A user on the VB Development Forum I’ve visited for a few years asked a question about creating file associations.  I chuckled and assumed that a Google search would return a wealth of information about the subject.  I was wrong; by my fifth combination of search terms I had found one site that had the minimal steps to get it done, and it wasn’t even correct.  The rest of the results were happy to discuss using the File Types tab of the Folder Options dialog to accomplish the task; this is not programmatic and not what the user wanted.  I remembered having a heck of a time trying to figure out how to make an association for all files (so I could have a right-click->Edit with hex editor option) so I decided I’d write up what I know.  Shame on you, internet, for letting the information get buried.

To start off, my example is technically incorrect.  I’m working with HKCR, but apparently the content here is generated based off of HKCU\Software and HKLM\Software. I assume this is so you can have per-user and global associations; I wrote my example using HKCR directly but you should probably use one of these two; please understand when I say HKCR I mean one of the other two locations.

I found the File Associations documentation, but haven’t had time to look over it properly.  For now, this should do.  In order to make it so that when you double-click a file your application opens you need three things:

1. Your application should support opening a file from the command-line in some way.
2. You need a registry key to associate a file type with the extension.
3. You need a registry key to associate a program with the file type.

The first one is up to you; I can’t tell you how to do it.  The registry key to associate a file type with an extension is pretty easy.  File extensions are listed in HKCR under keys named after the extension (including the period!) The value of these keys is a string that defines the file type associated with the extension.  I tend to not use spaces in these, but I have no idea if spaces are unsupported; it will end up being the name of a subkey later and I’m too lazy to check if spaces are supported in subkey names.  For example, text files have the extension txt, and the key that specifies the file type is HKCR\.txt.  It’s default value is txtfile.

Now that there’s a file type associated with the extension, you need a registry key that tells the shell how to handle operations on the file type.  There’s lots of things to implement here, but I only know the basics.  File types are declared in subkeys of HKCR named after the file type; their subkeys and default values vary based on exactly how the file type is to be handled.  All I’m going to discuss is handling the "open" verb, which is generally invoked by double-clicking a file in the shell.  To add support for this verb, a tree of subkeys need to be created under HKCR\filetype. Working with our Notepad example above, we see HKCR\txtfile\shell\open\command.  It has a value that represents the path to Notepad and the replacement string %1 for the file that was selected.

Example time

Suppose we want to associate Notepad with the file extension .zzz.  We’ll start by defining a key for the file type:

HKCR\.zzz
Value="MyEditorFile"

Now, we create the file type key and associate the "Open" verb with notepad:

HKCR\MyEditorFile\open\shell\command
Value="%SystemRoot%\system32\NOTEPAD.EXE %1"

Note that though there are no quotes in the value for the open key, I’ve had trouble in the past when this string didn’t have anything that could possibly contain a space quoted. For example, I’m used to using:
"C:\Program Files\My Program\program.exe" "%1"
I’m not sure why Notepad’s special and doesn’t require this.   That’s really all I have to say on this topic for now; I am still looking over the documentation.

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!

This is yet another thing that I found took me too long to find on Google. I’m using the .NET regular expressions, and looking to replace values in a string with numbers, and I need to use the special $1 symbols to keep some of my capture groups around.

So, suppose I have the text:
something.2005.txt

The pattern:
(.*)2005(.*)

And the replacment string:
$12008$2

This produces the undesirable result $12008.txt. Why? Well, the $1 has to be escaped in some way. Whatever it is that scans for variables is obviously interpreting this as $12008 instead of $1; either that’s invalid or the group doesn’t exist so it just uses the text. Personally I think I should be getting “2008.txt” in this case, as a raw $ should take $$ to create, but I won’t complain.

The solution? Surround the number of the group index in brackets:
${1}2008$2