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Can't open a FileSave dialog in Windows
f#

I'm trying to convert the little editor written by Microsoft as a sample into a class. The command handler - adapted slightly from the Microsoft sample - is shown below.

member x.saveFile file text =
let d = new SaveFileDialog() in
d.Filter <- "text files *.txt|*.txt|All files *.*|*.*";
d.FilterIndex <- 2;
d.FileName <- file;
d.AddExtension <- false;
d.OverwritePrompt <- true;
if d.ShowDialog() = DialogResult.OK then
let sw = new StreamWriter(d.FileName) in
sw.Write(text:string); (* type constrain text to resolve sw.Write(_) method overload *)
sw.Close();
Some d.FileName
else
None

This compiles, but when I try to get the File Save dialog box, I get the following complaint, even though I have marked STAThread.

A first chance exception of type 'System.Threading.ThreadStateException' occurred in System.Windows.Forms.dll
An unhandled exception of type 'System.Threading.ThreadStateException' occurred in System.Windows.Forms.dll
Additional information: Current thread must be set to single thread apartment (STA) mode before OLE calls can be made. Ensure that your Main function has STAThreadAttribute marked on it. This exception is only raised if a debugger is attached to the process.

Can anyone tell me why this is happening? The code stops at the ShowDialog statement.

I'm brand new at F# - formerly C++ - so please be gentle.

Thanks.

Bob

.

Hi Bob,

The problem is that calling the File dialog use com interop under the hood so the thread you're using needs to be a Single Thread Appartment thread (STA). This is true for a quite a few (but not all) of the winforms controls. The simpliest way round this is to add the STAThread attribute to the entry point of your program:

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#light
open System
open System.Windows.Forms
open System.IO

let main() =
  let text = "whatever"
  let d = new SaveFileDialog() in 
  d.Filter <- "text files *.txt|*.txt|All files *.*|*.*";
  d.FilterIndex <- 2;
  // d.FileName <- file;
  d.AddExtension <- false; 
  d.OverwritePrompt <- true;
  if d.ShowDialog() = DialogResult.OK then
    let sw = new StreamWriter(d.FileName) in
    sw.Write(text:string); (* type constrain text to resolve sw.Write(_) method overload *)
    sw.Close()
    
[<STAThread>]
do main()

Cheers,
Robert

By on 11/25/2007 1:09 AM ()

Hi, Robert.

Thanks for the reply. My program entry point is in a separate module. After opening the module with the ZTextWindow class, the code is as follows.

[<STAThread()>]
let form = new ZTextWindow()
do Application.Run( form.m_zForm )

The code in my post is a method from the ZTextWindow class.

So I thought I had set up a Single Thread Appartment thread (STA). Am I doing it wrong? Any other ideas?

Thanks for your help. By the way, thanks very much for the book; I'm enjoying it.

Bob

By on 11/25/2007 4:54 PM ()

Attach the attribute to a single do() binding. Unfortunately by specification STAThread can be attached to any function, though really has to go on the "main". 

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open System

[<STAThread>]
do()

However I note we could indeed give a warning for your code, where you're not attaching it to a function.

Kind regards

don

By on 11/25/2007 5:09 PM ()

I guess I answered my own question, in a way. I had the code below, which fails.

[<STAThread()>]
let form = new ZTextWindow()
do Application.Run( form.m_zForm )

Switch the top two statements and it works.

let form = new ZTextWindow()
[<STAThread()>]
do Application.Run( form.m_zForm )

I guess I didn't attach it to a function the first time, right?

Thanks to both Robert and Don for your help. I'll keep plugging.

By the way, this little exercise of converting the sample Microsoft editor to a class has taught me a lot.

Bob

By on 11/25/2007 5:21 PM ()

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