Checking nullable result of OpenFileDialog

Question

I have written some code in a WPF application, which looks like this:

var dialog = new OpenFileDialog { Filter = this.FileExtensionFilter };
var dialogResult = dialog.ShowDialog();
if (dialogResult.HasValue && dialogResult.Value)
{
    ... Process result of dialog ...
}

All well and good, I thought, but ReSharper has come up with a warning on the check for dialogResult.HasValue, "Expression is always true".

The first question is how ReSharper would even know that the dialogResult always has a result - it must have dived right down into the decompiled code of the Microsoft.Win32.OpenFileDialog class and observed that it never returns null. Either that, or it's just a hardcoded warning specifically for this class.

Secondly, it doesn't seem like good practice to just assume that the result will never be null. What if Microsoft release a future version of the library in which the null value does become available? The documentation on the matter says: "In the current implementation, the derived classes (OpenFileDialog and SaveFileDialog) will only return true or false" which does imply this is not a permanent arrangement that we can rely on forever.

Any thoughts on whether I'm being overcautious and should remove the line as ReSharper suggests?

Answer 1

It does seem odd. The MSDN spec states that it will return either true or false, but still there must be a reason for the Nullable.

I would absolutely agree with you, that assuming an implementation underneath is bad practise. I would code as per the interface, so in this case I think checking the HasValue is the correct way to go.

How does Re-sharper know? I'm afraid I can't answer that. It's not something I use, they may have hard-coded it.

This may be of interest to you: When does Microsoft.Win32.OpenFileDialog.ShowDialog() return null?

It seems the reason there is the possbility of null is because that's the result before the user has closed the dialog.