About a year ago, while designing a few windows forms that pulled in data from typed datasets, I came across a rather annoying forms designer issue that was a little obscure to solve. Lately some other members of the team have been confronted with the same issue.

When attempting to change a data property in the Properties window for some components, for instance the DataSource or DataMember properties. The following message pops up in a message box without any further information:   "Object reference not set to an instance of an object."

The solution is quite simple, by opening the 'Show Data Sources' window you are presented with a list of potential data sources to create bound controls from. It just so happens that this list will, on occasion, show an old data source that is no longer available. Luckily for us these 'dead' sources are easily identifiable by their icon (a small red cross from memory), and you can delete them. Doing so will correct the issue immediately.

This is the point where it starts making a little more sense I guess; the properties window uses an editor control to display a list of components in your project that can be bound to. This editor must use a similar method of discovery as the show data sources view, but its handling of null references during the enumeration of child objects obviously is less than sound.

Incidentally, if you are writing controls that need to be bound to data from something like a BindingSource, using the following pattern will allow your control to appear like other framework controls:

System.ComponentModel.AttributeProviderAttribute is the attribute which tells the property grid to use its IListSource editor. The RefreshPropertiesAttribute attribute tells property editors how to refresh other properties when the value of the marked property changes. The CategoryAttribute simply positions the property in the properties grid to the desired category, when the property grid is in category view.