This article was previously published under Q331979
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The Native Image Generator (Ngen.exe) creates a native image from a managed assembly and then installs the image into the native image cache on the local computer. When you use the images that you created with Ngen.exe from ASP.NET assemblies, the common language runtime ignores them and then reverts back to the original image for just-in-time compilation (JIT).
The common language runtime cannot load images that you create with NGEN into the shared application domain. Because ASP.NET standard assemblies are shared and are then loaded into a shared application domain, you cannot use Ngen.exe to install them into the native image cache on the local computer.