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I'm getting the following Exception when trying to use FolderBrowserDialog: System.Threading.ThreadStateException: 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.

I have Googled this problem extensively and the solutions that everybody suggests seem to be to put [STAThreadAttribute] above the Main method, to delete all dll's from the Debug folder, or to use the Invoke method. I have tried all of these, and I still get the same exception.

Here's the code:

public partial class Form1 : Form
{
    public event EventHandler ChooseLocationHandler = null;

    public string DestFolder
    {
        set { textBox1.Text = value; }
        get { return textBox1.Text; }
    }

    public Form1()
    {
        InitializeComponent();
    }

    private void ChooseLocationButton_Click(object sender, EventArgs e)
    {
        if (ChooseLocationHandler != null)
            ChooseLocationHandler(this, e);
    }
}

And in my presenter is the following:

public partial class Presenter
{
    Form1 myForm;
    public Presenter()
    {
        myForm = new Form1();
        myForm.ChooseLocationHandler += ChooseLocationHandler;
        myForm.Show();
    }

    public void ChooseLocationHandler(object obj, EventArgs e)
    {
        Form1 sender = (Form1)obj;

        FolderBrowserDialog fbd = new FolderBrowserDialog();
        fbd.RootFolder = System.Environment.SpecialFolder.MyComputer;
        fbd.ShowNewFolderButton = true;
        if (fbd.ShowDialog() == DialogResult.Cancel)
            return;

        sender.DestFolder = fbd.SelectedPath;
    }
}

I'm getting the Exception on fbd.ShowDialog().

Daniel
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7 Answers7

66

A thread is either STA or MTA it can't be specified just for one method so the attribute must be present on the entry point.

From STAThreadAttribute in MSDN :

Apply this attribute to the entry point method (the Main() method in C# and Visual Basic). It has no effect on other methods.

If this code is called from a secondary thread you have 3 choices :

IMPORTANT NOTE: Running (as you seem to do) System.Windows.Forms code inside an MTA thread is unwise, some functionalities like file open dialogs (not only folder) require a MTA thread to work.

Changing your secondary thread apartment

If you create the thread yourself (and don't use the specificity of MTA) you could just change it's apartment before starting it :

var t = new Thread(...);
t.SetApartmentState(ApartmentState.STA);

 

Creating a thread just for it

If you don't control the thread creation you could do it in a temporary thread :

string selectedPath;
var t = new Thread((ThreadStart)(() => {
    FolderBrowserDialog fbd = new FolderBrowserDialog();
    fbd.RootFolder = System.Environment.SpecialFolder.MyComputer;
    fbd.ShowNewFolderButton = true;
    if (fbd.ShowDialog() == DialogResult.Cancel)
        return;

    selectedPath = fbd.SelectedPath;
}));

t.SetApartmentState(ApartmentState.STA);
t.Start();
t.Join();
Console.WriteLine(selectedPath);

 

Invoking in another(STA) thread

If your main thread also contain System.Windows.Forms code you could invoke in it's message loop to execute your code :

string selectedPath = null;
Form f = // Some other form created on an STA thread;
f.Invoke(((Action)(() => {
    FolderBrowserDialog fbd = new FolderBrowserDialog();
    fbd.RootFolder = System.Environment.SpecialFolder.MyComputer;
    fbd.ShowNewFolderButton = true;
    if (fbd.ShowDialog() == DialogResult.Cancel)
        return;

