When does Microsoft.Win32.OpenFileDialog.ShowDialog() return null?

Question

OpenFileDialog's ShowDialog method returns a nullable boolean, set to true if the user clicked OK or false if he clicked Cancel. When does it return null? The documentation does not say.

Brougham

"Curiosity" is a perfectly good answer, but I'm curious to know why you ask? Why not just compare what it returns to "true" (TryParse, etc, etc) and roll on? – Maice

Just wondering why they use a nullable boolean over a standard one. Comparing against true works fine, as you suggested. – Brougham

Now you've got me curious, too. Hopefully someone will come along with a good answer! – Maice

Is a nullable boolean returned only by WPF's ShowDialog (and not WinForms')? – Maice

Answer 1

This is stated in the questions linked below, but I'll mention here that Programming WPF (Chris Sells, Ian Griffiths) says:

ShowDialog will always return true or false. ... Only after a dialog has been shown but before it's been closed is DialogResult null.

Similar question: When would ShowDialog() return null?

And: Why is DialogResult a nullable bool in WPF?

Maice

Answer 2

According to the .NET reflector, Microsoft.Win32.OpenFileDialog.ShowDialog is implemented by a base class, Microsoft.Win32.CommonDialog. That implementation has only one return clause:

return new bool?(this.RunDialog(activeWindow));

RunDialog returns a bool, not a bool?.

bool? is just a C# shorthand for System.Nullable<bool>. The constructor of System.Nullable<bool>, according to reflector again, sets the value of the nullable to its parameter, and marks its hasValue property as true.

So... you shouldn't ever get a null result. A quick test confirms that closing the dialog without canceling (red x button) indeed returns a false value, not a null.

The Windows Forms version of OpenFileDialog returns a DialogResult, which has a wider range of values.

Dasya

Answer 3

My guess is that OpenFileDialog returns bool? to be consistent with other WPF dialogs that actually can return a null result.

Finagle