Is this the correct way of using Tasking & Dispatcher in WPF?

Question

First of all, I want to mention that I am kinda new to WPF, I just start learning it a few months ago and I am using code behind atm, going to MVVM later... So, Im trying to create a simple program for my discord bot but I want to keep it responsive whenever a database action is going on, meaning my program cannot block while i am -for example- logging into my program. I have a way to archive this but it feels like i am doing something wrong or using Tasking wrong.

I use await Task.Run(() => { code here }) but then inside this task I also need to use the Dispatcher every time I want to update a control and there is where I wonder if i do this all the correct way, is it normal to use the dispatcher inside a Task.Run every time you need to update controls or am I overusing this / doing it wrong?

Currently it works as intented, but I am worried I am overdoing something or going into the wrong direction because for me it seems silly to use the dispatcher every time for updating a control but maybe this is just the way it has to be, i am still new to WPF, this is also my first program i ever did in WPF

Sorry if my english is not so good, i try my best to explain this

I have a sort of solution but the use of Dispatcher inside Tasking seems to me not the correct way, this is the first time im doing this in WPF, I googled around a lot but see people using background workers or threading instead but this still seems to block my UI

// This button I use to -for this example- login in my program
// I load a ContentPresenter over my program showing its 'loading'
// Then I try login


private async void BtnLogin_Click(object sender, RoutedEventArgs e)
{
  cpLoader.Visibility = Visibility.Visible;
  await TryLoginAsync();
}


// Then here i do the login with tasking
// But here i also need to use dispatcher when updating controls
// So I wonder if this is done correctly
private async Task TryLoginAsync()
{
  var username = txtUsername.Text.Trim();
  var password = txtPassword.Password.Trim();

  if (!string.IsNullOrWhiteSpace(username) && !string.IsNullOrWhiteSpace(password))
  {
    await Task.Run(() =>
    {
      try
      {
        var programAdmin = _dbAdmins.FindBy(x => x.Username.ToLower() == username.ToLower());
        if (programAdmin != null)
        {
          if (string.Equals(EncryptionManager.Decrypt(BotDBConfiguration.EncryptionKey, programAdmin.Password), password))
          {
            programAdmin.DateLastLogin = DateTime.Now;
            programAdmin.TotalLogins += 1;

            _dbAdmins.Update(programAdmin);

            Dispatcher.BeginInvoke(new Action(() =>
            {
              var firstTimeLoginPanel = new UpdatePasswordWindow(programAdmin);
              firstTimeLoginPanel.Show();

              Close();
            }));
          }
          else
          {
            Dispatcher.BeginInvoke(new Action(() =>
            {
              cpLoader.Visibility = Visibility.Collapsed;
              AlertManager.ShowAlertMessage("Password error", "The given password is incorrect, please try again...");
            }));
          }
        }
        else
        {
          Dispatcher.BeginInvoke(new Action(() =>
          {
            cpLoader.Visibility = Visibility.Collapsed;
            AlertManager.ShowAlertMessage("Admin not found", "The given username cannot be found in the admin database, please try again...");
          }));
        }
      }
      catch (Exception)
      {
        Dispatcher.BeginInvoke(new Action(() =>
        {
          cpLoader.Visibility = Visibility.Collapsed;
          AlertManager.ShowAlertMessage("Connection error", "Cannot connect to the database, please try again later...");
        }));
      }
    });
  }
  else
  {
    cpLoader.Visibility = Visibility.Collapsed;
    AlertManager.ShowAlertMessage("Missing information", "Please fill in all the required information...");
  }
  }

user_1783619 · Answer

There is one simple rule in WPF when it comes to UI controls:

All changes must happen on the UI thread.

By running a background worker, thread, or Task you are getting off the UI thread (which is the whole point, so you can avoid blocking it and making your program unresponsive). So if you need to run UI logic inside a task then marshalling back to the UI thread is necessary. This is what the Dispatcher does.

That said, you almost never have to do this in practice (at least when using Task) because of how async/await work. When you use await it automatically returns control to the calling thread after the Task completes, so if you structured your code as:

// Do some UI stuff
await dbTask;
// Do more UI stuff

instead of one giant task, you would find that your need for Dispatcher goes down dramatically.

As noted in the comments, this post provides a lot of super useful general information on dealing with async/await and tasks: How to run and interact with an async Task from a WPF gui

user_5086631 · Answer

I think you have the right idea, but I agree that your implementation could use a bit of work.

In your sample above, it seems like you're doing one "background" activity (validating the username and password) and then either continuing with the program if it succeeds, or displaying one of several error messages. To make things cleaner, reduce code duplication and minimize the number of places where you jump between threads, I would just use a string variable to store the error message you want to show, and then at the end, tell the main thread the result of the background activity and have it either continue or display the error:

        private async Task TryLoginAsync()
        {
            var username = txtUsername.Text.Trim();
            var password = txtPassword.Password.Trim();

            string errorTitle;
            string errorDetail;
            /*Your Type*/ object programAdmin;
            bool result;

            if (!string.IsNullOrWhiteSpace(username) && !string.IsNullOrWhiteSpace(password))
            {
                result = await Task.Run(new Func(() =>
                {
                    try
                    {
                        programAdmin = _dbAdmins.FindBy(x => x.Username.ToLower() == username.ToLower());
                        if (programAdmin != null)
                        {
                            if (string.Equals(EncryptionManager.Decrypt(BotDBConfiguration.EncryptionKey, programAdmin.Password), password))
                            {
                                programAdmin.DateLastLogin = DateTime.Now;
                                programAdmin.TotalLogins += 1;

                                _dbAdmins.Update(programAdmin);

                                return true;
                            }
                            else
                            {
                                errorTitle = "Password error";
                                errorDetail = "The given password is incorrect, please try again...";
                            }
                        }
                        else
                        {
                            errorTitle = "Admin not found";
                            errorDetail = "The given username cannot be found in the admin database, please try again...";
                        }
                    }
                    catch (Exception)
                    {
                        errorTitle = "Connection error";
                        errorDetail = "Cannot connect to the database, please try again later...";
                    }

                    return false;
                }));
            }
            else
            {
                errorTitle = "Missing information";
                errorDetail = "Please fill in all the required information...";
            }

            if (result)
            {
                var firstTimeLoginPanel = new UpdatePasswordWindow(programAdmin);
                firstTimeLoginPanel.Show();

                Close();
            }
            else
            {
                cpLoader.Visibility = Visibility.Collapsed;
                AlertManager.ShowAlertMessage(errorTitle, errorDetail);
            }
        }

Notice I changed the the internal Task to a run a Func instead of an Action. This way it can return a value bool to indicate the result. Because you are using a lambda function, you can also access variables like errorTitle from both threads, which I am using to pass the details of any error back to the main thread. Instead of sharing variables between threads, you could also change the return type of Task to be a more complex class or a Tuple.

These changes don't really alter the functionality of the code, but in my opinion they make it more readable/maintainable by separating out the main-thread code from the background-thread code.

Is this the correct way of using Tasking & Dispatcher in WPF?

2 Answers