Clearing the Cache of a WebBrowser Control

11/19/2008

I've been doing a bit of work with the Web Browser control in .Net. I have to say that while it's extremely easy to use, it has some flaws. One of the big flaws (at least to me) is the inability to clear the cache. There is no function or property that allows you to do it, which is really rather annoying. However there is a way to do it, that's rather simple actually:

   1: /**

   2:  * Modified from code originally found here: http://support.microsoft.com/kb/326201

   3:  **/

   4: #region Usings

   5: using System;

   6: using System.Runtime.InteropServices;

7:

   8: #endregion

9:

  10: namespace Utilities.Web.WebBrowserHelper

  11: {

  12:     /// <summary>

  13:     /// Class for clearing the cache

  14:     /// </summary>

  15:     public static class WebBrowserHelper

  16:     {

  17:         #region Definitions/DLL Imports

  18:         /// <summary>

  19:         /// For PInvoke: Contains information about an entry in the Internet cache

  20:         /// </summary>

  21:         [StructLayout(LayoutKind.Explicit, Size = 80)]

  22:         public struct INTERNET_CACHE_ENTRY_INFOA

  23:         {

  24:             [FieldOffset(0)]

  25:             public uint dwStructSize;

  26:             [FieldOffset(4)]

  27:             public IntPtr lpszSourceUrlName;

  28:             [FieldOffset(8)]

  29:             public IntPtr lpszLocalFileName;

  30:             [FieldOffset(12)]

  31:             public uint CacheEntryType;

  32:             [FieldOffset(16)]

  33:             public uint dwUseCount;

  34:             [FieldOffset(20)]

  35:             public uint dwHitRate;

  36:             [FieldOffset(24)]

  37:             public uint dwSizeLow;

  38:             [FieldOffset(28)]

  39:             public uint dwSizeHigh;

  40:             [FieldOffset(32)]

  41:             public System.Runtime.InteropServices.ComTypes.FILETIME LastModifiedTime;

  42:             [FieldOffset(40)]

  43:             public System.Runtime.InteropServices.ComTypes.FILETIME ExpireTime;

  44:             [FieldOffset(48)]

  45:             public System.Runtime.InteropServices.ComTypes.FILETIME LastAccessTime;

  46:             [FieldOffset(56)]

  47:             public System.Runtime.InteropServices.ComTypes.FILETIME LastSyncTime;

  48:             [FieldOffset(64)]

  49:             public IntPtr lpHeaderInfo;

  50:             [FieldOffset(68)]

  51:             public uint dwHeaderInfoSize;

  52:             [FieldOffset(72)]

  53:             public IntPtr lpszFileExtension;

  54:             [FieldOffset(76)]

  55:             public uint dwReserved;

  56:             [FieldOffset(76)]

  57:             public uint dwExemptDelta;

  58:         }

59:

  60:         // For PInvoke: Initiates the enumeration of the cache groups in the Internet cache

  61:         [DllImport(@"wininet",

  62:             SetLastError = true,

  63:             CharSet = CharSet.Auto,

  64:             EntryPoint = "FindFirstUrlCacheGroup",

  65:             CallingConvention = CallingConvention.StdCall)]

  66:         public static extern IntPtr FindFirstUrlCacheGroup(

  67:             int dwFlags,

  68:             int dwFilter,

  69:             IntPtr lpSearchCondition,

  70:             int dwSearchCondition,

  71:             ref long lpGroupId,

  72:             IntPtr lpReserved);

73:

  74:         // For PInvoke: Retrieves the next cache group in a cache group enumeration

  75:         [DllImport(@"wininet",

  76:             SetLastError = true,

  77:             CharSet = CharSet.Auto,

  78:             EntryPoint = "FindNextUrlCacheGroup",

  79:             CallingConvention = CallingConvention.StdCall)]

  80:         public static extern bool FindNextUrlCacheGroup(

  81:             IntPtr hFind,

  82:             ref long lpGroupId,

  83:             IntPtr lpReserved);

84:

  85:         // For PInvoke: Releases the specified GROUPID and any associated state in the cache index file

  86:         [DllImport(@"wininet",

  87:             SetLastError = true,

  88:             CharSet = CharSet.Auto,

  89:             EntryPoint = "DeleteUrlCacheGroup",

  90:             CallingConvention = CallingConvention.StdCall)]

  91:         public static extern bool DeleteUrlCacheGroup(

  92:             long GroupId,

  93:             int dwFlags,

  94:             IntPtr lpReserved);

95:

  96:         // For PInvoke: Begins the enumeration of the Internet cache

  97:         [DllImport(@"wininet",

  98:             SetLastError = true,

  99:             CharSet = CharSet.Auto,

 100:             EntryPoint = "FindFirstUrlCacheEntryA",

 101:             CallingConvention = CallingConvention.StdCall)]

