PartialViewResult不鸟_ViewStart.cshtml

概述

　　在ASP.NET MVC中，对于Action中得到的ActionResult如果是一个ViewResult对象，那么在进行View呈现时，则会先执行_ViewStart.cshtml，然后再去执行被请求的视图页，但是如果在控制器的Action中得到的ActionResult是一个PartialViewResult对象，那么在进行View呈现时，则不会执行_ViewStart.cshtml。

举例验证

例如：控制器代码如下：

public class HomeController : Controller
{
    public ActionResult Index()
    {
        return View("Index");
    }
 
    public ActionResult PartView()
    {
        return PartialView("Index");
    }
}

⑴ 如果视图中设置如下，即：将 Layout = "~/Views/Shared/_Layout.cshtml";定义在Index.cshtml中。

则ViewResult和PartialViewResult的输出结果相同：

⑵ 如果视图设置如下，即：将 Layout = "~/Views/Shared/_Layout.cshtml";定义在_ViewStart.cshtml中。

则ViewResult和PartialViewResult的输出结果不同：

　　以上的实例，我们可见断定，对于PartialViewResult对象进行View呈现时，不会执行 “_ViewStart.cshtml”（因为第二种情况下没有执行母板页中的代码），为了更加具有说服力，我们再来看看ASP.NET MVC源代码，并对比ViewResult和PartialViewResult来检查我们的猜想是否正确！！

源码验证

　　由于对于ASP.NET MVC来说，进行View呈现的入口是执行ActionResult的ExecuteResult方法，而ViewResult和PartialViewResult都是继承自ViewResultBase类，在ViewResultBase中定义了ExecuteResult 方法！

public abstract class ViewResultBase : ActionResult
{
    //省略其他代码...
     
    public override void ExecuteResult(ControllerContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException("context");
        }
        if (string.IsNullOrEmpty(this.ViewName))
        {
            this.ViewName = context.RouteData.GetRequiredString("action");
        }
        ViewEngineResult viewEngineResult = null;
        if (this.View == null)
        {
            //执行FindView方法(在派生类中实现)，通过视图引擎来创建视图对象！
            viewEngineResult = this.FindView(context);
            this.View = viewEngineResult.View;
        }
        TextWriter output = context.HttpContext.Response.Output;
        ViewContext viewContext = new ViewContext(context, this.View, this.ViewData, this.TempData, output);
        this.View.Render(viewContext, output);
        if (viewEngineResult != null)
        {
            viewEngineResult.ViewEngine.ReleaseView(context, this.View);
        }
    }
    //该方法在派生类ViewResult和PartialViewResult中实现
    protected abstract ViewEngineResult FindView(ControllerContext context);
}

　　上图可以看出，在ViewResult和PartialViewResult的FindView方法中，分别通过base.ViewEngineCollection的FindView和FindPartialView来创建ViewEngineResult对象(用于封装当前请求的视图对象和视图引擎对象)，我们知道base.ViewEngineCollection其实就是一个视图引擎集合(默认情况下有：RazorViewEngine、WebFormViewEngine)，而视图引擎集合的FindView和FindPartialView方法，本质上就是遍历执行每个视图引擎的FindView和FindPartialView方法。

　　由于我们使用的是Razor引擎，所有就以RazorViewEngine为例来介绍：

this.CreatePartialView(...)和this.CreateView(...)方法都实现在派生类中！

　　上图中，我们可以看出在创建RazorView对象时，ViewResult和PartialViewResult的区别在于参数：runViewStartPages，正式由于它，决定了在之后执行进行视图页处理时，也就是执行RazorView对象的Render(viewContext, output)方法时，是否执行“_ViewStar.cshtml”。

public abstract class BuildManagerCompiledView : IView
{
    //省略其他代码...
     
    public void Render(ViewContext viewContext, TextWriter writer)
    {
        if (viewContext == null)
        {
            throw new ArgumentNullException("viewContext");
        }
        object obj = null;
        Type compiledType = this.BuildManager.GetCompiledType(this.ViewPath);
        if (compiledType != null)
        {
            obj = this._viewPageActivator.Create(this._controllerContext, compiledType);
        }
        if (obj == null)
        {
            throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, MvcResources.CshtmlView_ViewCouldNotBeCreated, new object[]
            {
                this.ViewPath
            }));
        }
        this.RenderView(viewContext, writer, obj);
    }
    //实现在派生类中
    protected abstract void RenderView(ViewContext viewContext, TextWriter writer, object instance);
}
 
public class RazorView : BuildManagerCompiledView
{
    //省略其他代码...
     
    protected override void RenderView(ViewContext viewContext, TextWriter writer, object instance)
    {
        if (writer == null)
        {
            throw new ArgumentNullException("writer");
        }
        WebViewPage webViewPage = instance as WebViewPage;
        if (webViewPage == null)
        {
            throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, MvcResources.CshtmlView_WrongViewBase, new object[]
            {
                base.ViewPath
            }));
        }
        webViewPage.OverridenLayoutPath = this.LayoutPath;
        webViewPage.VirtualPath = base.ViewPath;
        webViewPage.ViewContext = viewContext;
        webViewPage.ViewData = viewContext.ViewData;
        webViewPage.InitHelpers();
        if (this.VirtualPathFactory != null)
        {
            webViewPage.VirtualPathFactory = this.VirtualPathFactory;
        }
        if (this.DisplayModeProvider != null)
        {
            webViewPage.DisplayModeProvider = this.DisplayModeProvider;
        }
        WebPageRenderingBase startPage = null;
        //在这里这里这里这里....
        if (this.RunViewStartPages)
        {       //执行“_ViewStart.cshtml”中内容！
            startPage = this.StartPageLookup(webViewPage, RazorViewEngine.ViewStartFileName, this.ViewStartFileExtensions);
        }
        WebPageBase arg_D3_0 = webViewPage;
        HttpContextBase httpContext = viewContext.HttpContext;
        WebPageRenderingBase page = null;
        object model = null;
        arg_D3_0.ExecutePageHierarchy(new WebPageContext(httpContext, page, model), writer, startPage);
    }
}