Assembly Context Load
Assembly Context Load
Assembly Context Load
Represents the runtime's concept of a scope for binding. This class is abstract.
public ref class AssemblyLoadContext abstract
public abstract class AssemblyLoadContext
type AssemblyLoadContext = class
Public MustInherit Class AssemblyLoadContext
The AssemblyLoadContext represents a load context. Conceptually, a load context creates a scope for loading, resolving, and potentially unloading a set of assemblies.
The AssemblyLoadContext exists primarily to provide assembly binding isolation. It allows multiple versions of the same assembly to be loaded within a single process. It replaces the isolation mechanisms provided by multiple AppDomain instances in the .NET Framework.
AssemblyLoadContext does not provide any security features. All code has full permissions of the process.
Usage in the runtime
The runtime implements two assembly load contexts:
- AssemblyLoadContext.Default represents the runtime's default binding context which is used for the application main assembly and its static dependencies.
- The Assembly.LoadFile(String) method isolates the assemblies it loads by instantiating the most basic AssemblyLoadContext. It has a simplistic isolation scheme which loads each assembly in its own AssemblyLoadContext with no dependency resolution.
An application can create its own AssemblyLoadContext to create a custom solution for advanced binding scenarios. The customization focuses on defining dependency resolution mechanisms.
The AssemblyLoadContext provides two extension points to implement managed assembly resolution:
- The AssemblyLoadContext.Load(AssemblyName) provides the first chance for the AssemblyLoadContext to resolve the assembly, load it and return it. If the AssemblyLoadContext.Load(AssemblyName) returns
null, the loader will try to load the assembly into the AssemblyLoadContext.Default.
- If the AssemblyLoadContext.Default is unable to resolve the assembly, the original AssemblyLoadContext gets a second chance to resolve the assembly. The runtime raises the Resolving event.
Additionally the AssemblyLoadContext.LoadUnmanagedDll(String) virtual method allows customization of the default unmanaged assembly resolution. The default implementation returns
null, which causes the runtime search to use its default search policy which is sufficient for most scenarios.
It is not possible to load multiple versions of the runtime in a single process.
Loading multiple copies or different versions of framework assemblies can lead to unexpected and hard to diagnose behavior.
Use process boundaries with remoting and/or interprocess communication to solve this isolation problem.
The timing of assembly loading can make testing and debugging difficult. Assemblies are typically loaded without their dependencies immediately being resolved. The dependencies are loaded as they are needed:
- When code branches into a dependent assembly.
- When code loads resources.
- When code explicitly loads assemblies.
The implementation of AssemblyLoadContext.Load(AssemblyName) can add new dependencies which may need to be isolated to allow different versions to exist. The most natural implementation would place these dependencies in the default context. Careful design can isolate the new dependencies.
The binding isolation process creates multiple types with the same type name.
- This can lead to confusing error messages, for example "type Foo.Foo cannot be cast to type Foo.Foo."
- Marshalling across isolation boundaries is non-trivial. A typical solution would be to use an interface defined in an assembly which is only loaded into the default load context.
|Default Default Default Default||
Gets the default AssemblyLoadContext. The default context contains the main application assembly and its static dependencies.
|IsCollectible IsCollectible IsCollectible IsCollectible|
|Resolving Resolving Resolving Resolving||
Occurs when the resolution of an assembly fails when attempting to load into this assembly load context.
|ResolvingUnmanagedDll ResolvingUnmanagedDll ResolvingUnmanagedDll ResolvingUnmanagedDll|
|Unloading Unloading Unloading Unloading||
Occurs when the AssemblyLoadContext is unloaded .
We'd love to hear your thoughts. Choose the type you'd like to provide:
Our feedback system is built on GitHub Issues. Read more on our blog.