Managed assembly loading algorithm

Managed assemblies are located and loaded with an algorithm involving various stages.

All managed assemblies except satellite assemblies and WinRT assemblies use the same algorithm.

When are managed assemblies loaded?

The most common mechanism to trigger a managed assembly load is a static assembly reference. These references are inserted by the compiler whenever code uses a type defined in another assembly. These assemblies are loaded (load-by-name) as needed by the runtime.

The direct use of specific APIs will also trigger loads:

API Description Active AssemblyLoadContext
AssemblyLoadContext.LoadFromAssemblyName Load-by-name The this instance.
AssemblyLoadContext.LoadFromAssemblyPath

AssemblyLoadContext.LoadFromNativeImagePath

Load from path. The this instance.
AssemblyLoadContext.LoadFromStream Load from object. The this instance.
Assembly.LoadFile Load from path in a new AssemblyLoadContext instance The new AssemblyLoadContext instance.
Assembly.LoadFrom Load from path in the AssemblyLoadContext.Default instance.

Adds a Resolving handler to AssemblyLoadContext.Default. The handler will load the assembly's dependencies from its directory.

The AssemblyLoadContext.Default instance.
Assembly.Load(AssemblyName)

Assembly.Load(String)

Assembly.LoadWithPartialName

Load-by-name. Inferred from caller.

Prefer AssemblyLoadContext methods.

Assembly.Load(Byte[])

Assembly.Load(Byte[], Byte[])

Load from object. Inferred from caller.

Prefer AssemblyLoadContext methods.

Type.GetType(String)

Type.GetType(String, Boolean)

Type.GetType(String, Boolean, Boolean)

Load-by-name. Inferred from caller.

Prefer Type.GetType methods with an assemblyResolver argument.

Assembly.GetType If type name describes an assembly qualified generic type, trigger a Load-by-name. Inferred from caller.

Prefer Type.GetType when using assembly qualified type names.

Activator.CreateInstance(String, String)

Activator.CreateInstance(String, String, Object[])

Activator.CreateInstance(String, String, Boolean, BindingFlags, Binder, Object[], CultureInfo, Object[])

Load-by-name. Inferred from caller.

Prefer Activator.CreateInstance methods taking a Type argument.

Algorithm

The following algorithm describes how the runtime loads a managed assembly.

  1. Determine the active AssemblyLoadContext.

  2. For the Load-by-name methods, the active AssemblyLoadContext loads the assembly. In priority order by:

  3. For the other types of loads, the active AssemblyLoadContext loads the assembly. In priority order by:

    • Checking its cache-by-name.

    • Loading from the specified path or raw assembly object.

  4. In either case, if an assembly is newly loaded, then:

  5. If the assembly is found, a reference is added as needed to the active AssemblyLoadContext instance's cache-by-name.