What is the purpose of Ngen.exe in .NET?

On compiling your source code, it will be converted to intermediate language which is then converted to machine code. The role and purpose of Ngen.exe in this process is shown below:

Image

As shown in the diagram above, you can use Ngen.exe to convert MSIL to machine code. This machine code will then be stored in native image cache. When there is a need for restoring it, it loads it from cache and not from the hard disk. Hence it will make the execution faster and improve performance.

You can execute Ngen.exe using the following command:
ngen.exe install sampleAssembly
Here sampleAssembly is the assembly name.

C# gives you yet another provision to schedule your assembly for precompilation. This is done using the command shown below:
ngen.exe install sampleAssembly / queue: 1

This 1 refers to the priority. Priority takes the value of 1 or 2 or 3. Precompilation of assemblies queued with priority 1 and 2 will be precompiled forcefully. But if you mention the priority as 3 then the precompilation will be done only when the machine is idle.

So far you have seen how to install assemblies using Ngen.exe. Similarly you can also uninstall an assembly that is already installed using Ngen.exe. This is established using the following command:
ngen.exe uninstall sampleAssembly

But one thing interesting and most important about this utility is that, native images generated using ngen.exe cannot be deployed in the server. They have to be created at the client side. For that, you have to include the above shown commands into your setup file which will be executed at the client side.

Characteristics of Ngen.exe:

• Not any user can run this Ngen.exe. Only users who have administrative privileges can execute this executable

• Loading is comparatively faster when compared to MSIL because loading is done from cache and not from hard disk

• The images generated using Ngen.exe facilitate code sharing across processes

• After generating native image, if you perform any change in the assembly then you have to ensure that you update the image as well

• A drawback while using Ngen.exe is that it requires more time to generate and the image that is created requires considerably more memory for storage

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