c# - 连接时如何放置取消按钮

Question

我使用登录名和密码在 C# 中建立连接，我想如果连接太多，请放置一个按钮以取消与数据库的连接长。

我想知道怎么做，如果可能的话把它放在一个线程中。

代码如下:

private void btncon_Click(object sender, EventArgs e)
{
    string strLogin = tblogin.Text.Trim();
    string pass  = tbpwd.Text;
    bool success = false;

    if (String.IsNullOrWhiteSpace(strLogin) || String.IsNullOrWhiteSpace(pass))
    {
        MessageBox.Show("Veuillez remplir tous les champs SVP ");
    }
    else if(String.IsNullOrEmpty(strLogin) || String.IsNullOrEmpty(pass)){
        MessageBox.Show("Veuillez remplir tous les champs SVP ");
    }
    else
    {
        model.Connexion cm = new model.Connexion();
        pass = Snippets.SHA1Util.SHA1HashStringForUTF8String(pass).ToString();
        string[] user = cm.login(strLogin, pass);    

        if(user[0] != null)
        {
            Int32.TryParse(user[0], out iduser);
            Int32.TryParse(user[1], out idGrp);
            Int32.TryParse(user[2], out idbtq);
            nom = user[3];

            if (idGrp != 3)
                success = true;
            else
            {
                MessageBox.Show("Accès Non autorisé , Veuillez contacter l'administrateur");
                success = false;
            }
        }
        else
        {
            MessageBox.Show("Email ou mot de passe incorrect.");
            success = false;
        }
    }

    if (success)
    {                
        main Principale = new main();
        Principale.Show();
        Hide();`
    }
}

Answer 1

问题是，当用户点击Cancel 按钮时，无法正常取消正在执行的cm.login() 方法。您可以使用 Thread.Abort() 强制终止登录，但这是不安全的并且强烈建议不要这样做，因为正确执行需要在另一个 AppDomain 中执行代码并且会使代码非常复杂。

幸运的是，如果满足以下条件，您仍然可以实现 Cancel 按钮:

• 在另一个线程上调用cm.login()是安全的
• 让 cm.login() 在用户点击 Cancel 按钮后在后台(在另一个线程上)完成不会产生不良影响。

此代码还假设它是一个 Winform 应用程序(但 WPF 应用程序的解决方案非常相似)。它还假设主窗体有一个隐藏的按钮 btnCancel (Visible=false) 并且它是 Click 事件处理程序设置为 btnCancel_Click 方法。

TaskCompletionSource<Object> CancelLoginTcs = new TaskCompletionSource<Object>();

// Make button click handler method async
private async void btncon_Click(object sender, EventArgs e)
{
    try
    {
        // Make Cancel button visible, so that user can click on it
        btnCancel.Visible = true;

        // Prepare everything needed to start login
        //var strLogin = ...;
        //var pass = ...;
        //model.Connexion cm = new model.Connexion();
        // ...

        // Start login on another thread
        var loginTask = Task<string[]>.Run(() => cm.login(strLogin, pass));

        // Create task that is used to wake-up main thread, when user clicks
        // on the Cancel button before login finishes.
        CancelLoginTcs = new TaskCompletionSource<Object>();

        // Wait for login task or cancel task to complete, whichever finishes first
        await Task.WhenAny(loginTask, CancelLoginTcs.Task);

        if (CancelLoginTcs.Task.IsCanceled)
        {
            // User clicked on the Cancel button.
            // Login method will be running in the background, until it
            // finishes. This assumes that it is safe to do so.
            // Here you should do neccessary clean-up, inform user, etc.
            // ...
        }
        else
        {
            // Login finished and user did NOT click on the Cancel button.

            try
            {
                // Simply read result of login. If an Exception occured during login,
                // it will be rethrow now, so you should handle it appropriatelly
                var user = loginTask.Result;

                // Here program continues in a usual way
                // ...
            }
            catch(Exception E)
            {
                // Handle login exception
                // ...
            }
        }
    }
    finally
    {
        // Hide Cancel button again
        btnCancel.Visible = false;
        CancelLoginTcs = null;
    }
}

private void btnCancel_Click(object sender, EventArgs e)
{
    // Set cancel task to cancelled state. 
    // This will wake-up main thread and let it continue
    CancelLoginTcs.SetCanceled();
}