WPF/MVVM:'Could not load file or assembly' and 'Type reference cannot find type'

user_2368655 0
Viewed 323

So I've been working on a project and things were going fine until a NullReferenceException: Object reference not set to an instance of an object error began popping up only in the XAML designer for a View along with the stack trace. The project would run fine however.

To find the cause of this error I began commenting out code in both the offending View and ViewModel. Doing this would give inconsistent and odd results where I ended up with nearly all of the code commented out.

For instance, this would cause the error:

private async void FirmwareUpdateAsync()
{
    bool isOK = true;
    isOK = await Task.Run(() => _usb.DeviceHandshakeAsync());

    if (!isOK)
    {
       // Left Empty
    }
}

But this would not:

private async void FirmwareUpdateAsync()
{
    bool isOK = true;
    isOK = await Task.Run(() => _usb.DeviceHandshakeAsync());

    /*if (!isOK)
    {
       // Left Empty
    }*/
}

I spent two days trying to figure this out and trying to track down the exact cause when the error seemed to evolve into Could not load file or assembly'[ProjectName], version=0.0.0.1, Culture=neutral, PublicKeyToken=null". The system cannot find the specified folder and Type reference cannot find type named'{clr-namespace:[ProjectName].ViewModels.SubViewModels;assembly = [ProjectName]}ErrorViewModel.

Again, this only occurs in the XAML designer and the line numbers for these errors are the declarations of the viewModels and subviews:

<Page.DataContext>
        <vm:Pg7FirmwareUpdateViewModel/>   <= Here
    </Page.DataContext>

    <Page.Resources>
        <!-- Sets Progress Bar View -->
        <DataTemplate DataType="{x:Type svm:ErrorViewModel}">   <= Here
            <sv:ErrorView/>
        </DataTemplate>
    </Page.Resources>

Oddly, the number of these errors depends on how many views I open (always in the declarations), plus I can still run the project.

No amount of building or rebuilding or cleaning seems to matter, I've verified the project's dll is in the bin folder, I tried but failed to use fuslogvw (Can't figure out how to get a log for the project, I only managed to capture something with PowerShell.exe that I wasn't using), and I've looked at all of the 150+ questions on these errors I could find. Any clue what I should do to fix this?

UPDATE: After thinking about Mark's answer below, I excluded from project & moved the suspect View and ViewModels, and replaced them with brand new files of the same name. Everything seems OK until I add back the DataContext again. Is there a way the project can get corrupted so even though I'm cleaning and rebuilding the solution, VS is using bad data or something? This is maddening.

UPDATE2: Rather than blindly stab in the dark I figured I'd just copy all the files to a new folder and revert to my last commit which was working fine, then just copy the changes in and try again. After reverting I built old project and I have the same exact problem before attempting to copy in the new files!

That has to point to a corrupt file or something no?

wpf c#
2 Answers

The view is being created by the XAML designer, and since you've declared your data context in XAML it's creating an instance of that as well. Something in your vm:Pg7FirmwareUpdateViewModel isn't running properly, and that's what causing the error. Can't really help any further without seeing the source code for that class.

This is why things like MVVM Light expose a IsInDesignMode property, so you can ensure that problematic portions of your code won't be executed in designer.

user_11771470

I finally gave up and just started a project and copied in all of the old files. When I finished this, I was back at the old "NullReferenceException" where the XMAL wouldn't display. Progress, ha.

What the problem appears to be was a constructor for displaying error messages. In the constructor I was passing an enum as an argument without doing anything with it:

public class ErrorMessageModel
{
   public ErrorMessageModel(ErrorCodes errorcode, string errorName, 
                            string errorDescription, string errorTips)
   {
       ErrorName = errorName;
       ErrorDescription = errorDescription;
       ErrorTips = errorTips;
   } 

}

After removing ErrorCodes errorcode the XMAL Designer is working again and things seem stable.

user_23686550
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