I'm having a problem where I cannot await an asynchronous function inside of the FormClosing event which will determine whether the form close should continue. I have created a simple example that prompts you to save unsaved changes if you close without saving (much like with notepad or microsoft word). The problem I ran into is that when I await the asynchronous Save function, it proceeds to close the form before the save function has completed, then it comes back to the closing function when it is done and tries to continue. My only solution is to cancel the closing event before calling SaveAsync, then if the save is successful it will call the form.Close() function. I'm hoping there is a cleaner way of handling this situation.
To replicate the scenario, create a form with a text box (txtValue), a checkbox (cbFail), and a button (btnSave). Here is the code for the form.
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace TestZ
{
public partial class Form1 : Form
{
string cleanValue = "";
public Form1()
{
InitializeComponent();
}
public bool HasChanges()
{
return (txtValue.Text != cleanValue);
}
public void ResetChangeState()
{
cleanValue = txtValue.Text;
}
private async void btnSave_Click(object sender, EventArgs e)
{
//Save without immediate concern of the result
await SaveAsync();
}
private async Task<bool> SaveAsync()
{
this.Cursor = Cursors.WaitCursor;
btnSave.Enabled = false;
txtValue.Enabled = false;
cbFail.Enabled = false;
Task<bool> work = Task<bool>.Factory.StartNew(() =>
{
//Work to do on a background thread
System.Threading.Thread.Sleep(3000); //Pretend to work hard.
if (cbFail.Checked)
{
MessageBox.Show("Save Failed.");
return false;
}
else
{
//The value is saved into the database, mark current form state as "clean"
MessageBox.Show("Save Succeeded.");
ResetChangeState();
return true;
}
});
bool retval = await work;
btnSave.Enabled = true;
txtValue.Enabled = true;
cbFail.Enabled = true;
this.Cursor = Cursors.Default;
return retval;
}
private async void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (HasChanges())
{
DialogResult result = MessageBox.Show("There are unsaved changes. Do you want to save before closing?", "Unsaved Changes", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Question);
if (result == System.Windows.Forms.DialogResult.Yes)
{
//This is how I want to handle it - But it closes the form while it should be waiting for the Save() to complete.
//bool SaveSuccessful = await Save();
//if (!SaveSuccessful)
//{
// e.Cancel = true;
//}
//This is how I have to handle it:
e.Cancel = true;
bool SaveSuccessful = await SaveAsync();
if (SaveSuccessful)
{
this.Close();
}
}
else if (result == System.Windows.Forms.DialogResult.Cancel)
{
e.Cancel = true;
}
//If they hit "No", just close the form.
}
}
}
}
Edit 05/23/2013
Its understandable that people would ask me why I would be trying to do this. The data classes in our libraries will often have Save, Load, New, Delete functions that are designed to be run asynchronously (See SaveAsync as an example). I do not actually care that much about running the function asynchronously in the FormClosing Event specifically. But if the user wants to save before closing the form, I need it to wait and see if the save succeds or not. If the save fails, then I want it to cancel the form closing event. I'm just looking for the cleanest way to handle this.
await Task.Yield();before closing the form programmatically, because otherwise you may get an exception in case theSaveAsync()completes synchronously. – Gaucho