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Redirecting Assembly Loads at Runtime

Posted on Wednesday, December 25, 2013

.Net’s assembly resolver (Fusion) takes assembly names (eg, MyCompany.MyProduct, Version=, , Culture=neutral, PublicKeyToken=3bf5f017df1a30a5) and resolves them to actual DLL files, searching in the Global Assembly Cache, the folder containing the entry assembly, and any additional PrivateBinPaths registered for the AppDomain.

You can change the way it resolves specific assemblies using <bindingRedirect> tags in your App.config (or Web.config) file, giving it a different name to use instead if it tries to resolve a specific range of versions for an assembly. This is useful if you want to support references to multiple versions of the same assembly (eg, from older plugins), but only want to use a single version at runtime.

However, these redirects must be specified statically in the config file, and cannot be changed at runtime. If you don’t control the config file (eg, if you’re writing a plugin), or if you don’t know what redirects you will want until runtime, this doesn’t help.

Instead, you can handle the AppDomain.AssemblyResolve event, which lets you run your own code to load an assembly if the loader fails to find it.

This event is meant to be used if you put your assemblies somewhere where the loader can’t find them (eg, as embedded resources in your EXE for a single-file application, in a subfolder, or even over HTTP). However, you can also use this to catch loads of older versions of an assembly and load the newest version instead.

There are a few traps to beware of when writing AssemblyResolve handlers:

I wrote code to handle AssemblyResolve and simulate a single binding redirect:

///<summary>Adds an AssemblyResolve handler to redirect all attempts to load a specific assembly name to the specified version.</summary>
public static void RedirectAssembly(string shortName, Version targetVersion, string publicKeyToken) {
	ResolveEventHandler handler = null;

	handler = (sender, args) => {
		// Use latest strong name & version when trying to load SDK assemblies
		var requestedAssembly = new AssemblyName(args.Name);
		if (requestedAssembly.Name != shortName)
			return null;

		Debug.WriteLine("Redirecting assembly load of " + args.Name
					  + ",\tloaded by " + (args.RequestingAssembly == null ? "(unknown)" : args.RequestingAssembly.FullName));

		requestedAssembly.Version = targetVersion;
		requestedAssembly.SetPublicKeyToken(new AssemblyName("x, PublicKeyToken=" + publicKeyToken).GetPublicKeyToken());
		requestedAssembly.CultureInfo = CultureInfo.InvariantCulture;

		AppDomain.CurrentDomain.AssemblyResolve -= handler;

		return Assembly.Load(requestedAssembly);
	AppDomain.CurrentDomain.AssemblyResolve += handler;

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