Button.DialogResult、Form.ShowDialog 和异步事件处理程序超出预期的执行顺序

Question

I gonna try to be brief. Two Forms:

Form1 (button1, textBox1)

---

public partial class Form1 : Form
{
    private readonly Form2 fr2 = new Form2();

    public Form1()
    {
        InitializeComponent();
        fr2.textBox1 = textBox1;
    }

    private async void button1_Click(object sender, EventArgs e)
    {
        textBox1.Text = string.Empty;
        if (fr2.ShowDialog() == DialogResult.OK)
        textBox1.AppendText("Form1" + Environment.NewLine);
    }
}

---

Form2 (button1 with button1.DialogResult == DialogResult.OK )

---

public partial class Form2 : Form
{
    public TextBox textBox1;

    public Form2()
    {
        InitializeComponent();
    }

    private async void button1_Click(object sender, EventArgs e)
    {
        textBox1.AppendText("Form2 A" + Environment.NewLine);
        await Task.Delay(1000);
        textBox1.AppendText("Form2 B" + Environment.NewLine);
        Hide();
    }
}

---

• Run
• Click on Form1.Button1
• Click on Form2.Button1

WHY Textbox.Text ==

@"Form2 A
Form1
Form2 B
";

instead of

@"Form2 A
Form2 B
Form1
"?

WHY the form "returns" to the caller without even finishing the event handler and finish later?

Is this a documented behaviour or a bug?

Update:

Apparently the original winform's form event handing code wasn't updated to account for an async event handler in case button.DialogResult is preset, it's just treated as a "fire and forget" async method call. I have to study some more to see what happens in case an exception is thrown. It seems that is "fired and forgotten" too by the async machinery (no Task to save it for later)?. It is a very different behaviour from the serial case and it's not documented anywhere. Spot on comment about await , Hans Passant!

Answer 1

This is expected behaviour, as you wait asynchronously for the Task.Delay task to finish. While that is being done, the method returns to its caller, and this is by design. Take this example:

private void button1_Click(object sender, EventArgs e)
{
   method();
   textBox1.Text += "C";
}

async void method()
{
   textBox1.Text += "A";
   await Task.Delay(1000);
   textBox1.Text += "B";
}

This is pretty much the same situation, the text of the TextBox will be ACB after pressing button1, as the method method() returns to its caller when await Task.Delay(1000); is called.

To get the desired output, do it as Hans Passant suggested in the comments: Set the DialogResult property of the button to None , so that the method does not return to the Form1 . Then, whenever you wish to hand control over to Form1 again, simply call this.DialogResult = DialogResult.OK in the method on runtime, which will have the same effect.