Question

According to the official documentation, the KeyDown event on a Windows Forms control occurs only once, but it is easy to demonstrate that the event fires continually aslong as a key is held down:

    private void textBox1_KeyDown(object sender, KeyEventArgs e)
    {
        label1.Text = string.Format("{0}", globalCounter++);
    }

How can you consume the event so that it fires only once?

Answer 1

I’m generally a VB guy, but this seems to work for me as demo code, using the form itself as the input source:

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private bool _keyHeld;
        public Form1()
        {
            InitializeComponent();
            this.KeyUp += new KeyEventHandler(Form1_KeyUp);
            this.KeyDown += new KeyEventHandler(Form1_KeyDown);
            this._keyHeld = false;
        }

        void Form1_KeyUp(object sender, KeyEventArgs e)
        {
            this._keyHeld = false;
        }

        void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if (!this._keyHeld)
            {
                this._keyHeld = true;
                if (this.BackColor == Control.DefaultBackColor)
                {
                    this.BackColor = Color.Red;
                }
                else
                {
                    this.BackColor = Control.DefaultBackColor;
                }
            }
            else
            {
                e.Handled = true;
            }
        }
    }   
}

I think the logic gets a little sketchy if you’re holding down multiple keys at a time, but that seems to only fire the event from the last key that was pressed anyway, so I don’t think it becomes an issue.

I tested this in a TextBox in VB, and it worked fine. Wasn’t sure on the inheritance conventions I should follow in c#, so I left it as a straight Form for this answer.

Apologies for any gross code formatting errors, again, this isn’t my usual language.