Will my assembly be Jit-ted multiple times if loaded into multiple appdomains?

Question

My assembly is loaded into multiple application domains. Is Jit-ting for my assembly performed once per process? Or is it once per appdomain?

Or is it something more complex altogether?

Answer 1

It is complex. This msdn article says

If an assembly is loaded as domain neutral, it means its code can be reused in another AppDomain. If the assembly is loaded in more than one AppDomain as domain bound (which is the default) each AppDomain gets its own copy of the code. This has several bad performance characteristics. First there's CPU cost. If there is a native image for the assembly, only the first AppDomain can use the native image. All other AppDomains will have to JIT-compile the code which can result in a significant CPU cost.

Next, the JIT-compiled code resides in private memory, so it cannot be shared with other processes or AppDomains. If the assembly did have an NGEN image, then the first AppDomain uses the image. All the other AppDomains have to JIT-compile the code, which means that the MSIL DLL for that assembly is also loaded. This is the worst possible scenario from a cold startup perspective because disk access for that assembly would double.

Loading the assembly as domain neutral ensures that the native image, if one exists, gets used in all AppDomains created in the application. If a native image does not exist there is still a benefit in loading assemblies as domain neutral because code gets compiled just once and then shared by all AppDomains in the application.