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,
    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:

Please add some!

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");
     }

     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");
     }

     return sb.ToString();
    }