 102:         public static extern IntPtr FindFirstUrlCacheEntry(

 103:             [MarshalAs(UnmanagedType.LPTStr)] string lpszUrlSearchPattern,

 104:             IntPtr lpFirstCacheEntryInfo,

 105:             ref int lpdwFirstCacheEntryInfoBufferSize);

 106:

 107:         // For PInvoke: Retrieves the next entry in the Internet cache

 108:         [DllImport(@"wininet",

 109:             SetLastError = true,

 110:             CharSet = CharSet.Auto,

 111:             EntryPoint = "FindNextUrlCacheEntryA",

 112:             CallingConvention = CallingConvention.StdCall)]

 113:         public static extern bool FindNextUrlCacheEntry(

 114:             IntPtr hFind,

 115:             IntPtr lpNextCacheEntryInfo,

 116:             ref int lpdwNextCacheEntryInfoBufferSize);

 117:

 118:         // For PInvoke: Removes the file that is associated with the source name from the cache, if the file exists

 119:         [DllImport(@"wininet",

 120:             SetLastError = true,

 121:             CharSet = CharSet.Auto,

 122:             EntryPoint = "DeleteUrlCacheEntryA",

 123:             CallingConvention = CallingConvention.StdCall)]

 124:         public static extern bool DeleteUrlCacheEntry(

 125:             IntPtr lpszUrlName);

 126:         #endregion

 127:

 128:         #region Public Static Functions

 129:

 130:         /// <summary>

 131:         /// Clears the cache of the web browser

 132:         /// </summary>

 133:         public static void ClearCache()

 134:         {

 135:             // Indicates that all of the cache groups in the user's system should be enumerated

 136:             const int CACHEGROUP_SEARCH_ALL = 0x0;

 137:             // Indicates that all the cache entries that are associated with the cache group

 138:             // should be deleted, unless the entry belongs to another cache group.

 139:             const int CACHEGROUP_FLAG_FLUSHURL_ONDELETE = 0x2;

 140:             // File not found.

 141:             const int ERROR_FILE_NOT_FOUND = 0x2;

 142:             // No more items have been found.

 143:             const int ERROR_NO_MORE_ITEMS = 259;

 144:             // Pointer to a GROUPID variable

 145:             long groupId = 0;

 146:

 147:             // Local variables

 148:             int cacheEntryInfoBufferSizeInitial = 0;

 149:             int cacheEntryInfoBufferSize = 0;

 150:             IntPtr cacheEntryInfoBuffer = IntPtr.Zero;

 151:             INTERNET_CACHE_ENTRY_INFOA internetCacheEntry;

 152:             IntPtr enumHandle = IntPtr.Zero;

 153:             bool returnValue = false;

 154:

 155:             // Delete the groups first.

 156:             // Groups may not always exist on the system.

 157:             // For more information, visit the following Microsoft Web site:

 158:             // http://msdn.microsoft.com/library/?url=/workshop/networking/wininet/overview/cache.asp

 159:             // By default, a URL does not belong to any group. Therefore, that cache may become

 160:             // empty even when the CacheGroup APIs are not used because the existing URL does not belong to any group.

 161:             enumHandle = FindFirstUrlCacheGroup(0, CACHEGROUP_SEARCH_ALL, IntPtr.Zero, 0, ref groupId, IntPtr.Zero);

 162:             // If there are no items in the Cache, you are finished.

 163:             if (enumHandle != IntPtr.Zero && ERROR_NO_MORE_ITEMS == Marshal.GetLastWin32Error())

 164:                 return;

 165:

 166:             // Loop through Cache Group, and then delete entries.

 167:             while (true)

 168:             {

 169:                 if (ERROR_NO_MORE_ITEMS == Marshal.GetLastWin32Error() || ERROR_FILE_NOT_FOUND == Marshal.GetLastWin32Error()) { break; }

 170:                 // Delete a particular Cache Group.

 171:                 returnValue = DeleteUrlCacheGroup(groupId, CACHEGROUP_FLAG_FLUSHURL_ONDELETE, IntPtr.Zero);

 172:                 if (!returnValue && ERROR_FILE_NOT_FOUND == Marshal.GetLastWin32Error())

 173:                 {

 174:                     returnValue = FindNextUrlCacheGroup(enumHandle, ref groupId, IntPtr.Zero);

 175:                 }

 176:

 177:                 if (!returnValue && (ERROR_NO_MORE_ITEMS == Marshal.GetLastWin32Error() || ERROR_FILE_NOT_FOUND == Marshal.GetLastWin32Error()))

 178:                     break;

 179:             }

 180:

 181:             // Start to delete URLs that do not belong to any group.

