Recently, I was adding the ability to open files from an application at work. Luckily, I had run into this situation before, so I knew that you could run a
Process.Start on the file in question and have Explorer open it for you. What I didn’t know was that it would throw a
Win32Exception if the file didn’t have an associated program to open it. My next step was what any good developer would do: Google it.
Fairly early on in my searching, I came across a post by Ned Batchelder that described the EXACT same scenario I was working on! Ned wanted to open a file with an unknown extension from a managed application just like me. He details his Google search and explains how he discovered information about the API functions,
ShellExecuteEx. Basically, you can call
ShellExecuteEx with an “open” verb and Explorer will attempt to open the file. If it fails with an
SE_ERR_NOASSOC error, then call
ShellExecuteEx again but using an “openas” verb. Doesn’t sound too bad, though some code snippets still would’ve sped the process up for me :-) I did learn it better this way, though.
Anyway, here’s where I started. First, I pulled in the definition for the
Friend Structure SHELLEXECUTEINFO Public cbSize As Integer Public fMask As SEE_MASK Public hwnd As IntPtr <MarshalAs(UnmanagedType.LPTStr)> _ Public lpVerb As String <MarshalAs(UnmanagedType.LPTStr)> _ Public lpFile As String <MarshalAs(UnmanagedType.LPTStr)> _ Public lpParameters As String <MarshalAs(UnmanagedType.LPTStr)> _ Public lpDirectory As String Dim nShow As Integer Dim hInstApp As SE_ERR Dim lpIDList As IntPtr <MarshalAs(UnmanagedType.LPTStr)> _ Public lpClass As String Public hkeyClass As IntPtr Public dwHotKey As Integer Public hIcon As IntPtr Public hProcess As IntPtr End Structure
Next, I created definitions for the
#Region " SW Constants " Friend Enum SW As Integer HIDE = 0 SHOWNORMAL = 1 NORMAL = 1 SHOWMINIMIZED = 2 SHOWMAXIMIZED = 3 MAXIMIZE = 3 SHOWNOACTIVATE = 4 SHOW = 5 MINIMIZE = 6 SHOWMINNOACTIVE = 7 SHOWNA = 8 RESTORE = 9 SHOWDEFAULT = 10 FORCEMINIMIZE = 11 MAX = 11 End Enum #End Region #Region " SEE_MASK Constants " Friend Enum SEE_MASK As Integer CLASSNAME = &H1 CLASSKEY = &H3 IDLIST = &H4 INVOKEIDLIST = &HC ICON = &H10 HOTKEY = &H20 NOCLOSEPROCESS = &H40 CONNECTNETDRV = &H80 FLAG_DDEWAIT = &H100 DOENVSUBST = &H200 FLAG_NO_UI = &H400 UNICODE = &H4000 NO_CONSOLE = &H8000 ASYNCOK = &H100000 HMONITOR = &H200000 NOZONECHECKS = &H800000 NOQUERYCLASSSTORE = &H1000000 WAITFORINPUTIDLE = &H2000000 FLAG_LOG_USAGE = &H4000000 End Enum #End Region #Region " SE_ERR Constants " Friend Enum SE_ERR As Integer SE_ERR_FNF = 2 ' file not found SE_ERR_PNF = 3 ' path not found SE_ERR_ACCESSDENIED = 5 ' access denied SE_ERR_OOM = 8 ' out of memory SE_ERR_DLLNOTFOUND = 32 SE_ERR_SHARE = 26 SE_ERR_ASSOCINCOMPLETE = 27 SE_ERR_DDETIMEOUT = 28 SE_ERR_DDEFAIL = 29 SE_ERR_DDEBUSY = 30 SE_ERR_NOASSOC = 31 End Enum #End Region
Finally, I created my definition for the
<DllImport("shell32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _ Friend Shared Function ShellExecuteEx(ByRef lpExecInfo As SHELLEXECUTEINFO) As Boolean End Function
I stuck all of this into a NativeMethods class and tried the code below:
Dim info As New NativeMethods.SHELLEXECUTEINFO info.cbSize = Marshal.SizeOf(info) info.lpDirectory = Path.GetDirectoryName(fileToStart) info.lpFile = Path.GetFileName(fileToStart) info.nShow = NativeMethods.SW.SHOWDEFAULT info.lpVerb = "open" info.fMask = NativeMethods.SEE_MASK.FLAG_NO_UI Or NativeMethods.SEE_MASK.FLAG_DDEWAIT If Not NativeMethods.ShellExecuteEx(info) Then If info.hInstApp = NativeMethods.SE_ERR.SE_ERR_NOASSOC Then Dim sinfo As New NativeMethods.SHELLEXECUTEINFO sinfo.cbSize = Marshal.SizeOf(info) sinfo.lpVerb = "openas" sinfo.lpDirectory = Path.GetDirectoryName(fileToStart) sinfo.lpFile = Path.GetFileName(fileToStart) sinfo.nShow = NativeMethods.SW.SHOWDEFAULT NativeMethods.ShellExecuteEx(sinfo) End If End If
UPDATED (9/12/2006): Many thanks to Michael and his comment regarding using the
SEE_MASK.FLAG_DDEWAIT. That fixed all of the problems I was running into regarding the above code. (see the usage on
A quick note about the code: Ned mentioned in his post that he got the
ERROR_NO_ASSOCATION error instead of
SE_ERR_NOASSOC. Well, the
ERROR_NO_ASSOCATION is what is returned in the Win32 error (
SE_ERR_NOASSOC is returned in the
hInstApp (see MSDN documentation here).
See any problems with that? I certainly didn’t (and still don’t). It works like a charm for files with associations… however, it would only work one time for files without any associated program. Afterwards, it wouldn’t give me anything… no errors, nada, zilch. After a few tries, an
AccessViolationException would get thrown. Why? I have no idea. I tried various things to see if I should be cleaning up memory somewhere but I couldn’t find anything. I did find out that if I just called
ShellExecuteEx with the “openas” verb the first time, I wouldn’t get any problems at all. What in the world???
As a result of the strange behavior, I changed my code slightly to look like this:
Try Using p As New Process p.StartInfo.FileName = fileToStart p.StartInfo.UseShellExecute = True p.Start() End Using Catch win32Ex As Win32Exception Dim sinfo As New NativeMethods.SHELLEXECUTEINFO sinfo.cbSize = Marshal.SizeOf(sinfo) sinfo.lpVerb = "openas" sinfo.lpDirectory = Path.GetDirectoryName(fileToStart) sinfo.lpFile = Path.GetFileName(fileToStart) sinfo.nShow = NativeMethods.SW.SHOWDEFAULT If Not NativeMethods.ShellExecuteEx(sinfo) Then Throw New Win32Exception End If End Try
The above code is working like a charm. I still have no idea why my first example won’t work for me. If anyone has any ideas or suggestions, please let me know. I haven’t worked with Interop between managed and unmanaged code very much. My experience up to this point has primarily been an entirely managed project or an entirely unmanaged project (and that only in college).
NOTES: Here are some resources I found while researching this:
Also, be sure to look at the ShellAPI.h header file!