Mikel
MasterPage是ASP.NET2.0引入的一个非常实用的特性，怎么用，我想不用我说，基本上大家都会，这里要讲的是，它是如何实现的。
在深入源代码去探索MasterPage之前，我以为MasterPage的实现应该是比较复杂的，也一直纳闷为什么 MasterPage类会继承于UserControl类，感觉这两者好像差得很远。昨天晚上，我专门抽出时间，阅读了部分与MasterPage有关的 源代码，终于明白了是怎么回事，在那突然明白的那一刻，真有如醍醐灌顶，拍案叫绝，不得不佩服微软的那些guys。
下面就是我的探索之旅的过程（大家也可以跳过该部分，直接看后面的真相大白部分）：
1、我首先查看的是Page.ProcessRequestMain方法，我们知道，Page类大部分特性，包括LifeCycle、 PostBack、ProcessPostData等都是在该方法中实现，所以，我想，MasterPage的实现肯定在该方法中有不少的体现。然而，令 我惊讶的是，我居然没有在该方法中找到任何有关MasterPage的线索，而仅仅在this.PerformPreInit()中，找到唯一一个 ApplyMasterPage()方法。而该方法也出奇的简单，感觉仅仅是将递归的各级MasterPage的._masterPageApplied 字段设为true而已。当时，我忽略了一个重要的东西，就是代码中对this.Master这个属性的访问，实际上，奥秘就在对这个属性的访问上（下文将 叙述）。
private void PerformPreInit()
{
this.OnPreInit(EventArgs.Empty); this.InitializeThemes(); this.ApplyMasterPage(); this._preInitWorkComplete = true;
}
private void ApplyMasterPage(){ if (this.Master != null)
{
ArrayList appliedMasterFilePaths = new ArrayList(); appliedMasterFilePaths.Add(this._masterPageFile.VirtualPathString.ToLower(CultureInfo.InvariantCulture)); MasterPage.ApplyMasterRecursive(this.Master, appliedMasterFilePaths);
}}
2、我查看了MasterPage的源代码，出奇的是，竟也如此简单，以至于我也没有从该源代码中找到多少有价值的信息。
[ControlBuilder(typeof(MasterPageControlBuilder)), Designer("Microsoft.VisualStudio.Web.WebForms.MasterPageWebFormDesigner, Microsoft.VisualStudio.Web, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(IRootDesigner)), ParseChildren(false), AspNetHostingPermission(SecurityAction.LinkDemand, Level=AspNetHostingPermissionLevel.Minimal), AspNetHostingPermission(SecurityAction.InheritanceDemand, Level=AspNetHostingPermissionLevel.Minimal)]public class MasterPage : UserControl{ private IList _contentPlaceHolders; private IDictionary _contentTemplateCollection; private IDictionary _contentTemplates; private MasterPage _master; private bool _masterPageApplied; private VirtualPath _masterPageFile; internal TemplateControl _ownerControl; [EditorBrowsable(EditorBrowsableState.Advanced)] protected internal void AddContentTemplate(string templateName, ITemplate template) { if (this._contentTemplateCollection == null) { this._contentTemplateCollection = new Hashtable(10, StringComparer.OrdinalIgnoreCase); } try { this._contentTemplateCollection.Add(templateName, template); } catch (ArgumentException) { throw new HttpException(SR.GetString("MasterPage_Multiple_content", new object[] { templateName })); } } internal static void ApplyMasterRecursive(MasterPage master, IList appliedMasterFilePaths) { if (master.Master != null) { string str = master._masterPageFile.VirtualPathString.ToLower(CultureInfo.InvariantCulture); if (appliedMasterFilePaths.Contains(str)) { throw new InvalidOperationException(SR.GetString("MasterPage_Circular_Master_Not_Allowed", new object[] { master._masterPageFile })); } appliedMasterFilePaths.Add(str); ApplyMasterRecursive(master.Master, appliedMasterFilePaths); } master._masterPageApplied = true; } internal static MasterPage CreateMaster(TemplateControl owner, HttpContext context, VirtualPath masterPageFile, IDictionary contentTemplateCollection) { MasterPage child = null; if (masterPageFile == null) { if ((contentTemplateCollection != null) && (contentTemplateCollection.Count > 0)) { throw new HttpException(SR.GetString("Content_only_allowed_in_content_page")); } return null; } VirtualPath virtualPath = VirtualPathProvider.CombineVirtualPathsInternal(owner.TemplateControlVirtualPath, masterPageFile); ITypedWebObjectFactory vPathBuildResult = (ITypedWebObjectFactory) BuildManager.GetVPathBuildResult(context, virtualPath); if (!typeof(MasterPage).IsAssignableFrom(vPathBuildResult.InstantiatedType)) { throw new HttpException(SR.GetString("Invalid_master_base", new object[] { masterPageFile })); } child = (MasterPage) vPathBuildResult.CreateInstance(); child.TemplateControlVirtualPath = virtualPath; if (owner.HasControls()) { foreach (Control control in owner.Controls) { LiteralControl control2 = control as LiteralControl; if ((control2 == null) || (Util.FirstNonWhiteSpaceIndex(control2.Text) >= 0)) { throw new HttpException(SR.GetString("Content_allowed_in_top_level_only")); } } owner.Controls.Clear(); } if (owner.Controls.IsReadOnly) { throw new HttpException(SR.GetString("MasterPage_Cannot_ApplyTo_ReadOnly_Collection")); } if (contentTemplateCollection != null) { foreach (string str in contentTemplateCollection.Keys) { if (!child.ContentPlaceHolders.Contains(str.ToLower(CultureInfo.InvariantCulture))) { throw new HttpException(SR.GetString("MasterPage_doesnt_have_contentplaceholder", new object[] { str, masterPageFile })); } } child._contentTemplates = contentTemplateCollection; } child._ownerControl = owner; child.InitializeAsUserControl(owner.Page); owner.Controls.Add(child); return child; } [Browsable(false), EditorBrowsable(EditorBrowsableState.Advanced)] protected internal IList ContentPlaceHolders { get { if (this._contentPlaceHolders == null) { this._contentPlaceHolders = new ArrayList(); } return this._contentPlaceHolders; } } [Browsable(false), EditorBrowsable(EditorBrowsableState.Advanced)] protected internal IDictionary ContentTemplates { get { return this._contentTemplates; } } [WebSysDescription("MasterPage_MasterPage"), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden), Browsable(false)] public MasterPage Master { get { if ((this._master == null) && !this._masterPageApplied) { this._master = CreateMaster(this, this.Context, this._masterPageFile, this._contentTemplateCollection); } return this._master; } } [WebSysDescription("MasterPage_MasterPageFile"), WebCategory("Behavior"), DefaultValue("")] public string MasterPageFile { get { return VirtualPath.GetVirtualPathString(this._masterPageFile); } set { if (this._masterPageApplied) { throw new InvalidOperationException(SR.GetString("PropertySetBeforePageEvent", new object[] { "MasterPageFile", "Page_PreInit" })); } if (value != VirtualPath.GetVirtualPathString(this._masterPageFile)) { this._masterPageFile = VirtualPath.CreateAllowNull(value); if ((this._master != null) && this.Controls.Contains(this._master)) { this.Controls.Remove(this._master); } this._master = null; } } }}
3、我又查看了ContentPlaceHolder类、Content类，心想，难道奥秘在这两个控件上？打开源代码一看，彻底晕 倒，这两个类简单得简直不能让人相信，ContentPlaceHolder居然是一个空类，仅仅起到一个标识的作用，而Content居然也仅仅只有 ContentPlaceHolderID唯一一个string属性。
public class ContentPlaceHolder : Control, INonBindingContainer, INamingContainer{}
Content
4、此时，我几乎已经没法再从ASP.NET源代码中找到其他有关MasterPage的有价值的信息了。于是，我决定写一个简单的 Web应用程序，该应用程序仅仅只有一个MasterPage页与Default页，并将其编译，查看编译后的代码，看看是否能找到有价值的信息。
源代码如下：
<%@ Master Language="C#" AutoEventWireup="true" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head> <title></title></head><body> <form id="form1" runat="server"> <div> <asp:ContentPlaceHolder id="ContentPlaceHolder" runat="server"> <asp:Button runat="server" ID="master" Text="master"/> </asp:ContentPlaceHolder> </div> </form></body></html>
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" %><%@ MasterType VirtualPath="~/MasterPage.master" %><asp:Content ID="Content" ContentPlaceHolderID="ContentPlaceHolder" Runat="Server"> <asp:Button runat="server" ID="default" Text="default" /></asp:Content>
编译后代码如下：
