Finding the default application for opening a particular file type on Windows

I'm developing an application targeting .NET Framework 2.0 using C# for which I need to be able to find the default application that is used for opening a particular file type.

I know that, for example, if you just want to open a file using that application you can use something like:

System.Diagnostics.Process.Start( "C:\...\...\myfile.html" );

to open an HTML document in the default browser, or

System.Diagnostics.Process.Start( "C:\...\...\myfile.txt" );

to open a text file in the default text editor.

However, what I want to be able to do is to open files that don't necessarily have a .txt extension (for example), in the default text editor, so I need to be able to find out the default application for opening .txt files, which will allow me to invoke it directly.

I'm guessing there's some Win32 API that I'll need to P/Invoke in order to do this, however a quick look with both Google and MSDN didn't reveal anything of much interest; I did find a very large number of completely irrelevant pages, but nothing like I'm looking for.

All current answers are unreliable. The registry is an implementation detail and indeed such code is broken on my Windows 8.1 machine. The proper way to do this is using the Win32 API, specifically AssocQueryString:

using System.Runtime.InteropServices;

[DllImport("Shlwapi.dll", CharSet = CharSet.Unicode)]
public static extern uint AssocQueryString(
    AssocF flags, 
    AssocStr str,  
    string pszAssoc, 
    string pszExtra, 
    [Out] StringBuilder pszOut, 
    ref uint pcchOut
); 

[Flags]
public enum AssocF
{
    None = 0,
    Init_NoRemapCLSID = 0x1,
    Init_ByExeName = 0x2,
    Open_ByExeName = 0x2,
    Init_DefaultToStar = 0x4,
    Init_DefaultToFolder = 0x8,
    NoUserSettings = 0x10,
    NoTruncate = 0x20,
    Verify = 0x40,
    RemapRunDll = 0x80,
    NoFixUps = 0x100,
    IgnoreBaseClass = 0x200,
    Init_IgnoreUnknown = 0x400,
    Init_Fixed_ProgId = 0x800,
    Is_Protocol = 0x1000,
    Init_For_File = 0x2000
}

public enum AssocStr
{
    Command = 1,
    Executable,
    FriendlyDocName,
    FriendlyAppName,
    NoOpen,
    ShellNewValue,
    DDECommand,
    DDEIfExec,
    DDEApplication,
    DDETopic,
    InfoTip,
    QuickTip,
    TileInfo,
    ContentType,
    DefaultIcon,
    ShellExtension,
    DropTarget,
    DelegateExecute,
    Supported_Uri_Protocols,
    ProgID,
    AppID,
    AppPublisher,
    AppIconReference,
    Max
}

Relevant documentation:

• AssocQueryString
• ASSOCF
• ASSOCSTR

Sample usage:

static string AssocQueryString(AssocStr association, string extension)
{
    const int S_OK = 0;
    const int S_FALSE = 1;

    uint length = 0;
    uint ret = AssocQueryString(AssocF.None, association, extension, null, null, ref length);
    if (ret != S_FALSE)
    {
        throw new InvalidOperationException("Could not determine associated string");
    }

    var sb = new StringBuilder((int)length); // (length-1) will probably work too as the marshaller adds null termination
    ret = AssocQueryString(AssocF.None, association, extension, null, sb, ref length);
    if (ret != S_OK)
    {
        throw new InvalidOperationException("Could not determine associated string"); 
    }

    return sb.ToString();
}

You can check under registry section HKEY_CLASSES_ROOT for the extension and action details. Documentation for this is on MSDN. Alternatively, you can use the IQueryAssociations interface.

Doh! Of course.

HKEY_CLASSES_ROOT\.txt

includes a reference to

HKEY_CLASSES_ROOT\txtfile

which contains a subkey

HKEY_CLASSES_ROOT\txtfile\shell\open\command

which references Notepad.

Sorted, many thanks!

Bart

Here is a blog post with about this topic. The code samples are in VB.net, but it should be easy to port them to C#.