維護 ASP.NET MVC 專案遇上巢狀 Layout 引用 Partial View 的情境，無法斷定執行先後順序，想必是自己觀念不清，做了以下實驗驗證，順手分享之。
假設有 ASP.NET MVC 巢狀 Layout 並混用 Partial View 結構如下：
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_Layout.cshtml
@{
System.Diagnostics.Debug.WriteLine("_Layout.cshtml");
}<!DOCTYPE html><html><head><meta charset="utf-8" /><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>@ViewBag.Title</title></head><body>
@Html.Partial("_PartView1")
@RenderBody()
</body></html>_PartView1.cshtml (_PartView2.cshtml 及 _PartView3.cshtml 做法相同，只有數字不同)
@{
System.Diagnostics.Debug.WriteLine("PartialView1");
}<div>Partial View 1</div>_NestedLayout.cshtml
@{
Layout = "~/Views/Shared/_Layout.cshtml";
System.Diagnostics.Debug.WriteLine("_NestedLayout.cshtml");
}
@Html.Partial("_PartView2")
@RenderBody()Index.cshtml
@{
Layout = "~/Views/Shared/_NestedLayout.cshtml";
System.Diagnostics.Debug.WriteLine("Index.cshtml");
}
@Html.Partial("_PartView3")<h2>Index</h2>HomeController.cs
public class HomeController : Controller
{
// GET: Home
public ActionResult Index()
{
System.Diagnostics.Debug.WriteLine("Index Action");
return View();
}
}執行結果不難預期：
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問題來了，HomeController.cs、Index.cshtml、_PartView1.cshtml、_PartView2.cshtml、_PartView3.cshtml、_NestedLayout.cshtml、_Layout.cshtml 都埋了 System.Diagnostics.Debug.WriteLine()，將以什麼順序執行？
給大家 20 秒自我測驗。
答案揭曉：
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HomeController Index Action –> Index.cshtml –> Partial View 3 –> _NestedLayout –> Partial View 2 -> _Layout –> Partial View 1
這順序不難理解，基本上就是從 HomeController.cs 開始，從 Index.cshtml、_NestedLayout.cshtml 到 _Layout.cshtml，由內而外的順序將 Razor View 轉為 HTML，生成 HTML 過程才載入 Partial View。
(原想找到官方文件證實，搜索未獲，十方大德如有知悉懇請不吝補充)
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