这个问题是我在MSDN中文技术论坛中遇到的，现象十分诡异，仔细的研究了一下源代码，现在分享给大家，大家先看一段Demo：

1. FileStream fs = new FileStream(@"D:\aa.jpg", FileMode.Open, FileAccess.Read); 
2.             Image image = Image.FromStream(fs); 
3.              
4.             image.Tag = "txtbmp"; 
5.             ImageList bigHeadPicList = new ImageList(); 
6.             bigHeadPicList.Images.Add("1",image); 
7.             Console.WriteLine(bigHeadPicList.Images["1"].Tag);  //这里Tag是null,相当意外！ 
8. 直观上感觉image和 bigHeadPicList.Images["1"]并不是同一对象，让我们通过源代码来验证这一点： 
9. 先看ImageCollection 类的索引器的源代码： 
10.                 public Image this[string key] { 
11.                 get {  
12.                     // We do not support null and empty string as valid keys.  
13.                     if ((key == null) || (key.Length == 0)){ 
14.                         return null;  
15.                     } 
16.  
17.                     // Search for the key in our collection 
18.                     int index = IndexOfKey(key);  
19.                     if (IsValidIndex(index)) { 
20.                         return this[index];  
21.                     }  
22.                     else { 
23.                         return null;  
24.                     } 
25.  
26.                 } 
27.             }  

关键的实现都在另外一个索引器中：

1. [Browsable(false), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)] 
2.             public Image this[int index] {  
3.                 get { 
4.                     if (index < 0 || index >= Count) 
5.                         throw new ArgumentOutOfRangeException("index", SR.GetString(SR.InvalidArgument, "index", index.ToString(CultureInfo.CurrentCulture))); 
6.                     return owner.GetBitmap(index);   //这行代码比较关键 
7.                 } 
8.                 set {  
9.                     if (index < 0 || index >= Count)  
10.                         throw new ArgumentOutOfRangeException("index", SR.GetString(SR.InvalidArgument, "index", index.ToString(CultureInfo.CurrentCulture))); 
11.   
12.                     if (value == null) { 
13.                         throw new ArgumentNullException("value"); 
14.                     } 
15.   
16.                    if (!(value is Bitmap)) 
17.                         throw new ArgumentException(SR.GetString(SR.ImageListBitmap));  
18.   
19.                     AssertInvariant(); 
20.                     Bitmap bitmap = (Bitmap)value;  
21.  
22.                     bool ownsImage = false; 
23.                     if (owner.UseTransparentColor) { 
24.                         // Since there's no ImageList_ReplaceMasked, we need to generate  
25.                         // a transparent bitmap 
26.                         Bitmap source = bitmap;  
27.                         bitmap = (Bitmap) bitmap.Clone();  
28.                         bitmap.MakeTransparent(owner.transparentColor); 
29.                         ownsImage = true;  
30.                     } 
31.  
32.                     try { 
33.                         IntPtr hMask = ControlPaint.CreateHBitmapTransparencyMask(bitmap);  
34.                         IntPtr hBitmap = ControlPaint.CreateHBitmapColorMask(bitmap, hMask); 
35.                         bool ok = SafeNativeMethods.ImageList_Replace(new HandleRef(owner, owner.Handle), index, new HandleRef(null, hBitmap), new HandleRef(null, hMask));  
36.                         SafeNativeMethods.DeleteObject(new HandleRef(null, hBitmap));  
37.                         SafeNativeMethods.DeleteObject(new HandleRef(null, hMask)); 
38.   
39.                         if (!ok) 
40.                             throw new InvalidOperationException(SR.GetString(SR.ImageListReplaceFailed)); 
41.  
42.                     } finally {  
43.                         if(ownsImage) { 
44.                             bitmap.Dispose();  
45.                         }  
46.                     } 
47.                 }  
48.           } 

这行代码比较关键：

1. return owner.GetBitmap(index);   

