How to associate a file extension to the current executable in C#

Question

I\'d like to to associate a file extension to the current executable in C#. This way when the user clicks on the file afterwards in explorer, it\'ll run my executable with t

Answer 1

Also, if you decide to go the registry way, keep in mind that current user associations are under HKEY_CURRENT_USER\Software\Classes. It might be better to add your application there instead of local machine classes.

If your program will be run by limited users, you won't be able to modify CLASSES_ROOT anyway.

Answer 2

There doesn't appear to be a .Net API for directly managing file associations but you can use the Registry classes for reading and writing the keys you need to.

You'll need to create a key under HKEY_CLASSES_ROOT with the name set to your file extension (eg: ".txt"). Set the default value of this key to a unique name for your file type, such as "Acme.TextFile". Then create another key under HKEY_CLASSES_ROOT with the name set to "Acme.TextFile". Add a subkey called "DefaultIcon" and set the default value of the key to the file containing the icon you wish to use for this file type. Add another sibling called "shell". Under the "shell" key, add a key for each action you wish to have available via the Explorer context menu, setting the default value for each key to the path to your executable followed by a space and "%1" to represent the path to the file selected.

For instance, here's a sample registry file to create an association between .txt files and EmEditor:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.txt]
@="emeditor.txt"

[HKEY_CLASSES_ROOT\emeditor.txt]
@="Text Document"

[HKEY_CLASSES_ROOT\emeditor.txt\DefaultIcon]
@="%SystemRoot%\\SysWow64\\imageres.dll,-102"

[HKEY_CLASSES_ROOT\emeditor.txt\shell]

[HKEY_CLASSES_ROOT\emeditor.txt\shell\open]

[HKEY_CLASSES_ROOT\emeditor.txt\shell\open\command]
@="\"C:\\Program Files\\EmEditor\\EMEDITOR.EXE\" \"%1\""

[HKEY_CLASSES_ROOT\emeditor.txt\shell\print]

[HKEY_CLASSES_ROOT\emeditor.txt\shell\print\command]
@="\"C:\\Program Files\\EmEditor\\EMEDITOR.EXE\" /p \"%1\""

Answer 3

Here's a complete example:

public class FileAssociation
{
    public string Extension { get; set; }
    public string ProgId { get; set; }
    public string FileTypeDescription { get; set; }
    public string ExecutableFilePath { get; set; }
}

public class FileAssociations
{
    // needed so that Explorer windows get refreshed after the registry is updated
    [System.Runtime.InteropServices.DllImport("Shell32.dll")]
    private static extern int SHChangeNotify(int eventId, int flags, IntPtr item1, IntPtr item2);

    private const int SHCNE_ASSOCCHANGED = 0x8000000;
    private const int SHCNF_FLUSH = 0x1000;

    public static void EnsureAssociationsSet()
    {
        var filePath = Process.GetCurrentProcess().MainModule.FileName;
        EnsureAssociationsSet(
            new FileAssociation
            {
                Extension = ".binlog",
                ProgId = "MSBuildBinaryLog",
                FileTypeDescription = "MSBuild Binary Log",
                ExecutableFilePath = filePath
            },
            new FileAssociation
            {
                Extension = ".buildlog",
                ProgId = "MSBuildStructuredLog",
                FileTypeDescription = "MSBuild Structured Log",
                ExecutableFilePath = filePath
            });
    }

    public static void EnsureAssociationsSet(params FileAssociation[] associations)
    {
        bool madeChanges = false;
        foreach (var association in associations)
        {
            madeChanges |= SetAssociation(
                association.Extension,
                association.ProgId,
                association.FileTypeDescription,
                association.ExecutableFilePath);
        }

        if (madeChanges)
        {
            SHChangeNotify(SHCNE_ASSOCCHANGED, SHCNF_FLUSH, IntPtr.Zero, IntPtr.Zero);
        }
    }

    public static bool SetAssociation(string extension, string progId, string fileTypeDescription, string applicationFilePath)
    {
        bool madeChanges = false;
        madeChanges |= SetKeyDefaultValue(@"Software\Classes\" + extension, progId);
        madeChanges |= SetKeyDefaultValue(@"Software\Classes\" + progId, fileTypeDescription);
        madeChanges |= SetKeyDefaultValue($@"Software\Classes\{progId}\shell\open\command", "\"" + applicationFilePath + "\" \"%1\"");
        return madeChanges;
    }

    private static bool SetKeyDefaultValue(string keyPath, string value)
    {
        using (var key = Registry.CurrentUser.CreateSubKey(keyPath))
        {
            if (key.GetValue(null) as string != value)
            {
                key.SetValue(null, value);
                return true;
            }
        }

        return false;
    }