Native Image Generator
Encyclopedia
The Native Image Generator, or simply NGEN is the Ahead-of-time compilation service of the .NET Framework
. It allows a .NET assembly
to be pre-compiled instead of letting the Common Language Runtime
do a Just-in-time compilation
at runtime. In some cases the execution will be significally faster with AOT than with JIT.
automatically reverts to using JIT
. Once NGEN is run against an assembly, the resulting native image is placed into the Native Image Cache for use by all other .NET assemblies. This makes it possible, for example, to use NGEN to process .NET assemblies at installation time, saving processor time later on, when the end-user invokes the application on their system.
NGEN is intended to make the assembly execute faster by removing the JIT compilation process at runtime, but this does not always improve performance because some optimizations can be done only by a JIT compiler (e.g., if the JIT compiler knows that the code is already running with full trust, it can skip certain expensive security checks). Because of this fact, it makes sense to use NGEN only after benchmarking the application performance before and after it.
.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
. It allows a .NET assembly
.NET assembly
In the .NET framework, an assembly is a compiled code library used for deployment, versioning, and security. There are two types: process assemblies and library assemblies . A process assembly represents a process that will use classes defined in library assemblies...
to be pre-compiled instead of letting the Common Language Runtime
Common Language Runtime
The Common Language Runtime is the virtual machine component of Microsoft's .NET framework and is responsible for managing the execution of .NET programs. In a process known as just-in-time compilation, the CLR compiles the intermediate language code known as CIL into the machine instructions...
do a Just-in-time compilation
Just-in-time compilation
In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...
at runtime. In some cases the execution will be significally faster with AOT than with JIT.
General information
The Native Image Generator produces a native binary image for the current environment (i.e.; operating systems). This eliminates the JIT overhead at the expense of portability and disk space; whenever an NGEN-generated image is run in an incompatible environment, .NET framework.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
automatically reverts to using JIT
Just-in-time compilation
In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...
. Once NGEN is run against an assembly, the resulting native image is placed into the Native Image Cache for use by all other .NET assemblies. This makes it possible, for example, to use NGEN to process .NET assemblies at installation time, saving processor time later on, when the end-user invokes the application on their system.
NGEN is intended to make the assembly execute faster by removing the JIT compilation process at runtime, but this does not always improve performance because some optimizations can be done only by a JIT compiler (e.g., if the JIT compiler knows that the code is already running with full trust, it can skip certain expensive security checks). Because of this fact, it makes sense to use NGEN only after benchmarking the application performance before and after it.