如何在 C# 中创建按键表单事件

Question

I'm trying to make a KeyPress event on a form, but in this line I got an error MainWindow.KeyPress = new KeyPressEventArgs(Form_KeyPress); , I read the Microsoft Docs about events in C#, but I don't get the idea.

Do listeners like in Java exist in C# or not?

My code:

class PracticeEvent
{
    static void Main(String[] args)
    {
        Form MainWindow = new Form();
        MainWindow.Text = "Practice";
        MainWindow.MaximizeBox = false;
        MainWindow.MinimizeBox = false;
        MainWindow.FormBorderStyle = FormBorderStyle.FixedSingle;
        MainWindow.StartPosition = FormStartPosition.CenterScreen;
        MainWindow.Size = new Size(1000, 700);
        MainWindow.KeyPreview = true;
        MainWindow.KeyPress = new KeyPressEventArgs(Form_KeyPress); 

        MainWindow.ShowDialog();

    }

    private void Form_KeyPress(object sender, System.Windows.Forms.KeyEventArgs e)
    {
        if(e.KeyCode == Keys.A){
            MessageBox.Show("You pressed the A key.");
        }
    }

}

Answer 1

Your main method is static, your event handler is not. You need to provide it an object reference, that's what the error message is trying to say. Another error is that you are assigning rather than attaching the handler, use += operator for that.

Specifically, change this line:

MainWindow.KeyPress = new KeyPressEventArgs(Form_KeyPress);

to be

var instance = new PracticeEvent();
MainWindow.KeyPress += instance.Form_KeyPress;

Answer 2

You have several errors in your code.

MainWindow.KeyPress = new KeyPressEventArgs(Form_KeyPress);

1) KeyPress has KeyPressEventHandler type. Not KeyPressEventArgs . In C# classes which called ...EventArgs are usually used as special objects that contains data about a raised event and them are inherited from EventArgs system class. And classes which called ...EventHandlers are usually define wrapper for delegates and called events.

2) So KeyPress is event. If you want to subscribe this event you should use += operator. And method that you want to specify as handler should have signature void(object, KeyPressEventArgs) . Typical signature for events is void(object, ...EventArgs)

private void Form_KeyPress(object sender, System.Windows.Forms.KeyEventArgs e)

3) As I said this method has wrong signature ( KeyPressEventArgs instead KeyEventArgs ).

4) It should be static . You can not use non-static class members in static method.

So your code should look like this:

    class PracticeEvent
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        static void Main()
        {
            Form MainWindow = new Form();
            MainWindow.Text = "Practice";
            MainWindow.MaximizeBox = false;
            MainWindow.MinimizeBox = false;
            MainWindow.FormBorderStyle = FormBorderStyle.FixedSingle;
            MainWindow.StartPosition = FormStartPosition.CenterScreen;
            MainWindow.Size = new Size(1000, 700);
            MainWindow.KeyPreview = true;
            MainWindow.KeyPress += Form_KeyPress;
            MainWindow.ShowDialog();
        }

        private static void Form_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            if (e.KeyChar == 'a')
            {
                MessageBox.Show("You pressed the A key.");
            }
        }
    }

Use listeners in C# is not good practice, but some frameworks use it. Usually events and callbacks is used.

And my last advice. May be you want to use the KeyDown event? The KeyPress is used for working with char input.