标签文本没有'；直到整个循环完成，才能更新

我有一个Winform程序，当用户单击按钮时，它会进行一些计算，然后调用picturebox绘制事件，根据计算结果绘制新的BMP。这很好用。

现在我想这样做100次，每次图片框刷新时，我都想通过更新标签上的文本来查看它当前的迭代，如下所示：

 private void button2_Click(object sender, EventArgs e)
        {
        for (int iterations = 1; iterations <= 100; iterations++)
        {
            // do some calculations to change the cellmap parameters
            cellMap.Calculate();
            // Refresh picturebox1
            pictureBox1.Invalidate();
            pictureBox1.Update();
            // Update label with the current iteration number
            label1.Text = iterations.ToString();
        }
    }
    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        Bitmap bmp = new Bitmap(cellMap.Dimensions.Width, cellMap.Dimensions.Height);
        Graphics gBmp = Graphics.FromImage(bmp);
        int rectWidth = scaleFactor;
        int rectHeight = scaleFactor;
         // Create solid brushes
        Brush blueBrush = new SolidBrush(Color.Blue);
        Brush greenBrush = new SolidBrush(Color.Green);
        Brush transparentBrush = new SolidBrush(Color.Transparent);
        Graphics g = e.Graphics;
        for (int i = 0; i < cellMap.Dimensions.Width; i++)
        {
                for (int j = 0; j < cellMap.Dimensions.Height; j++)
                {
                    // retrieve the rectangle and draw it
                    Brush whichBrush;
                    if (cellMap.GetCell(i, j).CurrentState == CellState.State1)
                    {
                        whichBrush = blueBrush;
                    }
                    else if (cellMap.GetCell(i, j).CurrentState == CellState.State2)
                    {
                        whichBrush = greenBrush;
                    }
                    else
                    {
                        whichBrush = transparentBrush;
                    }
                    // draw rectangle to bmp
                    gBmp.FillRectangle(whichBrush, i, j, 1f, 1f);
                }
         }
         g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.NearestNeighbor;
         g.DrawImage(bmp, 0, 0, pictureBox1.Width, pictureBox1.Height);
    }

我遇到的问题是，标签文本只有在最后一次图片框更新完成后才会显示。所以本质上，它不显示1到99。我可以看到图片框在每次刷新后都会更新，因为BMP在每次迭代中都会发生变化。知道吗？

// Code fragement...
// 5 cent solution, add Invalidate/Update
label1.Text = iterations.ToString();
label1.Invalidate();
label1.Update();