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

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.