下面我们看看GetBitmap方法的实现：

1. private Bitmap GetBitmap(int index) { 
2.             if (index < 0 || index >= Images.Count) 
3.                 throw new ArgumentOutOfRangeException("index", SR.GetString(SR.InvalidArgument, "index", index.ToString(CultureInfo.CurrentCulture)));  
4.  
5.             Bitmap result=null;  
6.  
7.             if(ColorDepth == ColorDepth.Depth32Bit) {  
8.   
9.                 NativeMethods.IMAGEINFO imageInfo = new NativeMethods.IMAGEINFO(); // review? do I need to delete the mask and image inside of imageinfo? 
10.                 if(SafeNativeMethods.ImageList_GetImageInfo(new HandleRef(this, this.Handle), index, imageInfo)) {  
11.                     Bitmap tmpBitmap = null; 
12.                     BitmapData bmpData = null; 
13.                     BitmapData targetData = null; 
14.                     IntSecurity.ObjectFromWin32Handle.Assert();  
15.                     try { 
16.                         tmpBitmap = Bitmap.FromHbitmap(imageInfo.hbmImage);  
17.                         //  
18.  
19.   
20.  
21.  
22.                         bmpData = tmpBitmap.LockBits(new Rectangle(imageInfo.rcImage_left,imageInfo.rcImage_top, imageInfo.rcImage_right-imageInfo.rcImage_left, imageInfo.rcImage_bottom-imageInfo.rcImage_top), ImageLockMode.ReadOnly, tmpBitmap.PixelFormat); 
23.   
24.                         int offset =  bmpData.Stride * imageSize.Height   * index; 
25.                         // we need do the following if the image has alpha because otherwise the image is fully transparent even though it has data  
26.                         if(BitmapHasAlpha(bmpData)) {  
27.                             result = new Bitmap(imageSize.Width, imageSize.Height, PixelFormat.Format32bppArgb);  //注意这里是新创建的Bitmap实例 
28.                             targetData = result.LockBits(new Rectangle(0, 0, imageSize.Width, imageSize.Height), ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);  
29.                             CopyBitmapData(bmpData, targetData);//仅仅拷贝图像数据 
30.                         } 
31.                     } finally { 
32.                         CodeAccessPermission.RevertAssert();  
33.                         if(tmpBitmap != null && bmpData != null) { 
34.                             tmpBitmap.UnlockBits(bmpData);  
35.                         }  
36.                         if(result != null && targetData != null) { 
37.                             result.UnlockBits(targetData);  
38.                         } 
39.                     } 
40.                 } 
41.             }  
42.  
43.             if(result == null) { // paint with the mask but no alpha...  
44.                 result = new Bitmap(imageSize.Width, imageSize.Height); //新创建Bitmap实例 
45.  
46.                 Graphics graphics = Graphics.FromImage(result);  
47.                 try { 
48.                     IntPtr dc = graphics.GetHdc(); 
49.                     try { 
50.                         SafeNativeMethods.ImageList_DrawEx(new HandleRef(this, Handle), index, new HandleRef(graphics, dc), 0, 0,  
51.                                                 imageSize.Width, imageSize.Height, NativeMethods.CLR_NONE, NativeMethods.CLR_NONE, NativeMethods.ILD_TRANSPARENT); 
52.   
53.                     }  
54.                     finally { 
55.                         graphics.ReleaseHdcInternal(dc);  
56.                     } 
57.                 } 
58.                 finally { 
59.                     graphics.Dispose();  
60.                 } 
61.             }  
62.   
63.             // gpr: See Icon for description of fakeTransparencyColor 
64.             result.MakeTransparent(fakeTransparencyColor);  
65.             return result; 
66.         }  

代码比较长，关键的部分我使用注释标记出来了，大家看我标记的注释就可以了，大家可以发现，无论BitmapHasAlpha是否为真，都是新创建Bitmap对象然后返回的，所以在mage和 bigHeadPicList.Images["1"]并不是同一对象，所以Tag是null的！