new Thread(() => GetMessage(userId, false)).Start();

using System;
using System.Windows.Forms;

namespace Tools
{
    public static class InvokeHelper
    {
        public static void SafeInvoke(this Control control, Action action)
        {
            if(control.InvokeRequired) {
                control.Invoke(action);
                return;
            }
            action();
        }

        public static void SafeInvoke(this Control control, Action action, T obj)
        {
            if(control.InvokeRequired) {
                control.Invoke(action, obj);
                return;
            }
            action(obj);
        }

        public static void SafeInvoke(this Control control, Action action, T1 obj1, T2 obj2, T3 obj3)
        {
            if(control.InvokeRequired) {
                control.Invoke(action, obj1, obj2, obj3);
                return;
            }
            action(obj1, obj2, obj3);
        }

        public static void SafeAsyncInvoke(this Control control, Action action)
        {
            if(control.InvokeRequired) {
                control.BeginInvoke(action);
                return;
            }
            action();
        }

        public static void SafeAsyncInvoke(this Control control, Action action, T obj)
        {
            if(control.InvokeRequired) {
                control.BeginInvoke(action, obj);
                return;
            }
            action(obj);
        }

        public static void SafeAsyncInvoke(this Control control, Action action, T1 obj1, T2 obj2)
        {
            if(control.InvokeRequired) {
                control.BeginInvoke(action, obj1, obj2);
                return;
            }
            action(obj1, obj2);
        }

        public static void SafeAsyncInvoke(this Control control, Action action, T1 obj1, T2 obj2, T3 obj3)
        {
            if(control.InvokeRequired) {
                control.BeginInvoke(action, obj1, obj2, obj3);
                return;
            }
            action(obj1, obj2, obj3);
        }
    }
}

private string GetMessage(int userId, bool expression)
{
    if(this.InvokeRequired) {
        this.SafeInvoke(SetText, userId, expression);
        return;
    }
    // . . .
    return _richtextbox.Text;
}