Warning as Error: Possible unintended reference comparison when upgrading from .Net 3.5 to .Net 4.5

Question

Currently I am in the process of migrating a WPF application from .Net Framework 3.5 to .Net Framework 4.5. Next to the upgrade of the .Net Framework, the application will now be compiled 64-bits instead of 32-bits. I get the following error when compiling the application:

Warning as Error: Possible unintended reference comparison; to get a value comparison, cast the left hand side to type 'System.Type'

My question is: why does this error occur now that I have upgraded to .Net 4.5 and never before on .Net 3.5? I have not made any changes to the build properties of the project, both have the setting Treat warnings as errors set to All . Below is the code that produces the error, I have added some comments about the changes that I have made, the uncommented part compiles under .Net 4.5.

private void FoldersListBoxMouseDoubleClick(object sender, MouseButtonEventArgs e)
{
    // Get typed sender
    ListBox typedSender = sender as ListBox;
    if (typedSender != null)
    {
        // Check if an item was double clicked
        //The line below worked in .Net 3.5, but not in .Net 4.5
        //if ((typedSender.SelectedItem != null) && (typedSender.InputHitTest(e.GetPosition(typedSender)) != typedSender.GetType()))
        //And this is the line that I have changed (I added GetType()!).
        if ((typedSender.SelectedItem != null) && (typedSender.InputHitTest(e.GetPosition(typedSender)).GetType() != typedSender.GetType()))
        {
            // Yes, set the new path
            SelectedPath = typedSender.SelectedValue as string;
        }
    }
}

Edit: I have looked up when this code was added to the application, but was unable to go find it, because the Subversion repository only goes back 6 years.

Answer 1

Your previous code was broken, basically. You say it worked, but I can't see that the condition would ever be false. I suspect the compiler is just smarter now than it was before. The condition was:

typedSender.InputHitTest(e.GetPosition(typedSender)) != typedSender.GetType()

Now InputHitTest returns a value of type IInputElement . So the condition (ignoring the first part) can be rewritten as:

IInputElement element = typedSender.InputHitTest(e.GetPosition(typedSender));
Type type = typedSender.GetType();
if (element != type)
{
    ...
}

The only way in which the two references could be equal would be if they were both null , and typedSender.GetType() will never return null , so the condition is pointless.

You're now comparing whether the input element which is the result of the hit test is of the same type as typedSender , which at least makes sense - although it doesn't check that it is typedSender .

It's worth noting that the documentation for InputHitTest includes:

This method typically is not called from your application code. Calling this method is only appropriate if you intend to re-implement a substantial amount of the low level input features that are already present, such as recreating mouse device logic.

Are you sure you want to call it?