一、效果

二、代码
1.定义一个MyParam类。
文件MyParam.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.Windows.Controls;

namespace w
{
    class MyParam
    {
        public TextBox txt{ set; get; }
        public String s { set; get; }
        public int x { set; get; }

        public void outAB()
        {
           // MyParam ss = (MyParam)s;//将Object型强制转换为MyParam型
            StringBuilder sb = new StringBuilder();
            int cs = 100;
            int i = 0;
            while (i < cs)
            {
                this.txt.Dispatcher.Invoke(
                  () =>
                  {
                      txt.Text += this.s + this.x;//ss.s？？？
                  }
                  );
                Thread.Sleep(100);
                i++;
            }
        }
    }
}

2.文件MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace w
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }   
     
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            MyParam a = new MyParam();
            a.s = "张三";
            a.x = 21;
            a.txt = this.txt;
            MyParam b = new MyParam();
            b.s = "李四";
            b.x = 22;
            b.txt = this.txt;
            ThreadStart startA = new ThreadStart(a.outAB);
            Thread thA = new Thread(startA);
            ThreadStart startB = new ThreadStart(b.outAB);
            Thread thB = new Thread(startB); 
            thA.Start();
            thB.Start();
        }
    }
}

3.文件MainWindow.xaml

<Window x:Class="w.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:w"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <TextBox x:Name="txt" HorizontalAlignment="Center" Margin="0,53,0,0" Text="" TextWrapping="Wrap" VerticalAlignment="Top" Width="419" Height="285"/>
        <Button Content="多线程" HorizontalAlignment="Center" Margin="0,372,0,0" VerticalAlignment="Top" Height="28" RenderTransformOrigin="0.192,0.548" Width="55" Click="Button_Click"/>

    </Grid>
</Window>