RTFM:
managed code
Code that is executed by the common language runtime environment rather than directly by the operating system. Managed code applications gain common language runtime services such as automatic garbage collection, runtime type checking and security support, and so on. These services provide uniform platform- and language-independent behavior of managed-code applications.

unmanaged code
Code that is executed directly by the operating system, outside the common language runtime environment. Unmanaged code must provide its own garbage collection, type checking, security support, and so on, unlike managed code, which receives these services from the common language runtime.

Native Image Generator (Ngen.exe)
The Native Image Generator creates a native image from a managed assembly and installs it into the native image cache on the local computer. The native image cache is a reserved area of the global assembly cache. Once you create a native image for an assembly, the runtime automatically uses that native image each time it runs the assembly. You do not have to perform any additional procedures to cause the runtime to use a native image. Running Ngen.exe on an assembly allows the assembly to load and execute faster, because it restores code and data structures from the native image cache rather than generating them dynamically.

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Не е важно да си добър, важното е да си ман-и-як!