在窗体中添加两个Lable控件和一个Button控件。单击Button控件在两个Lable控件中显示计算结果。代码是：
using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace LXK
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Thread th1, th2;
        double x1, x2;

        private void button1_Click(object sender, EventArgs e)
        {
            th1 = new Thread(new ThreadStart(fangfa1));
            th2 = new Thread(new ThreadStart(fangfa2));
            th1.Start();
            th2.Start();
        }

        private void fangfa1()
        {
            for (int i = 0; i < 50; i++)
            {
                x1 = i * 2;
                label1.Text += x1.ToString();
            }
        }

        private void fangfa2()
        {
            for (int i = 0; i < 50; i++)
            {
                x2 = i + 2;
                label2.Text += x2.ToString();
            }
        }

        private void Form3_Load(object sender, EventArgs e)
        {
            button1.Text = "OK";
            label1.BorderStyle = BorderStyle.Fixed3D;
            label2.BorderStyle = BorderStyle.FixedSingle;
        }
    }
}
运行提示错误，请帮忙解决下。谢谢。

跨线程调用控件肯定要调用委托,很基础的问题了

Form3_load，哪来的？
具体错误是什么，贴出来

刚才又上网搜了下，发现你的问题确实是2楼所说：
挂线程调用控件必须要用委托

那个没有改过来。是在From3中写的，在From1中调用的。调用那部分没写，把它改了下，没改彻底。很抱歉。

该如何调用委托？

public delegate void SetControl();

将代码
private void fangfa1()
        {
            for (int i = 0; i < 50; i++)
            {
                x1 = i * 2;
                label1.Text += x1.ToString();
            }
        }
改成
private void fangfa1()
        {
            if(label1.InvokeRequired)
            {
                label1.BeginInvoke(new SetControl(fangfa1));
                return;
            }

            for (int i = 0; i < 50; i++)
            {
                x1 = i * 2;
                label1.Text += x1.ToString();
            }
        }
方法fangfa2类似的改一下

如果为了图省事,可以在实例化Form1的时候设置忽略跨线程调用UI的错误
Form1.CheckForIllegalCrossThreadCalls = false;
不过这就没线程安全了

       Thread th1, th2;
        double x1, x2;

        public delegate void SetControl();
        private void button1_Click(object sender, EventArgs e)
        {
            th1 = new Thread(new ThreadStart(ffa1));
            th2 = new Thread(new ThreadStart(ffa2));
            th1.Start();
            th2.Start();

        }

        public void ffa1()
        {
            if (label1.InvokeRequired)
            {
                SetControl sc = new SetControl(fangfa1);
                this.BeginInvoke(sc, new object[] { });
            }
        }
        public void ffa2()
        {
            if (label2.InvokeRequired)
            {
                SetControl sc = new SetControl(fangfa2);
                this.BeginInvoke(sc, new object[] { });
            }
        }

和7楼的基本原理都是一样的，使用委托。
但是没有修改你的两个方法