Examples of Dynamic Binding in.net

Transcription

1 Examples of Dynamic Binding in.net Alex Buckley Imperial College London June 00 1 Context-locality of dependencies We produce two strongly named assemblies, Server and Server. Code in Server calls code in Server. We wish to show that if both assemblies are loaded in the second context, then a dependency from one to the other is automatically resolved. Server is built first (producing Server.dll), then Server is built (referencing Server.dll and producing Server.dll). Server.dll and Server.dll exist in separate directories, both unknown to Fusion. Listing 1: Server assembly using System. R e f l e c t i o n ; 4 [ assembly : AssemblyVersion ( ) ] [ assembly : AssemblyKeyFile ( Server. key ) ] 7 p u b l i c c l a s s Server { 8 p u b l i c void UseServer ( ) { 9 Console. WriteLine ( C a l l i n g other s e r v e r ) ; 10 Server other = new Server ( ) ; 11 other. p r i n t ( ) ; 1 } 1 } Listing : Server assembly using System. R e f l e c t i o n ; 4 [ assembly : AssemblyVersion ( ) ] [ assembly : AssemblyKeyFile ( Server. key ) ] 7 p u b l i c c l a s s Server { 8 p u b l i c void p r i n t ( ) { 9 Console. WriteLine ( I am Server ) ; 10 } 11 } The Client class in listing is placed in an assembly that has no static dependencies on the Server or Server assemblies. The LoadFrom statements place both these assemblies in the second context. The different hash codes in the output indicate that the different.dll files are distinct Assembly objects in memory. 1

2 Listing : Dependencies are context-local 1 // Dependencies are context l o c a l using System ; p u b l i c c l a s s C l i e n t { 8 Assembly a = Assembly. LoadFrom (@ c : \ s c e n a r i o 1 \ hidden1 \ Server. d l l ) ; 9 Console. WriteLine ( a+ at +a. Location ) ; 10 Console. WriteLine ( a. GetHashCode ( ) ) ; 11 1 Assembly b = Assembly. LoadFrom (@ c : \ s c e n a r i o 1 \ hidden \ Server. d l l ) ; 1 // Assembly b = Assembly. Load ( Server, Version = , Culture=neutral, PublicKeyToken =4ED1A1ADB ) ; 14 Console. WriteLine ( b+ at +b. Location ) ; 1 Console. WriteLine ( b. GetHashCode ( ) ) ; 1 17 Object o = a. C r e a t e I n s t a n c e ( Server ) ; 18 Type t = o. GetType ( ) ; 19 MethodInfo m = t. GetMethod ( UseServer ) ; 0 m. Invoke ( o, n u l l ) ; 1 } } By indirect invocation at line 0 of the client, code in the Server code is invoked. (We need to use indirect invocation to avoid a static reference to the Server assembly from the Client. Since the Client executable is in the first context, having been loaded by the operating system, it would not be able to use the Server assembly in the second context.) Now, code in the Server assembly (class Server, method UseServer) calls code in the Server assembly. The static dependency that Server has on Server is successfully resolved within the second context. No attempt to load a Server assembly takes place. Listing 4: Output of listing 1 C: \ s c e n a r i o 1 >c l i e n t Server, Version = , Culture=neutral, PublicKeyToken=fc71b7cbb8a9d at c : \ s c e n a r i o 1 \ hidden1 \ s e r v e r. d l l 4 Server, Version = , Culture=neutral, PublicKeyToken=4ed1a1adb at c : \ s c e n a r i o 1 \ hidden \ s e r v e r. d l l C a l l i n g other s e r v e r 7 I am Server Note that if we comment out lines 1 and uncomment line 1 in the Client class, then we see cross-context dependency resolution. Assuming that Server.dll is able to be found (e.g. it is in the same directory as the Client executable), it will be loaded into the first context. The code in the Server assembly, loaded in the second context, is able to call code in the Server assembly, loaded in the first context.

3 Unification This example shows that trying to bind to an arbitrary core assembly results in the bind succeeding, but to the approved core assembly. This program tries to load some unavailable versions of the mscorlib core assembly. Listing : Unification 1 // R e f e r e n c e s to m s c o r l i b are a u t o m a t i c a l l y u n i f i e d using System ; p u b l i c c l a s s C l i e n t { 8 Assembly a = Assembly. Load ( Mscorlib, Version = , Culture=neutral, PublicKeyToken=B77AC194E089 ) ; 9 Console. WriteLine ( a+ at +a. Location ) ; 10 Assembly b = Assembly. Load ( Mscorlib, Version = , Culture=neutral, PublicKeyToken=B77AC194E089 ) ; 11 Console. WriteLine ( b+ at +b. Location ) ; 1 } 1 } Running it on the CLR v , we see unification happening, so only the correctly versioned assembly is loaded. 1 C: \ s c e n a r i o >c l i e n t mscorlib, Version = , Culture=neutral, PublicKeyToken= b77ac194e089 at c : \ windows\ m i c r o s o f t. net \ framework \v \ m s c o r l i b. d l l mscorlib, Version = , Culture=neutral, PublicKeyToken= b77ac194e089 at c : \ windows\ m i c r o s o f t. net \ framework \v \ m s c o r l i b. d l l