[CompilerGlobalScope]public class masterpage_master : MasterPage{ private static bool __initialized; private ITemplate __Template_ContentPlaceHolder; protected ContentPlaceHolder ContentPlaceHolder; protected HtmlForm form1; protected Button master; [DebuggerNonUserCode] public masterpage_master() { base.AppRelativeVirtualPath = "~/MasterPage.master"; if (!__initialized) { __initialized = true; } base.ContentPlaceHolders.Add("contentplaceholder"); } [DebuggerNonUserCode] private ContentPlaceHolder __BuildControlContentPlaceHolder() { ContentPlaceHolder container = new ContentPlaceHolder(); this.ContentPlaceHolder = container; container.ID = "ContentPlaceHolder"; if (base.ContentTemplates != null) { this.__Template_ContentPlaceHolder = (ITemplate) base.ContentTemplates["ContentPlaceHolder"]; } if (this.__Template_ContentPlaceHolder != null) { this.__Template_ContentPlaceHolder.InstantiateIn(container); return container; } IParserAccessor accessor = container; accessor.AddParsedSubObject(new LiteralControl("\r\n ")); Button button = this.__BuildControlmaster(); accessor.AddParsedSubObject(button); accessor.AddParsedSubObject(new LiteralControl("\r\n ")); return container; } [DebuggerNonUserCode] private HtmlForm __BuildControlform1() { HtmlForm form = new HtmlForm(); this.form1 = form; form.ID = "form1"; IParserAccessor accessor = form; accessor.AddParsedSubObject(new LiteralControl("\r\n <div>\r\n ")); ContentPlaceHolder holder = this.__BuildControlContentPlaceHolder(); accessor.AddParsedSubObject(holder); accessor.AddParsedSubObject(new LiteralControl("\r\n </div>\r\n ")); return form; } [DebuggerNonUserCode] private Button __BuildControlmaster() { Button button = new Button(); this.master = button; button.ApplyStyleSheetSkin(this.Page); button.ID = "master"; button.Text = "master"; return button; } [DebuggerNonUserCode] private void __BuildControlTree(masterpage_master __ctrl) { IParserAccessor accessor = __ctrl; accessor.AddParsedSubObject(new LiteralControl("\r\n\r\n<!DOCTYPE html PUBLIC \"–//W3C//DTD XHTML 1.0 Transitional//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\">\r\n\r\n<html xmlns=\"http://www.w3.org/1999/xhtml\">\r\n<head>\r\n <title></title>\r\n</head>\r\n<body>\r\n ")); HtmlForm form = this.__BuildControlform1(); accessor.AddParsedSubObject(form); accessor.AddParsedSubObject(new LiteralControl("\r\n</body>\r\n</html>\r\n")); } [DebuggerNonUserCode] protected override void FrameworkInitialize() { base.FrameworkInitialize(); this.__BuildControlTree(this); } protected HttpApplication ApplicationInstance { get { return this.Context.ApplicationInstance; } } protected DefaultProfile Profile { get { return (DefaultProfile) this.Context.Profile; } } protected override bool SupportAutoEvents { get { return false; } } [TemplateContainer(typeof(MasterPage)), TemplateInstance(TemplateInstance.Single)] public virtual ITemplate Template_ContentPlaceHolder { get { return this.__Template_ContentPlaceHolder; } set { this.__Template_ContentPlaceHolder = value; } }}
[CompilerGlobalScope]public class default_aspx : Page, IRequiresSessionState, IHttpHandler{ private static object __fileDependencies; private static bool __initialized; protected Button @default; [DebuggerNonUserCode] public default_aspx() { base.AppRelativeVirtualPath = "~/Default.aspx"; if (!__initialized) { string[] virtualFileDependencies = new string[] { "~/Default.aspx", "~/MasterPage.master" }; __fileDependencies = base.GetWrappedFileDependencies(virtualFileDependencies); __initialized = true; } base.Server.ScriptTimeout = 0x1c9c380; } [DebuggerNonUserCode] private void __BuildControlContent(Control __ctrl) { IParserAccessor accessor = __ctrl; accessor.AddParsedSubObject(new LiteralControl("\r\n ")); Button button = this.__BuildControldefault(); accessor.AddParsedSubObject(button); accessor.AddParsedSubObject(new LiteralControl("\r\n")); } [DebuggerNonUserCode] private Button __BuildControldefault() { Button button = new Button(); this.