    selectedPath = fbd.SelectedPath;
})), null);
Console.WriteLine(selectedPath);
Julien Roncaglia
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  • How is your application started ? Just double clicking on it ? If yes where is this code called from ? A background worker, a secondary thread or something like that ? (If you don't know break in visual studio on the line calling the dialog and paste here the entire call stack) – Julien Roncaglia Jul 28 '11 at 14:34
  • The call stack is far to large to fit in this text box. But it is being called from a different thread from the main thread. – Daniel Jul 28 '11 at 14:48
  • So it is where the problem come from, the apartment is a thread state, i'll add 2 or 3 possible solutions to my answer. – Julien Roncaglia Jul 28 '11 at 14:50
  • Done but as noted in my answer a thread creating forms should be STA in most cases. – Julien Roncaglia Jul 28 '11 at 15:05
  • The temporary thread did the trick! Thanks so much. I'd upvote 100 times if I could. – Daniel Jul 28 '11 at 15:28
  • @VirtualBlackFox Thank you so very much for posting your methods on correcting this error. I searched online for a few hours before i found this thread and your post. your suggestion of "Changing your secondary thread apartment If you create the thread yourself (and don't use the specificity of MTA) you could just change it's apartment before starting it : var t = new Thread(...); t.SetApartmentState(ApartmentState.STA);" was just what i needed and fixed my code. Thank you! – toosweetnitemare Nov 26 '12 at 15:40
  • Can I also use this with vb.net? I tried creating temp thread `Using topForm As New System.Windows.Forms.Form() Dim tempThread As New Thread(New ThreadStart( Sub() Dim dialog As New Windows.Forms.FolderBrowserDialog() Dim result As Windows.Forms.DialogResult = dialog.ShowDialog()..... ))tempThread.Start() tempThread.Join() tempThread.SetApartmentState(ApartmentState.STA) End Using` but still it is giving me the same error. :( – Priya May 04 '22 at 10:38
  • Thanks for the 3 diffrnt typs. Creating a temp thread worked nw. Format was different in vb.net but I managed it. For the ones, who are interested in vb.net, `Using topForm As New System.Windows.Forms.Form() Dim tempThread = New Thread(CType((Sub() Dim dialog As New Windows.Forms.FolderBrowserDialog() ......... End Sub), ThreadStart)) tempThread.SetApartmentState(ApartmentState.STA) tempThread.Start() tempThread.Join() End Using` – Priya May 04 '22 at 12:20
4

This fixed my issue. [STAThread] static void Main()

Just an extra question: why can't microsoft make things simple? Are they trying to disgust people to do some coding?

Mat M
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Eric Mariacher
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    This doesn't really answer his specific question... At least it doesn't add to the accepted answer. And complaining or asking "Why can't XXX to XXXX" isn't appropriate for the site. – David Oct 26 '12 at 15:24
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    This don't answer the question, and please limit to just answer the question. – Eric Javier Hernandez Saura Oct 26 '12 at 16:25
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    Sorry but this DID fix my issue. I was using a console app and needed access to the clipboard. Adding the [STAThread] worked wonderful for me. – da_jokker Nov 10 '18 at 16:50
2

As simple as the below : 

using System.Windows.Forms;
namespace fileConverterBaset64
{
    class Program
    {
        [STAThread]
        static void Main(string[] args)

Add the command [STAThread] before your main method. That's it, it would work.

Quan VO
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Shankar K Arun
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    Could you please rework the code markup and add a simple explanation to your answer? – creyD Jul 23 '19 at 16:01
  • That's exactly what [@Eric answered](https://stackoverflow.com/a/13089468/4256973) 7 years earlier. – n00dles Sep 11 '20 at 04:27
2

I had the same issue with ASP.NET MVC project. When I export my crystal report to some format it shows me the error. What I have done is replace

This:

            SaveFileDialog browser = new SaveFileDialog();
            string fileName = "";

            browser.Filter = "Pdf|*.pdf|Txt|.txt";

            if (browser.ShowDialog() == DialogResult.OK)
            {
                ExportFormatType formatType = ExportFormatType.NoFormat;
                switch (browser.FilterIndex)
                {
                    case 2:
                        formatType = ExportFormatType.WordForWindows;
                        break;
                    case 1:
                        formatType = ExportFormatType.PortableDocFormat;
                        break;
                }

                fileName = browser.FileName;
                crReportDocument.ExportToDisk(formatType, fileName);

Into:

Thread thread = new Thread((ThreadStart)(() =>
            {
                SaveFileDialog browser = new SaveFileDialog();
                string fileName = "";

                browser.Filter = "Pdf|*.pdf|Txt|.txt";

                if (browser.ShowDialog() == DialogResult.OK)
                {
                    ExportFormatType formatType = ExportFormatType.NoFormat;
                    switch (browser.FilterIndex)
                    {
                        case 2:
                            formatType = ExportFormatType.WordForWindows;
                            break;
                        case 1:
                            formatType = ExportFormatType.PortableDocFormat;
                            break;
                    }

                    fileName = browser.FileName;
                    crReportDocument.ExportToDisk(formatType, fileName);
                }
            }));

            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
            thread.Join();
Chamila Maddumage
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1

The STAThread attribute must be in front of main as far as i know.

Mario The Spoon
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-1

I Had This Same Issue, I Remove 3 Un-Used Dll's And it Fixed... Thank's So Much!

CripWalked
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-4

Now, check all dll in Reference and delete dll not use.

That was unbelievable. I could have never imagined those dll's are causing this problem.

user10590035
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