System.Threading.ThreadStateException Using OpenFileDialog

I'm trying to make a dialog to get a path through the OpendFileDialog, but I skip the exception System.Threading.ThreadStateException .

The application is a WebAplication , hosted on a server. Therefore, the PDF must be generated in a local directory of the user who is using it. For this I want to get the Path desired.

The code is as follows:

private string nombreFile()
{
    OpenFileDialog ofg = new OpenFileDialog();
    DialogResult rst = ofg.ShowDialog();
    string n = "";
    if (rst == DialogResult.OK)
    {
        n = ofg.FileName;
    }
    return n;
}
private void imprimir()
{
    string nomF = nombreFile();

    string nombre = nomF + Guid.NewGuid().ToString() + ".pdf";
    Document doc = new Document(PageSize.A4, 50, 50, 25, 25);
    doc.Open();
    //GENERAMOS EL CONTENIDO DEL PDF
    doc.Close();
    //Imprimimos el doc
    Process prc = new Process();
    prc.StartInfo.FileName = nombre;
    prc.Start();
}

What I want to achieve is that the nombre that is passed to the Process is the nomF , path generated by the nombreFile() but I skip that exception.