4 Casting across assemblies This example shows that casting an object from a class in one assembly, to the same class in another assembly, fails when the assemblies have different versions. We create two assemblies, both called Server.dll, that differ only in their version number. We install both to the GAC. Listing : Server assembly v using System. R e f l e c t i o n ; 4 [ assembly : AssemblyVersion ( ) ] [ assembly : AssemblyKeyFile ( Server. key ) ] 7 p u b l i c c l a s s Server {} Listing 7: Server assembly v using System. R e f l e c t i o n ; 4 [ assembly : AssemblyVersion ( ) ] [ assembly : AssemblyKeyFile ( Server. key ) ] 7 p u b l i c c l a s s Server {} The client loads both assemblies (into the first context), then tries to cast an instance of the Server class from the v assembly, to the Server class from the v assembly. The Client executable statically references v of the Server assembly, due to the instantiation of a Server object at line 8 in listing 8. Listing 8: No casting between assembly versions 1 // A c l a s s isn t compatible with i t s e l f i f i t comes from d i f f e r e n t v e r s i o n o f the same assembly using System ; p u b l i c c l a s s C l i e n t { 8 Server s = new Server ( ) ; 9 Console. WriteLine ( S t a t i c a l l y r e f e r e n c e d Server c l a s s comes from assembly : + s. GetType ( ). Assembly. GetName ( ) + at + s. GetType ( ). Assembly. Location ) ; Assembly a = Assembly. Load ( Server, Version = , Culture=neutral, PublicKeyToken=17aa9acb191 ) ; 1 Object o = a. C r e a t e I n s t a n c e ( Server ) ; 1 Console. WriteLine ( Dynamically r e f e r e n c e d Server c l a s s comes from +o. GetType ( ). Assembly+ at +o. GetType ( ). Assembly. Location ) ; 14 1 // Try c a s t i n g from the Server c l a s s in assembly to the Server c l a s s i n assembly t r y { 17 Server s = ( Server ) o ; 18 } catch ( I nvalidcastexception ex ) { 19 Console. WriteLine ( ex ) ; 4

5 0 } 1 } } The cast fails and a InvalidCastException is thrown. Even though the Server class itself is identical in both the Server and Server assemblies, the fact that the assemblies have different versions is important. 1 C: \ s c e n a r i o >c l i e n t S t a t i c a l l y r e f e r e n c e d Server c l a s s comes from assembly : Server, Version = , Culture=neutral, PublicKeyToken=17aa9acb191 at c : \ windows \ assembly \ gac \ s e r v e r \ aa9acb191 \ s e r v e r. d l l Dynamically r e f e r e n c e d Server c l a s s comes from Server, Version = , Culture=n e u t r a l, PublicKeyToken=17aa9acb191 at c : \ windows\ assembly \ gac \ s e r v e r \ aa9acb191 \ s e r v e r. d l l 4 System. I nvalidcastexception : S p e c i f i e d c a s t i s not v a l i d. at C l i e n t. Main ( S t r i n g [ ] a r g s )

