如何修复仅在某些计算机上发生的 NullReferenceException

Question

I create a setup file of a winform app using VS 2017 installer. When I test this setup file on my PC, the login runs fine. So I run this setup file on other PCs.

However, when I login the app in these PCs, the app say "Object reference not set to an instance of an object", though I am sure that the username and password were correct in the db.

I have seen What is a NullReferenceException, and how do I fix it? , but I do not think it is the null exception, because the installed app did not throw above error in my PC. This error only happens when I install the app in other PCs.

I also try replacing the.exe and.config files in folder of the installed app in other PCs with the same files of the app in my PC (which work well), and the app runs ok. But then I restart those PCs, and try to login the app, the same error happens.

This is the code for login. I believe that I have checked for the null exception properly. Am I right?

private void btnLogin_Click(object sender, EventArgs e)
    {
        try
        {
            var info = UsersBiz.Login(txtUserName.Text.Trim(), txtPassWord.Text);

            if (info != null && info.user.Id > 0)
            {
                Constants.USERINFO = info;

                this.Hide();
                var frm = new frmMain();
                frm.Show();

                if (ckbRemember.Checked)
                {
                    ManagementInventory.Properties.Settings.Default.User = txtUserName.Text;
                    ManagementInventory.Properties.Settings.Default.Pass = txtPassWord.Text;
                    ManagementInventory.Properties.Settings.Default.Save();
                }
            }
            else
            {
                MessageBox.Show("UserName or Password not correct!");
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

Answer 1

The most easy way would be to use visual studio debugger on the computer where the program runs. If there is no visual studio on that computer, consider using remote debugging. See how to start debugging on a remote computer

If for security reasons remote debugging is out of the question, consider to log the exception, especially the stack trace. If you know where it occurs, simply surround it with a try-catch

try
{
    // Here are the statements that cause your ArgumentNullException
}
catch (ArgumentNullException exc)
{
    // log the exception
}

If your program has no logging facility, consider adding something like NLOG. Another possibility is to append it to a text file.

void WriteExceptionToTextFile(Exception exc, string fileName)
{
    using (var writer = File.AppendText(fileName))
    {
        writer.WriteLine("Exception {0}", exc.GetType());
        write.WriteLine("Stack trace" + exc.StackTrace);
        ... // etc
    }
}

If you don't know where the exception occurs, you can't try-catch it. In that case consider to catch the unhandled exception and log it:

See: catching unhandled exception

In your forms program is a file program.cs which contains the main:

public static void Main()
{
    AppDomain currentDomain = AppDomain.CurrentDomain;
    currentDomain.UnhandledException += OnUnhandledException;
    ...
}

static void OnUnhandledException(object sender, UnhandledExceptionEventArgs args)
{
    Exception undhandledException = (Exception) args.ExceptionObject;
    // TODO log the unhandled exception

}