how to use delegates correctly in multi thread application

Question

I am trying to show a message box on top of my form and get the result. I am using backgroundworker and in my dowork function, I am calling below function. I used delegates based on my past experience for show the message box but I am not sure what is wrong it is giving me below error.

e.Message = "Cross-thread operation not valid: Control 'Form1' accessed from a thread other than the thread it was created on."

Would you mind check why I am getting above error even after using delegates?

        public delegate DialogResult Del(Form1 fr,string text, string title, MessageBoxButtons mbb, MessageBoxIcon mbi);

        public static DialogResult showMessageBox(Form1 fr, string text, string title, MessageBoxButtons mbb, MessageBoxIcon mbi)
        {
            return MessageBox.Show(fr, text, title, mbb, mbi);
        }



  public static OutputEventArgs execSync(string exe, string arguments, string standardInput,Form1 fr)
        {
            OutputEventArgs oea = new OutputEventArgs();
            try
            {
                using (Process myProcess = new Process())
                {
                    ProcessStartInfo startInfo = new ProcessStartInfo();
                    startInfo.WindowStyle = ProcessWindowStyle.Hidden;
                    startInfo.RedirectStandardOutput = true;
                    startInfo.RedirectStandardError = true;
                    startInfo.RedirectStandardInput = true;
                    startInfo.UseShellExecute = false;
                    startInfo.CreateNoWindow = true;

                    startInfo.FileName = exe;
                    startInfo.Arguments = arguments;
                    myProcess.StartInfo = startInfo;

                    myProcess.Start();
                    string line = string.Empty;

                    while (!myProcess.StandardOutput.EndOfStream)
                    {
                        line = myProcess.StandardOutput.ReadLine();
                        if (line.Contains("is already defined in archive"))
                        {
                            Del handler = new Del(showMessageBox);

                            DialogResult dialogResult = handler(fr, "Do you want overwrite the version label?", "OverWrite",MessageBoxButtons.YesNo,MessageBoxIcon.Question);
                            if(dialogResult == DialogResult.Yes)
                                myProcess.StandardInput.WriteLine("y");
                            else
                                myProcess.StandardInput.WriteLine("n");
                        }

                    }

                    oea.Data = line;

                    myProcess.WaitForExit();
                    oea.exitCode = myProcess.ExitCode;
                }
            }
            catch (Exception e)
            {
                oea.Data = e.Message;
                oea.ExceptionHappened();
            }
            return oea;
        }