6 4 Casting across contexts This example shows that casting an object from a class in one assembly, to the same class in another assembly, fails when the assemblies are loaded in different contexts. We use the v Server assembly from scenario, and build Server.dll. We install it into the GAC, and also copy it to a directory unavailable to Fusion. Listing 9: Server assembly v using System. R e f l e c t i o n ; 4 [ assembly : AssemblyVersion ( ) ] [ assembly : AssemblyKeyFile ( Server. key ) ] 7 p u b l i c c l a s s Server {} The client references the Server class, so when we build the client executable, we must reference the hidden Server.dll that provides the class. This gives the client executable a static reference to an assembly called Server, of version v Such a reference causes the server to load the Server.dll assembly in the GAC. This assembly goes into the first context. The client also loads the hidden Server.dll via LoadFrom, so the assembly goes into the second context. The client then tries to cast an instance of the Server class from the Server.dll loaded from the filesystem, to the class from the Server.dll loaded from the GAC. Listing 10: No casting across contexts 1 // A c l a s s isn t compatible with i t s e l f i f i t comes from a s s e m b l i e s in d i f f e r e n t c o n t e x t s ( hence t h e i r paths are d i f f e r e n t ) using System ; p u b l i c c l a s s C l i e n t { 8 Server s = new Server ( ) ; 9 Console. WriteLine ( S t a t i c a l l y r e f e r e n c e d Server c l a s s comes from assembly : + s. GetType ( ). Assembly+ at + s. GetType ( ). Assembly. Location ) ; Assembly a = Assembly. LoadFrom (@ C: \ s c e n a r i o 4 \ hidden \ Server. d l l ) ; 1 Object o = a. C r e a t e I n s t a n c e ( Server ) ; 1 Console. WriteLine ( Dynamically r e f e r e n c e d Server c l a s s comes from assembly : + o. GetType ( ). Assembly+ at +o. GetType ( ). Assembly. Location ) ; 14 1 // Try c a s t i n g from the Server c l a s s in the LoadFrom context to the Server c l a s s in the Load context 1 t r y { 17 Server s = ( Server ) o ; 18 } catch ( I nvalidcastexception ex ) { 19 Console. WriteLine ( ex ) ; 0 } 1 } } Since the assemblies that declare Server classes are in different contexts, the cast fails, even though the Server classes are identical. (Indeed, the assemblies themselves are identical; only their location, and hence context, is different.)

7 1 C: \ s c e n a r i o 4 >c l i e n t S t a t i c a l l y r e f e r e n c e d Server c l a s s comes from assembly : Server, Version = , Culture=neutral, PublicKeyToken=17aa9acb191 at c : \ windows \ assembly \ gac \ s e r v e r \ aa9acb191 \ s e r v e r. d l l Dynamically r e f e r e n c e d Server c l a s s comes from assembly : Server, Version = , Culture=neutral, PublicKeyToken=17aa9acb191 at c : \ s c e n a r i o 4 \ hidden \ s e r v e r. d l l 4 System. I nvalidcastexception : S p e c i f i e d c a s t i s not v a l i d. at C l i e n t. Main ( S t r i n g [ ] a r g s ) 7

8 LoadFrom reuses assemblies This example shows that LoadFrom has the surprising behaviour that it does not always load the assembly specified. We use the v Server assembly from scenario. We copy it to two different locations, both unavailable to Fusion. The client then uses LoadFrom to attempt to load both files. Listing 11: LoadFrom reuses assemblies 1 // In the LoadFrom context, l o a d i n g the same assembly from d i f f e r e n t l o c a t i o n s r e u s e s the f i r s t one loaded using System ; p u b l i c c l a s s C l i e n t { 8 Assembly a = Assembly. LoadFrom (@ C: \ s c e n a r i o \ hidden1 \ Server. d l l ) ; 9 Console. WriteLine ( a+ at +a. Location ) ; 10 Console. WriteLine ( a. GetHashCode ( ) ) ; 11 1 Assembly b = Assembly. LoadFrom (@ C: \ s c e n a r i o \ hidden \ Server. d l l ) ; 1 Console. WriteLine ( b+ at +b. Location ) ; 14 Console. WriteLine ( b. GetHashCode ( ) ) ; 1 1 AppDomain currentdomain = AppDomain. CurrentDomain ; 17 Assembly [ ] assems = currentdomain. GetAssemblies ( ) ; 18 f o r e a c h ( Assembly assem in assems ) Console. WriteLine ( Loaded : + assem + +assem. Location ) ; 19 } 0 } However, since the assembly s strong name is identical in both locations, the second LoadFrom merely receives the same Assembly object as the first (evidenced by the same hash code, ). For good measure, the client prints out all assemblies loaded in the appdomain, regardless of context. We see that only one Server assembly is loaded, namely the first. 1 C: \ s c e n a r i o >c l i e n t Server, Version = , Culture=neutral, PublicKeyToken=17aa9acb191 at c : \ s c e n a r i o \ hidden1 \ s e r v e r. d l l 4 Server, Version = , Culture=neutral, PublicKeyToken=17aa9acb191 at c : \ s c e n a r i o \ hidden1 \ s e r v e r. d l l Loaded : mscorlib, Version = , Culture=neutral, PublicKeyToken= b77ac194e089 c : \ windows\ m i c r o s o f t. net \ framework \v \ m s c o r l i b. d l l 7 Loaded : Client, Version = , Culture=neutral, PublicKeyToken=n u l l C: \ s c e n a r i o \ C l i e n t. exe 8 Loaded : Server, Version = , Culture=neutral, PublicKeyToken=17 aa9acb191 c : \ s c e n a r i o \ hidden1 \ s e r v e r. d l l 8