C# Signature:

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

VB Signature:

<DllImport("Shlwapi.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
Private Shared Function AssocQueryString(ByVal flags As UInteger, ByVal str As UInteger, ByVal pszAssoc As String, ByVal pszExtra As String, ByVal pszOut As Text.StringBuilder, ByRef pcchOut As UInteger) As UInteger
End Function

User-Defined Types:

[Flags]
enum AssocF : uint
{
    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_FixedProgId = 0x800,
    IsProtocol = 0x1000,
    InitForFile = 0x2000,
}

enum AssocStr
{
    Command = 1,
    Executable,
    FriendlyDocName,
    FriendlyAppName,
    NoOpen,
    ShellNewValue,
    DDECommand,
    DDEIfExec,
    DDEApplication,
    DDETopic,
    InfoTip,
    QuickTip,
    TileInfo,
    ContentType,
    DefaultIcon,
    ShellExtension,
    DropTarget,
    DelegateExecute,
    SupportedUriProtocols,
    // The values below ('Max' excluded) have been introduced in W10 1511
    ProgID,
    AppID,
    AppPublisher,
    AppIconReference,
    Max
}

Alternative Managed API:

Do you know one? Please contribute it!

Notes:

Also see http://www.pinvoke.net/default.aspx/shell32.findexecutable (though this method is preferred, see http://visualstudiomagazine.com/articles/2009/10/13/finding-an-associated-executable.aspx ).

Tips & Tricks:

The result is based on the process owner. If your program is running elevated on a user session, the result will be valid for the administrator, not for the user.

Sample Code:

static string AssocQueryString(AssocStr association, string extension)
{
    uint length = 0;
    uint ret = AssocQueryString(AssocF.None, association, extension, null, null, ref length);
    if (ret != 1) // expected S_FALSE
    {
        throw new InvalidOperationException("Could not determine associated string, unable to get the required buffer length. Error code: " + ret);
    }

    var sb = new StringBuilder((int)length); // (length-1) will probably work too as null termination is added
    ret = AssocQueryString(AssocF.None, association, extension, null, sb, ref length);
    if (ret != 0) // expected S_OK
    {
        throw new InvalidOperationException("Could not determine associated string. Error code: " + ret);
    }

    return sb.ToString();
}

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