I’m working on a project with a custom control on which I want to implement the arrow keys.  I was completely mystified as to why I wasn’t getting KeyUp events when I pressed the arrow keys. I started using Reflector to hunt for where the event was raised in controls I was familiar with, and I stumbled upon the Control.IsInputKey method.  What does this method do? The remarks told me what I needed to know.

Call the IsInputKey method to determine whether the key specified by the keyData parameter is an input key that the control wants. This method is called during window message preprocessing to determine whether the specified input key should be preprocessed or sent directly to the control. If IsInputKey returns true, the specified key is sent directly to the control. If IsInputKey returns false, the specified key is preprocessed and only sent to the control if it is not consumed by the preprocessing phase. Keys that are preprocessed include the TAB, RETURN, ESCAPE, and the UP ARROW, DOWN ARROW, LEFT ARROW, and RIGHT ARROW keys.

I had no idea this existed. It turns out to get events from these keys in any control, you have to override this method and return true. Here’s an example control in C# 3.0 to demonstrate (conversion to C# 2.0 is trivial; just make a backing field for the property). When it has focus (you’ll know because it is white instead of gray), if you push an arrow key the text name of the key you pressed is displayed. If you press any other key, you’ll get “Unknown”. The documentation does seem a little off, as I’m not doing anything special for Esc or Return but they still cause the events to be raised. Still, if you set CustomInput to false, you won’t ever get an arrow key message displayed (the arrow keys seem to move the focus).

using System;
using System.Windows.Forms;
using System.Drawing;

namespace TestIsInput
{
    public class InputField : Control
    {
        public InputField()
        {
            this.SetStyle(ControlStyles.AllPaintingInWmPaint, true);
            this.SetStyle(ControlStyles.OptimizedDoubleBuffer, true);
            this.SetStyle(ControlStyles.Selectable, true);
        }

        public bool CustomInput
        {
            get;
            set;
        }

        protected override bool IsInputKey(Keys keyData)
        {
            if (CustomInput)
            {
                bool isInput = false;

                switch (keyData)
                {
                    case Keys.Up:
                    case Keys.Down:
                    case Keys.Left:
                    case Keys.Right:
                        isInput = true;
                        break;
                    default:
                        isInput = false;
                        break;
                }
                return isInput;
            }
            else
            {
                return base.IsInputKey(keyData);
            }
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);

            Brush backBrush = this.Focused ? Brushes.White : Brushes.LightGray;

            e.Graphics.FillRectangle(backBrush, this.ClientRectangle);
            ControlPaint.DrawBorder3D(e.Graphics, this.ClientRectangle, Border3DStyle.Sunken);
            StringFormat f = new StringFormat(StringFormatFlags.NoWrap);
            f.Alignment = StringAlignment.Center;
            f.LineAlignment = StringAlignment.Center;

            e.Graphics.DrawString(this.Text, this.Font, Brushes.Black, (RectangleF)this.ClientRectangle, f);
        }

        protected override void OnKeyUp(KeyEventArgs e)
        {
            base.OnKeyUp(e);

            switch(e.KeyCode)
            {
                case Keys.Up :
                    this.Text = "Up";
                    break;
                case Keys.Down :
                    this.Text = "Down";
                    break;
                case Keys.Left :
                    this.Text = "Left";
                    break;
                case Keys.Right :
                    this.Text = "Right";
                    break;
                default :
                    this.Text = "Unknown";
                    break;
            }

            this.Invalidate();
        }

        protected override void OnEnter(EventArgs e)
        {
            base.OnEnter(e);
            this.Invalidate();
        }

        protected override void OnLeave(EventArgs e)
        {
            base.OnLeave(e);
            this.Invalidate();
        }
    }
}

So, if you need to accept input from one of those keys, keep in mind that you need to override IsInputKey. This was completely new to me!