imagelist.images[零].tag无法赋值

imagelist.images[0].tag无法赋值
            imageList1.Images[0].Tag = "000";
            MessageBox.Show(imageList1.Images[0].Tag.ToString());//出错！！！

            注：已经为imagelist1添加了几张图片。
错误信息为：未将对象引用设置到对象的实例。

[解决办法]
在ImageList中获取某索引处的图片是创建新图，因此设置Tag无效，每次访问该属性得到的都是不同的Image对象，为什么每次都要创建新图而不是缓存图片就不知道了。

public sealed class ImageList : Component
{
    private ImageList owner;

    public Image this[int index]
    {
        get
        {
            if ((index < 0) 
[解决办法]
 (index >= this.Count))
            {
                throw new ArgumentOutOfRangeException("index", SR.GetString("InvalidArgument", new object[] { "index", index.ToString(CultureInfo.CurrentCulture) }));
            }
            return this.owner.GetBitmap(index);
        }
        set ......
    }
......
}

private Bitmap GetBitmap(int index)
{
    if ((index < 0) 
[解决办法]
 (index >= this.Images.Count))
    {
        throw new ArgumentOutOfRangeException("index", SR.GetString("InvalidArgument", new object[] { "index", index.ToString(CultureInfo.CurrentCulture) }));
    }
    Bitmap image = null;
    if (this.ColorDepth == ColorDepth.Depth32Bit)
    {
        NativeMethods.IMAGEINFO pImageInfo = new NativeMethods.IMAGEINFO();
        if (SafeNativeMethods.ImageList_GetImageInfo(new HandleRef(this, this.Handle), index, pImageInfo))
        {
            Bitmap bitmap2 = null;
            BitmapData bmpData = null;
            BitmapData targetData = null;
            IntSecurity.ObjectFromWin32Handle.Assert();
            try
            {
                bitmap2 = Image.FromHbitmap(pImageInfo.hbmImage); 
 
                bmpData = bitmap2.LockBits(new Rectangle(pImageInfo.rcImage_left, pImageInfo.rcImage_top, pImageInfo.rcImage_right - pImageInfo.rcImage_left, pImageInfo.rcImage_bottom - pImageInfo.rcImage_top), ImageLockMode.ReadOnly, bitmap2.PixelFormat);
                int stride = bmpData.Stride;
                int height = this.imageSize.Height;
                if (BitmapHasAlpha(bmpData))
                {
                    image = new Bitmap(this.imageSize.Width, this.imageSize.Height, PixelFormat.Format32bppArgb);
                    targetData = image.LockBits(new Rectangle(0, 0, this.imageSize.Width, this.imageSize.Height), ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);
                    this.CopyBitmapData(bmpData, targetData);
                }
            }
            finally
            {
                CodeAccessPermission.RevertAssert();
                if (bitmap2 != null)
                {
                    if (bmpData != null)
                    {
                        bitmap2.UnlockBits(bmpData);
                    }
                    bitmap2.Dispose();
                }
                if ((image != null) && (targetData != null))
                {
                    image.UnlockBits(targetData);
                }
            }
        }
    }
    if (image == null)
    {
        image = new Bitmap(this.imageSize.Width, this.imageSize.Height); 
 
        using (Graphics graphics = Graphics.FromImage(image))
        {
            IntPtr hdc = graphics.GetHdc();
            try
            {
                SafeNativeMethods.ImageList_DrawEx(new HandleRef(this, this.Handle), index, new HandleRef(graphics, hdc), 0, 0, this.imageSize.Width, this.imageSize.Height, -1, -1, 1);
            }
            finally
            {
                graphics.ReleaseHdcInternal(hdc);
            }
        }
    }
    image.MakeTransparent(fakeTransparencyColor);
    return image;
}