.net程序集標示與綁定上下文

2019-11-17 02:05:14

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.net程序集標示與綁定上下文

之前在實現Autofac掃描自加載程序集實現IOC時候遇到程序集依賴的問題，在網上搜了一下，沒有發現中文世界的相關描述。隨取google拿了幾篇文章，翻譯&自己的理解，之后會寫一些小demo，如下：

相關原文：

http://stackoverflow.com/questions/1477843/difference-between-loadfile-and-loadfrom-with-net-assemblies

http://blogs.msdn.com/b/suzcook/archive/2003/07/21/57232.aspx

http://blogs.msdn.com/b/suzcook/archive/2003/05/29/57143.aspx

1、程序集標示

程序集加載有兩種標示程序集的策略：綁定時和綁定后。標示用來探測一個程序集是否和另一程序集完全相同，或者是否是另外一個程序集所要引用的。

綁定時：用程序集名稱來探測是否相同。對非強命名程序集會忽略版本號；可以不用詳細指明程序集全名，但是建議總是給出全名，否則就是自己找麻煩。

綁定后即已經加載的程序集：程序集的manifest文件路徑是用來探測的標示。對于兩個相同的程序集，如果他們的加載路徑不同，即使他們內部有完全相同的類型，也不能相互轉換。

對于 Load(byte[]) 或者 created by Reflection Emit加載的程序集，總是被判斷為不同的程序集，即使他們從二進制上來說完全相同。

2、程序集綁定上下文

有三種綁定上下文：Load、Load From、其他：

Load context

從GAC、宿主目錄(如IIS Cache)、或者applicationBase / PRivateBinPaths加載的程序集.

Load From context

通常，用戶提供一個不能被加載到Load上下文的路徑來加載程序集, 可通過 LoadFrom(), CreateInstanceFrom(), ExecuteAssembly()等方法來加載成Load From上下文。

Neithercontext

非以上兩種，通過 Assembly.Load(byte[]) 或者Reflection Emit assemblies或者Assembly.LoadFile()加載的。

推薦使用Load上下文，當然另外兩種在某些情景下也很有用。

Title	Advantages	Disadvantages
Load	Gets the benefits of versioning and policy. Best avoids "Dll Hell."Dll Hell can break your app over time. Dependencies available in the Load context are automatically found.	Dependencies in other contexts are not available unless you subscribe to the AppDomain.AssemblyResolve event.
LoadFrom	Assemblies can be loaded from multiple paths, not just from beneath the ApplicationBase. Dependencies already loaded in this context will automatically be found. Dependencies in the same dir as the requesting LoadFrom context assembly will automatically be found.	If a Load context assembly tries to load this assembly by display name, it will fail to be found by default (e.g., when mscorlib.dll deserializes this assembly). Worse, an assembly with the same identity but at a different path could be found on the probing path, causing an InvalidCastException, MissingMethodException, or unexpected method behavior later on. If an assembly by the same identity is already loaded, you'll get that one back, even if you've specified a path to a different one. It does a FileIOPermission.Read+ PathDiscovery demand, or a WebPermission demand on the path/URL specified. The ngen image (if any) won't be used. Can't be loaded as domain-neutral. v1.0 and v1.1 CLR only: won'tbe affected bypolicy, so can't be centrally serviced.
Neither	Avoids probing costs for loading this assembly. You can load multiple assemblies with the same identity into the same appdomain. You get the bits from the locationyou specified (as opposed to LoadFrom). Starting in v2, policy/GAC overrides it, however.	Nothing can bind to this assembly unless you've subscribed to the AssemblyResolve event. Dependencies can only be loaded from the Load context or using the AssemblyResolve event. Passing this assembly to another AppDomain may become tricky (e.g., when this is a Reflection Emit assembly that hasn't been saved). The ngen image (if any) won't be used. Can't be loaded as domain-neutral. v1.0 and v1.1 CLR only: won'tbe affected bypolicy, so can't be centrally serviced.