Returning custom error from DokanNetMirror

[andreas-eriksson]

Is there a way to return custom errors from a function in DokanNetMirror?

NtStatus IDokanOperations.ReadFile(string fileName, byte[] buffer, out int bytesRead, long offset, IDokanFileInfo info)
{
    if (info.Context is not FileContext ctx)
    {
        return ReadFileMemoryMapped(fileName, buffer, out bytesRead, offset);
    }

    lock (ctx)
    {
        try
        {
            ctx.Stream.Position = offset;
            bytesRead = ctx.Stream.Read(buffer, 0, buffer.Length);
        }
        catch (CustomException e)
        {
            // Here I would like to set a custom error that can be read by a C# application trying to read the file.
            bytesRead = 0;
            SetLastError(e.Message);
            return CustomError;
        }
    }

    return DokanResult.Success;
}

[Liryna]

Hi @andreas-eriksson ,

Is your goal to have to pass some specific information to the reading app that is non standard ?
I wouldn't suggest doing that as the value will be read by a few components of the system and third party apps (antivirus etc). This could result in unwanted behavior.

[andreas-eriksson]

Thanks for the reply, I thought that might be the case.

Do you think it could be okay to return a different standard error code, for example DokanResult.InternalError?

NtStatus IDokanOperations.ReadFile(string fileName, byte[] buffer, out int bytesRead, long offset, IDokanFileInfo info)
{
    if (info.Context is not FileContext ctx)
    {
        return ReadFileMemoryMapped(fileName, buffer, out bytesRead, offset);
    }

    lock (ctx)
    {
        try
        {
            ctx.Stream.Position = offset;
            bytesRead = ctx.Stream.Read(buffer, 0, buffer.Length);
        }
        catch (CustomException)
        {           
            bytesRead = 0;            
            return DokanResult.InternalError;
        }
    }

    return DokanResult.Success;
}

I could then handle that specific error in the reading application

try
{
    using (Stream stream = File.OpenRead(filePath))
    using (var temp = new MemoryStream())
    {
        stream.CopyTo(temp);
    }
}
catch (IOException e) when ((e.HResult & 0x0000FFFF) == 87) // Invalid Parameters
{
    // Normal error
}
catch (IOException e) when ((e.HResult & 0x0000FFFF) == 1359) // Internal Error
{
   // Handle custom error
}

[Liryna]

Yes this would work but like I said this could create undefined behavior with system and third party apps. They might even rewrite the error code on the way.

I really wouldn't suggest doing that.

[andreas-eriksson]

Thank you very much for the detailed answer.