C#: Set File Type Association

April 14th, 2009 by Mel Leave a reply »

While searching for some simple C# code to set the program and icon associated with a file type in Windows I ran across these beautiful functions that illustrate how to accomplish this by using the registry.

All credit goes to cristiscu (source).

Here’s the code:

using Microsoft.Win32;
using System.Runtime.InteropServices;
 
public class FileAssociation
{
    // Associate file extension with progID, description, icon and application
    public static void Associate(string extension, 
           string progID, string description, string icon, string application)
    {
        Registry.ClassesRoot.CreateSubKey(extension).SetValue("", progID);
        if (progID != null && progID.Length > 0)
            using (RegistryKey key = Registry.ClassesRoot.CreateSubKey(progID))
            {
                if (description != null)
                    key.SetValue("", description);
                if (icon != null)
                    key.CreateSubKey("DefaultIcon").SetValue("", ToShortPathName(icon));
                if (application != null)
                    key.CreateSubKey(@"Shell\Open\Command").SetValue("", 
                                ToShortPathName(application) + " \"%1\"");
            }
    }
 
    // Return true if extension already associated in registry
    public static bool IsAssociated(string extension)
    {
        return (Registry.ClassesRoot.OpenSubKey(extension, false) != null);
    }
 
    [DllImport("Kernel32.dll")]
    private static extern uint GetShortPathName(string lpszLongPath, 
        [Out] StringBuilder lpszShortPath, uint cchBuffer);
 
    // Return short path format of a file name
    private static string ToShortPathName(string longName)
    {
        StringBuilder s = new StringBuilder(1000);
        uint iSize = (uint)s.Capacity;
        uint iRet = GetShortPathName(longName, s, iSize);
        return s.ToString();
    }
}

And here’s how you’d use it:

if (!FileAssociation.IsAssociated(".ext"))
   Associate(".ext", "ClassID.ProgID", "ext File", "YourIcon.ico", "YourApplication.exe");

A couple caveats to this code that I’ve noticed:

  1. The second parameter passed to Associate() can be any string you’d like to use to represent your program in the registry, just make sure you’re consistent with it.


  2. ToShortPathName() will return an empty string if the file path you pass it doesn’t exist, i.e. if you pass it a path to an icon file that doesn’t exist it won’t work.


  3. IsAssociated() will return true if the file extension is associated with any program, not just yours. If you want to be specific about it checking whether it’s associated with your program, then just check whether it’s default key is set to your programs ProgID.


Posted in C#, Computers, Software

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10 comments

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  1. Huy says:
    December 7, 2011 at 4:36 am

    the icon does not show for the file association.

    Reply
  2. MazQuick says:
    November 29, 2011 at 10:59 am

    Almost everything work, except icon being displayed.
    I gave a path for the icon, but it doesn’t work anyway.
    I tried using:
    Application.StartupPath + @”\icon.exe”;
    but it didn’t work, also I tried:
    Application.ExecutablePath //(and added ,1 after it (in associate function)), but it also didn’t work.
     
    Anyone know how to fix icon problem? :F

    Reply
  3. Robert says:
    April 8, 2011 at 7:21 pm

    it is muh easy and fast to get the path if you use Application.StartUpPath(); function. that is the better way instead of using the DllImport attribute

    Reply
  4. china法宝 says:
    November 3, 2009 at 10:53 pm

    非常好。

    Reply
  5. Darth Reisender says:
    October 6, 2009 at 8:36 pm

    Promising, but… when I have my file type associated, how do I open it within my code, I mean, how does my app know that there is a file waiting to be opened?

    Reply
    • Mel says:
      October 6, 2009 at 8:43 pm

      Hey Darth,

      If you’d like you’re application to handle files when it’s opened, such as if the user double-clicks on a file that you’ve associated with your program, you need to check for the path of the file in your program’s command-line arguments. Like so:

      string[] arguments = Environment.GetCommandLineArgs();
if (arguments.Length > 1)
{
    string filePath = arguments[1];
}

      If you’d like to loop through all the files that might be sent to your program you can do it like this:

      string[] arguments = Environment.GetCommandLineArgs();
for (int i = 1; i < arguments.Length; i++)
{
    string filePath = arguments[i];
}
      Reply
  6. haha lol says:
    October 2, 2009 at 7:14 pm

    kool

    Reply

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