cross thread operation not valid

 
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;
namespace example1
{
public partial class frmoperations : Form
{
private int increment;
private int square;
private int loopvalue;
private System.Threading.Thread objsquare;
private System.Threading.Thread objincrement;
private System.Threading.Thread objloop;
public frmoperations()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
 
}
private void btnsquare_Click(object sender, EventArgs e)
{
square = int.Parse(txtnum.Text);
btnsquare.Enabled = false;
ChooseThreads(1);
}
private void btnincrement_Click(object sender, EventArgs e)
{
increment = int.Parse(txtnum.Text);
btnincrement.Enabled = false;
ChooseThreads(2);
}
private void btnloop_Click(object sender, EventArgs e)
{
loopvalue = int.Parse(txtnum.Text);
btnloop.Enabled = false;
ChooseThreads(3);
}
public void ChooseThreads(int threadnumber)
{
switch (threadnumber)
{
case 1:
objsquare = new System.Threading.Thread(new System.Threading.ThreadStart(this.runsquare));
objsquare.Start();
break;
case 2:
objincrement = new System.Threading.Thread(new System.Threading.ThreadStart(this.runincrement));
objincrement.Start();
break;
case 3:
objloop = new System.Threading.Thread(new System.Threading.ThreadStart(this.runloop));
objloop.Start();
break;
}
}
public void runsquare()
{
double result;
Console.WriteLine("running square thread");
result = square * square;
lblsquare.Text = "the square is :" + result.ToString();
btnsquare.Enabled = true;
}
public void runincrement()
{
int result1 = increment + 9;
Console.WriteLine("running the increment thread");
lblinc.Text = " the incremented value is" + result1.ToString();
btnincrement.Enabled = true;
}
public void runloop()
{
int index; 
Console.WriteLine("loop");
for(index=1;index<=loopvalue;index++)
{
Console.WriteLine ("running loop");
System .Threading .Thread .Sleep(500);
}
btnloop.Enabled = true;
lblloop.Text = " executed the loop" + loopvalue.ToString()+"times";
 

}
}
}