@default = button; button.TemplateControl = this; button.ApplyStyleSheetSkin(this); button.ID = "default"; button.Text = "default"; return button; } [DebuggerNonUserCode] private void __BuildControlTree(default_aspx __ctrl) { __ctrl.Title = ""; __ctrl.MasterPageFile = "~/MasterPage.master"; this.InitializeCulture(); base.AddContentTemplate("ContentPlaceHolder", new CompiledTemplateBuilder(new BuildTemplateMethod(this.__BuildControlContent))); IParserAccessor accessor = __ctrl; accessor.AddParsedSubObject(new LiteralControl("\r\n\r\n")); } [DebuggerNonUserCode] protected override void FrameworkInitialize() { base.FrameworkInitialize(); this.__BuildControlTree(this); base.AddWrappedFileDependencies(__fileDependencies); base.Request.ValidateInput(); } [DebuggerNonUserCode] public override int GetTypeHashCode() { return –2002306427; } [DebuggerNonUserCode] public override void ProcessRequest(HttpContext context) { base.ProcessRequest(context); } protected HttpApplication ApplicationInstance { get { return this.Context.ApplicationInstance; } } public masterpage_master Master { get { return (masterpage_master) base.Master; } } protected DefaultProfile Profile { get { return (DefaultProfile) this.Context.Profile; } } protected override bool SupportAutoEvents { get { return false; } }}
我们首先观察default_aspx的BuildControlTree方法，该方法调用了 base.AddContentTemplate("ContentPlaceHolder", new CompiledTemplateBuilder(new BuildTemplateMethod(this.__BuildControlContent)));而该方法实质上只有一行代码，即 this._contentTemplateCollection.Add(templateName, template);其中，_contentTemplateCollection是IDictionary类型。因 此，base.AddContentTemplate只有一个功能，即在Page的_contentTemplateCollection中添加一个 CompliedTemplateBuilder类型的对象。那么该对象有啥作用呢？为了不岔开话题，这里不对该对象做详细描述，只给出结论：该对象实现 ITemplate接口，其接口方法InstantiateIn(Control container)具体实现为为container添加BuildTemplateMethod委托所创建的控件。这话有点拗口，简单地就刚才那个例子 来说，就是如果你调用该对象的InstantiateIn(Control container)方法，就为该container添加this.__BuildControlContent()方法所创建的控件做为子控件。
5、我们继续来看看masterpage_master的__BuildControlContentPlaceHolder()方 法，我们发现，该方法即调用了刚才讨论的那个InstantiateIn方法。哈，原来在这里，终于明白了，原来Page里Content控件中的所有内 容最终都将变成其对应的MasterPage中的ContentPlaceHolder的子控件。等一下，这个结论下得有点早，难道这里的 base.ContentTemplates属性就等于Page的_contentTemplateCollection字段吗？如果是，那么上面的结论 就是正确的。
6、我们重新回到1中的代码，查看Page.Master属性的实现，我们发现，它调用了MasterPage的CreateMaster静态方法，而该方法传送的其中一个参数就是._contentTemplateCollection字段。
public MasterPage Master{ get { if ((this._master == null) && !this._preInitWorkComplete) { this._master = MasterPage.CreateMaster(this, this.Context, this._masterPageFile, this._contentTemplateCollection); } return this._master; }}
我们再回头来看看MasterPage.CreateMaster方法，在倒数第6行，我们发现，该方法果然将 contentTemplateCollection赋给了MasterPage实例(child)的_contentTemplates字段。再往下 看，我们还看到了owner.Controls.Add(child)，什么意思呢？意思就是将该MasterPage实例作为普通控件加入到owner 实例的子控件集合中。而往上，我们又可以找到owner.Controls.Clear()语句，因此，该MasterPage将作为owner的唯一子 控件而存在。而该owner，常常就是Page实例。（在存在嵌套MasterPage的时候，该owner还可能是下一层次的MasterPage实 例）。
至此，真相大白，原来，MasterPage最终将作为Page的唯一子控件而存在，难怪它要继承自 UserControl，而Page中Content控件定义的各个子控件，又将作为该MasterPage的ContentPlaceHolder的子控件而存在，难 怪ContentPlaceHolder无需实现任何代码，因为它仅仅是一个容器。正因为MasterPage最终成为Page的唯一子控件，那么后来的 处理就与普通的控件没什么两样了，难怪ProcessRequestMain方法里无需为MasterPage单独编码，哈哈，一切都真相大白了。这里， 我们还发现一个比较有趣的现象，即Content控件本身却消失不见了，这应该是Asp.net解析器所做的优化，因为ContentPlaceHolder完全没必要先装上Content控件，然后再装上Content中的那些控件。
另外，从Page.Master属性与Page.MasterPageFile属性的实现上，我们也不难明白为什么MasterPageFile属性只能在 PreInit 事件中进行设置的原因。
如何证明以上所说都是正确的呢？呵呵，其实很简单，我们可以观察最终页面的控件树，就可证明上面分析是正确的。（写这篇blog时才想起看控件树，要是早想起，就能让我少走不少歪路了，唉，幸好打开控件树发现结果与预期完全一致。）
那么嵌套MasterPage是如何实现的呢？呵呵，其实也一样，即Top1MasterPage成为Top2MasterPage的 唯一子控件，Top2MasterPage成为Top3MasterPage的唯一子控件，……，直到TopNMasterPage成为Page的唯一子 控件。
最后我用两幅图来做总结。
下图为初始的控件树结构：
下图为最终的控件树结构：
