How to allow a very large object in .net application?

By | September 8, 2017

Since .NET 1.0 the memory limit of .NET object is 2GB. This means you cannot for example create array which contains elements with more than 2GB in total. If try to create such array, you will get the OutOfMemoryException.
Let’s see an example how to produce OutOfMemoryException.

Before that Open Visual Studio, and create C# Console Application. Lets create simple struct with two double members like example below:

public struct ComplexNumber {
public double Re;
public double Im;

public ComplexNumber(double re, double im) {
Re = re;
Im = im;
}
}

As we know this structure consumes about 16 bytes of memory. So if we want to create array of this type which consume more than 2GB we need to create array at least with 134217728 instances. So this sample program below creates 130000000 (about 1,97 GB) of array.

int maxCount = 130000000;
ComplexNumber[] arr = null;
try {
arr = new ComplexNumber[maxCount];
} catch (Exception ex) {
Console.WriteLine(ex.Message);
}

So if we run this sample program the output will be like picture below:

Let’s try to create 140000000 instances. This is about 2.08 GB. The output produces exception:

The new feature of .NET 4.5 allows to create object beyond this magical number. To enable creation of object more than 2 GB, you only need to set proper variable in configuration file. The new gcAllowVeryLargeObjects
Element enables creation of array that are greater than 2 gigabytes (GB) in total size on 64 bit platform. So the following configuration file allows us to create more that 140000000 instances of our sample.

Now,If we run our sample the output is:

Ok how about to create more than 3GB of instances. For that propose, we need to create at least 201326592 instances. If we include this number in our sample the output is:

Be careful with this feature, because it will suck all your computer memory.

This feature works only with 64bit OS, it will not work with 32 bit.

Before you use this feature please see official description, because it contains some limitation which you must know before you use.

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