In ngen’ed assemblies, static field initializers don’t run until the field is first accessed

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Interesting fact that I discovered today: usually, static field initializers run whenever the parent type is first loaded. However when you ngen the assembly, apparently you can load the type and work with it – static field initializers will only be executed immediately before you actually access the field.

I’ve run into this by having a static Stopwatch timer = Stopwatch.StartNew(); in one of my types. It was supposed to measure an application’s startup time, but I noticed that after NGEN’ing the assembly the timer always returned a value close to 0.

Here’s the code to verify this behavior:

 using System;
 
class Program
{
    private static int field = Initialize("Field initializer");
 
    static void Main(string[] args)
    {
        Initialize("Main");
        field = 1; // touch the field
    }
 
    private static int Initialize(string message)
    {
        Console.WriteLine(message + ": " + DateTime.Now.Ticks.ToString());
        return 0;
    }
}

This works the same for .NET 4.0 SP1 for Debug/Release and x86/x64. I didn’t try other frameworks/OS/platforms.

Here’s the output from the console:

 E:\>NgenedStaticFieldInitializers.exe
Field initializer: 634462768999291588
Main: 634462768999321594

E:\>NgenedStaticFieldInitializers.exe
Field initializer: 634462769013544438
Main: 634462769013574444

E:\>ngen install NgenedStaticFieldInitializers.exe
Microsoft (R) CLR Native Image Generator - Version 4.0.30319.1
Copyright (c) Microsoft Corporation.  All rights reserved.
Installing assembly E:\NgenedStaticFieldInitializers.exe
1>    Compiling assembly E:\NgenedStaticFieldInitializers.exe (CLR v4.0.30319) .
..
1>NgenedStaticFieldInitializers, Version=1.0.0.0, Culture=neutral, PublicKeyToke
n=null 

E:\>NgenedStaticFieldInitializers.exe
Main: 634462769095630852
Field initializer: 634462769095660858

E:\>NgenedStaticFieldInitializers.exe
Main: 634462769108903506
Field initializer: 634462769108933512