If you want to set a thread's apartment model via Thread.CurrentThread.ApartmentState, you need to act quickly

Welcome to CLR Week 2014. Don’t worry, it’ll be over in a few days.

A customer wanted to know why their Folder­Browser­Dialog was displaying the infamous Current thread must be set to single thread apartment (STA) mode before OLE calls can be made error.

private void btnBrowseFolder_Click(object sender, System.EventArgs e)
{
  Thread.CurrentThread.ApartmentState = ApartmentState.STA;
  FolderBrowserDialog fbd = new FolderBrowserDialog {
    RootFolder = System.Environment.SpecialFolder.MyComputer,
    ShowNewFolderButton = true,
    Description = "Select the awesome folder..."
  };
  DialogResult dr = fbd.ShowDialog();
  ...
}

“Even though we set the Apartment­State to STA, the apartment state is still MTA. Curiously, if we put the above code in a standalone test program, it works fine.”

The problem is that the customer is changing the apartment state too late.

On the first call to unmanaged code, the runtime calls Co­Initialize­Ex to initialize the COM apartment as either an MTA or an STA apartment. You can control the type of apartment created by setting the System.Threading.ApartmentState property on the thread to MTA, STA, or Unknown.

Notice that the value you specify in Current­Thread.Apartment­State is consulted at the point the runtime initializes the COM apartment (which occurs on the first call to unmanaged code). If you change it after the COM apartment has been initialized, you’re revising the blueprints of a house after it has been built.

The standard way to avoid this problem is to attach the [STAThread] attribute to your Main function, or if you need to set the apartment model of a thread you created yourself, call the Thread.Set­Apartment­State method before the thread starts.