 182:             enumHandle = FindFirstUrlCacheEntry(null, IntPtr.Zero, ref cacheEntryInfoBufferSizeInitial);

 183:             if (enumHandle != IntPtr.Zero && ERROR_NO_MORE_ITEMS == Marshal.GetLastWin32Error())

 184:                 return;

 185:

 186:             cacheEntryInfoBufferSize = cacheEntryInfoBufferSizeInitial;

 187:             cacheEntryInfoBuffer = Marshal.AllocHGlobal(cacheEntryInfoBufferSize);

 188:             enumHandle = FindFirstUrlCacheEntry(null, cacheEntryInfoBuffer, ref cacheEntryInfoBufferSizeInitial);

 189:

 190:             while (true)

 191:             {

 192:                 internetCacheEntry = (INTERNET_CACHE_ENTRY_INFOA)Marshal.PtrToStructure(cacheEntryInfoBuffer, typeof(INTERNET_CACHE_ENTRY_INFOA));

 193:                 if (ERROR_NO_MORE_ITEMS == Marshal.GetLastWin32Error()) { break; }

 194:

 195:                 cacheEntryInfoBufferSizeInitial = cacheEntryInfoBufferSize;

 196:                 returnValue = DeleteUrlCacheEntry(internetCacheEntry.lpszSourceUrlName);

 197:                 if (!returnValue)

 198:                 {

 199:                     returnValue = FindNextUrlCacheEntry(enumHandle, cacheEntryInfoBuffer, ref cacheEntryInfoBufferSizeInitial);

 200:                 }

 201:                 if (!returnValue && ERROR_NO_MORE_ITEMS == Marshal.GetLastWin32Error())

 202:                 {

 203:                     break;

 204:                 }

 205:                 if (!returnValue && cacheEntryInfoBufferSizeInitial > cacheEntryInfoBufferSize)

 206:                 {

 207:                     cacheEntryInfoBufferSize = cacheEntryInfoBufferSizeInitial;

 208:                     cacheEntryInfoBuffer = Marshal.ReAllocHGlobal(cacheEntryInfoBuffer, (IntPtr)cacheEntryInfoBufferSize);

 209:                     returnValue = FindNextUrlCacheEntry(enumHandle, cacheEntryInfoBuffer, ref cacheEntryInfoBufferSizeInitial);

 210:                 }

 211:             }

 212:             Marshal.FreeHGlobal(cacheEntryInfoBuffer);

 213:         }

 214:         #endregion

 215:     }

 216: }

That class right there is from the Help & Support section of MS's site and it was a pain to find. It does have a couple of modifications, but they're rather small and deal only with the case of having the cache empty already. Other than that though, there is really no change. All you need to do is call the ClearCache function and it does the rest. Anyway, I hope that this helps someone out. So try out the code, leave feedback (and probably send a thank you letter to Microsoft, because I never would have figured this one out on my own), and happy coding.

Oh, and before I forget. Left 4 Dead kicks ass... That is all...

Comments

Dmitry
February 10, 2011 3:24 AM
Code contains a bug, but code string DeleteUrlCacheEntry(internetCacheEntry.lpszSourceUrlName) is absolutly correct. Bug is contained in INTERNET_CACHE_ENTRY_INFOA structure definition. If you define this struct with [StructLayout(LayoutKind.Sequential)] attribute, see below: [StructLayout(LayoutKind.Sequential)]public struct InternetCacheEntryInfo{public UInt32 dwStructSize;public IntPtr lpszSourceUrlName;public IntPtr lpszLocalFileName;public UInt32 CacheEntryType;public UInt32 dwUseCount;public UInt32 dwHitRate;public UInt32 dwSizeLow;public UInt32 dwSizeHigh;public FILETIME LastModifiedTime;public FILETIME ExpireTime;public FILETIME LastAccessTime;public FILETIME LastSyncTime;public IntPtr lpHeaderInfo;public UInt32 dwHeaderInfoSize;public IntP

Jason
June 01, 2010 3:00 PM

The Microsoft example contains a bug that prevents the code from working on Windows Vista or Windows 7. Change this line:returnValue = DeleteUrlCacheEntry(internetCacheEntry.lpszSourceUrlName);toreturnValue = DeleteUrlCacheEntry(internetCacheEntry.lpszLocalFileName);will fix it so it works under Vista & Windows 7. This is because on Vista/Win7 the lpszSourceUrlName pointer will resolve to a null (empty) string. The Microsoft example should have used lpszLocalFileName to begin with.

Sreenivas
April 04, 2010 4:51 PM