C# WinForms: How to set Main function STAThreadAttribute

Question

I get the following exception when calling saveFileDialog.ShowDialog() in a background thread:

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.

According to this:

To fix the problem, insert the statement:

Threading.Thread.CurrentThread.ApartmentState = Threading.ApartmentState.STA;

in Main right before the Application.Run statement.

But the Application.Run statement is in Program.cs which seems to be generated code so any changes might be unexpectedly lost. Also, I could not find a way to set current thread to STA in the project or main form properties but maybe I am looking in the wrong place. What is the proper way to call saveFileDialog.ShowDialog() in a background thread?

http://stackoverflow.com/questions/428494/is-it-possible-to-use-showdialog-without-blocking-all-forms — ba__friend, Jun 16 '11 at 14:36
[Don't do UI in a background thread.](http://blogs.msdn.com/b/oldnewthing/archive/2007/10/18/5501378.aspx) — Craig Stuntz, Jun 16 '11 at 14:37
Program.cs is created by the wizard, but never regenerated (unlike *.Designer.cs, etc, where your changes really would be lost). — Ben Voigt, Mar 04 '15 at 15:05

score 58 · Answer 1 · edited Jan 23 '18 at 23:33

58

Solution very easy; Just add this on top of the Main method [STAThread]

So your main method should look like this

 [STAThread]
 static void Main(string[] args)
 {
     ....
 }

It works for me.

edited Jan 23 '18 at 23:33

Kirill Kobelev

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answered Dec 11 '12 at 18:06

MILAD

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score 31 · Accepted Answer · answered Jun 16 '11 at 14:35

31

ShowDialog() shouldn't be called from a background thread - use Invoke(..).

Invoke((Action)(() => { saveFileDialog.ShowDialog() }));

answered Jun 16 '11 at 14:35

Nathan

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1

Nathan, YDM! After 10 years - thanks. The error message was so misleading. – Chris W Dec 10 '21 at 10:40

score 14 · Answer 3 · answered Jun 16 '11 at 14:36

14

this should work if you are creating the thread in which you call the showDialog:

var thread = new Thread(new ParameterizedThreadStart(param => { saveFileDialog.ShowDialog(); }));
 thread.SetApartmentState(ApartmentState.STA);
thread.Start();

answered Jun 16 '11 at 14:36

Mg.

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score 7 · Answer 4 · edited Apr 12 '16 at 09:21

7

Add following code on FormLoad

private void Form1_Load(object sender, EventArgs e)
{
    Thread myth;
    myth = new Thread(new System.Threading.ThreadStart(CallSaveDialog)); 
    myth.ApartmentState = ApartmentState.STA;
    myth.Start();
}

Here CallSaveDialog is a thread and here you can call ShowDialog like this

void CallSaveDialog(){saveFileDialog.ShowDialog();}

edited Apr 12 '16 at 09:21

Breeze

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answered Jan 29 '13 at 13:48

santosh Kundkar

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+1, only one solution that worked for me (calling `ShowDialog()` from another thread in my .dll). Thanks. – icl7126 Mar 23 '13 at 23:14
Can you please add some explanation on what this does? – Draex_ Jul 31 '16 at 05:29

score 1 · Answer 5 · answered Jun 16 '11 at 14:42

On your MainForm:

if (this.InvokeRequired) { 
 this.Invoke(saveFileDialog.ShowDialog()); 
} else { 
 saveFileDialog.ShowDialog(); 
}

Or, if you will have other methods that need to be run from the UI thread:

  private void DoOnUIThread(MethodInvoker d) {
     if (this.InvokeRequired) { this.Invoke(d); } else { d(); }
  }

Then, call your method as such:

 DoOnUIThread(delegate() {
    saveFileDialog.ShowDialog();
 });

C# WinForms: How to set Main function STAThreadAttribute

5 Answers5

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