C# help needed - Current thread must be set to single thread apartment (STA) mode before OLE calls can be made

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.

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I am trying to run this method but get the above error. Can someone please send me a set of code that can let me do the work I need to do ?
I simply need to copy a string to the clipboard.

my method is below

   private void copyStringToClipBoard(string  sValue)
        {
            try
            {
                System.Windows.Forms.Clipboard.SetDataObject(sValue);               
               
            }
            catch (Exception ex)
            {
                // something 
            }
        }

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indikadAsked:
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käµfm³d 👽Commented:
Add the STAThread attribute to your main method.

e.g.

[STAThread]
public static void main()
{

}

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indikadAuthor Commented:
kaufmed ,
[STAThread] is already there

 - but I think because my application is called from another application I believe things aren't going right when I want to access the clipboard object. I don't have control over the caller application.
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käµfm³d 👽Commented:
When you say, "my application is called from another application," is your app accessed as a reference you added to the other application, or are you using something like Process.Start to run this application from the other application?
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indikadAuthor Commented:
I am not sure - I am actually assuming some of the things - with regards to how the caller app calls my app.
my app is an exe.  I dont know how the caller app calls my app, but I know that I have a class in my application that receives event messages from the caller application - hope that helps.
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käµfm³d 👽Commented:
When you said, "[STAThread] is already there," on which method did you place that attribute?
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indikadAuthor Commented:
<<When you said, "[STAThread] is already there," on which method did you place that attribute?>>


on the Main
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käµfm³d 👽Commented:
I am unsure then. I tested what I mentioned above in a quick Console application, and it was as I stated. The only thing I can think of is, as I alluded to earlier, if your app is being used as a referenced assembly, then it is not the main thread of the application, and it would be the responsibility of the caller to add the STAThreadAttribute to its main method. If your caller is using Process.Start, then your process has its own thread, and you would be the one required to add STAThreadAttribute to your own main.

I suggest researching how your assembly is being invoked.
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VoloxCommented:
You say that your application receives event messages.  Is this clipboard code being called from within the call stack of one of those event messages?

If so, then your problem may be that you're operating within the event thread of the instead of the STA thread.  Since event threads come from the system Thread Pool, they are not STA. If that's the case, you would need to either trigger the calling application to run it's event threads in STA or marshall the call into the STA thread of your application.
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indikadAuthor Commented:
Hi Volox and kaufmed ,

thanks for the comments from both of you.

Since this was not a very critical application - i resolved it using timer control  - but I am sure there are better techniques that can solve this.  The use of the timer control allowed me to run the  piece of the code without an issue (code that accesses the clipboard )

this is what I did ( not very elegant but did the job )
1.  when user clicks to add something to clipboard - the method call will simply set a global bool to true.
2. The timer that always runs  - checks the bool variable every few seconds.
3. if the bools is true - then the timer will add the desired text to clipboard.
there is a unnoticeable delay.

However like I said I am sure there can be better solutions and happy to get suggestions.
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VoloxCommented:
The Timer control doesn't have an issue accessing the clipboard because it runs on an STA thread that is within the events context.  The fact that this works for you and the other method didn't definitely supports my theory that you were calling the clipboard from a thread that was not STA.

Try this mechanism if you want to get rid of the less than optimal timer mechanism...

Thread t = new Thread(() =>
{
    Clipboard.SetDataObject(sValue);
});
t.SetApartmentState(ApartmentState.STA);
t.Start();
t.Join();

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