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, [In][Out] 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
  {
    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:

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Notes:

None.

Tips & Tricks:

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Sample Code:

  public string GetAssociation(string doctype)
  {
    uint pcchOut = 0;   // size of output buffer

    // First call is to get the required size of output buffer
    AssocQueryString(AssocF.Verify, AssocStr.Executable, doctype, null, null, ref pcchOut);

    if (pcchOut == 0)
    {
           return String.Empty;
    }

    // Allocate the output buffer
    StringBuilder pszOut = new StringBuilder((int)pcchOut);

    // Get the full pathname to the program in pszOut
    AssocQueryString(AssocF.Verify, AssocStr.Executable, doctype, null, pszOut, ref pcchOut);

    string doc = pszOut.ToString();
    return doc;
  }

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