty string to a DBNull value which is then propagated to the data source however the binding, since it is two-way, then attempts to re-populate the bound control (the Textbox) with appropriate formatting, and fails, causing the Textbox to display the strange behaviour of not allowing the focus to be removed from it!

This is happening due to the DataSourceNullValue property of the Binding class. This can be set using one of the Binding classes constructor overloads, or set separately via a property setting, however, if you do not explicitly set this property, it is important to note that:

The default is DBNull for value types and null for non-value types.

It appears that you're not explicitly setting this, so the default is applying, and with your DateTime being a value type, it is using DBNull.

Once the data source has been updated (to DBNull), the binding mechanism will attempt to then repopulate the Textbox with the newly updated data source value. When the underlying data source value is DBNull, the value used for the bound control is governed by the Binding class's NullValue property. Again, if this property is not explicitly set either via the relevant overloaded constructor argument or via the property setting itself, the default value will apply, which is:

The Object to be set as the control property when the data source contains a DBNull value. The default is null.

Of course, a Textbox's Text property can only be set to an object of type System.String and not a null value (Nothing in VB), so the TextBox fails to bind the representative value (null/nothing) of the data source's value (DBNull) to the bound control.

The way to correct this behaviour is to ensure that the Binding class's NullValue property is explicitly set to a suitable value. In this case, a zero-length string will suffice to correct the problem.

One way to achieve this is to change the line:

Me.txtRangeEnd.DataBindings.Add(New System.Windows.Forms.Binding("Text", Me.dvClientNos, "RangeEnd", True))

to:

Me.txtRangeEnd.DataBindings.Add(New System.Windows.Forms.Binding("Text", Me.dvClientNos, "RangeEnd", True, DataSourceUpdateMode.OnValidation, ""))

The key here is the very last parameter, which is the NullValue, set to a zero-length string (The DataSourceUpdateMode is also explicitly specified due to the arguments of the constructor, but it's being set to it's default value anyway).

Despite all of this, it does appear to be somewhat "odd" behaviour, if not an actual bug. This is also evidenced by others who appear to be experiencing the same issue (which is still prevalent in Visual Studio 2010/.NET 4.0!). This thread on the social.msdn.microsoft.com forums contains someone experiencing the same issue with some interesting possible explanations as to why this happens, and why Microsoft designed it this way.

There is also a Microsoft Connect suggestion that was reported back in 2005 that highlights the issue. This suggestion has been "Closed as Postponed". It appears that Microsoft do not consider it a bug, as a very reasonable workaround exists (the explicit setting of the Binding's NullValue property) which, arguably, should be done anyway for readability's sake. They will apparently consider the suggestion in the future.

Going back to why this didn't exist pre-.NET 2.0 (Visual Studio 2005) seems to be due to the fact that the entire data binding mechanism was completely revamped for the release of .NET Framework 2.0. Your original solution, being a VS2003 project was using .NET Framework 1.1 which did not have as rich a data binding feature-set. Although I no longer have a copy of VS2003 to hand to test this, I'm assuming the binding mechanism in .NET 1.1 made much more use of implicit conversions between the control's value and the data source's value. This appears to be supported when you examine the Binding class from .NET 1.1, compared with .NET 2.0 (or higher). For example, there was no way to (easily) control the actual two-way binding itself (and how values are converted between the form and the data source) or the formatting of said values.