A transient dynamic assembly that you generate at run time by using the classes in the System.Reflection.Emit namespace, subject to certain restrictions. When you have released all references to all instances of the types in the collectible assembly and to all objects that were used to create the assembly, the common language runtime unloads the assembly from memory without unloading the application domain. This is the only circumstance in which an assembly can be unloaded without unloading the application domain.