舉個例子，你的項目中所有的aspx頁面均存放在Website這個目錄下，那麼通常你可以使用相對路徑~/WebSite/WebPage.aspx來找到目錄下的WebPage頁面。
當你需要從某個程序集（DLL）中加載一個page到你的頁面上來。通常情況下必須要使用Assembly.load()方法來找到它，並且很有可能需要使用到反射來確定它的類型，並且將他返回給頁面，但是當你使用VirtualFile和VirtualPathProvider的時候，這時候對於父頁面來說，它只需要去讀取這個custom file的虛擬路徑即可，具體返回的stream由你自己來定義（例如你仍然可以使用Assembly.load來獲取文件）。那麼你可以自定義它的路徑來保證路徑的一致性。
舉個例子來看看如何來實現這個功能，能夠更清晰一些。
準備工作是建一個Web應用程序，創建一個頁面WebPage和一個用戶控件WebUserControl，同理新建一個另外一個項目也包含同名的這兩個文件，當然你最好在頁面上區分它們來自本項目還是其他項目。例如本項目的寫“I am from 本項目名”，其他項目則改爲其他項目的名字。
[本示例完整源碼下載(0分)] http://download.csdn.net/detail/aa466564931/4414671
首先我們創建一個CustomFile繼承自VirtualFile，這個類的作用是加載來自其他程序集的文件並且返回給VirtualFileProvider使用：
public class CustomFile: VirtualFile
{
string path
{
get;
set;
}
public CustomFile(string virtualPath)
: base(virtualPath)
{
path = VirtualPathUtility.ToAppRelative(virtualPath);
}
/// <summary>
/// Override Open method to load resource files of assembly.
/// </summary>
/// <returns></returns>
public override System.IO.Stream Open()
{
string[] strs = path.Split('/');
string name = strs[2];
string resourceName = strs[3];
name = Path.Combine(HttpRuntime.BinDirectory, name);
Assembly assembly = Assembly.LoadFile(name);
if (assembly != null)
{
Stream s = assembly.GetManifestResourceStream(resourceName);
return s;
}
else
{
return null;
}
}
}
接着創建一個CustomVirtualProvider類繼承自VirtualPathProvider，這裏我們必須實現以下幾個方法（Assembly是我自定義的文件名，你以爲改爲任意名稱），注意AssemblyPathExist這個方法，在使用base.FileExists判斷之前先進行我們的自定義路徑的判斷，如果存在則使用VirtualFile來取代默認的方法：
public class CustomPathProvider : VirtualPathProvider
{
public CustomPathProvider()
{
}
/// <summary>
/// Make a judgment that application find path contains specifical folder name.
/// </summary>
/// <param name="path"></param>
/// <returns></returns>
private bool AssemblyPathExist(string path)
{
string relateivePath = VirtualPathUtility.ToAppRelative(path);
return relateivePath.StartsWith("~/Assembly/", StringComparison.InvariantCultureIgnoreCase);
}
/// <summary>
/// If we can find this virtual path, return true.
/// </summary>
/// <param name="virtualPath"></param>
/// <returns></returns>
public override bool FileExists(string virtualPath)
{
if (this.AssemblyPathExist(virtualPath))
{
return true;
}
else
{
return base.FileExists(virtualPath);
}
}
/// <summary>
/// Use custom VirtualFile class to load assembly resources.
/// </summary>
/// <param name="virtualPath"></param>
/// <returns></returns>
public override VirtualFile GetFile(string virtualPath)
{
if (AssemblyPathExist(virtualPath))
{
return new CustomFile(virtualPath);
}
else
{
return base.GetFile(virtualPath);
}
}
/// <summary>
/// Return null when application use virtual file path.
/// </summary>
/// <param name="virtualPath"></param>
/// <param name="virtualPathDependencies"></param>
/// <param name="utcStart"></param>
/// <returns></returns>
public override CacheDependency GetCacheDependency(string virtualPath, IEnumerable virtualPathDependencies, DateTime utcStart)
{
if (AssemblyPathExist(virtualPath))
{
return null;
}
else
{
return base.GetCacheDependency(virtualPath, virtualPathDependencies, utcStart);
}
}
}OK, 接下來我們就可以使用我們自定義的path來加載程序集中的文件了，我們把一下代碼放在default頁面中：
注意~/WebSite/WebUserControl.ascx是本項目的文件， ~/Assembly/CSASPNETAssembly.dll/CSASPNETAssembly.WebUserControl.ascx是來自於CSASPNETAssembly.dll的文件：
public partial class Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
// Add relative web pages and user controls in assembly and this application.
DataTable tab = this.InitializeDataTable();
if (tab != null && tab.Rows.Count != 0)
{
for (int i = 0; i < tab.Rows.Count; i++)
{
Control control = Page.LoadControl(tab.Rows[i]["UserControlUrl"].ToString());
UserControl usercontrol = control as UserControl;
Page.Controls.Add(usercontrol);
HyperLink link = new HyperLink();
link.NavigateUrl = tab.Rows[i]["WebPageUrl"].ToString();
link.Text = tab.Rows[i]["WebPageText"].ToString();
Page.Controls.Add(link);
}
}
}
/// <summary>
/// Initialize a DataTable variable for storing URL and text properties.
/// </summary>
/// <returns></returns>
protected DataTable InitializeDataTable()
{
DataTable tab = new DataTable();
DataColumn userControlUrl = new DataColumn("UserControlUrl",Type.GetType("System.String"));
tab.Columns.Add(userControlUrl);
DataColumn webPageUrl = new DataColumn("WebPageUrl", Type.GetType("System.String"));
tab.Columns.Add(webPageUrl);
DataColumn webPageText = new DataColumn("WebPageText", Type.GetType("System.String"));
tab.Columns.Add(webPageText);
DataRow dr = tab.NewRow();
dr["UserControlUrl"] = "~/Assembly/CSASPNETAssembly.dll/CSASPNETAssembly.WebUserControl.ascx";
dr["WebPageUrl"] = "~/Assembly/CSASPNETAssembly.dll/CSASPNETAssembly.WebPage.aspx";
dr["WebPageText"] = "Assembly/WebPage";
DataRow drWebSite = tab.NewRow();
drWebSite["UserControlUrl"] = "~/WebSite/WebUserControl.ascx";
drWebSite["WebPageUrl"] = "~/WebSite/WebPage.aspx";
drWebSite["WebPageText"] = "WebSite/WebPage";
tab.Rows.Add(dr);
tab.Rows.Add(drWebSite);
return tab;
}
}