dotnet-exception-handling

.NET Exception Handling – System.OutOfMemoryException

Taking the next glorious step down the shining path of our .NET Exception Handling series, today we’ll be looking over the amazing System.OutOfMemoryException. As the name implies, the System.OutOfMemoryException typically occurs when the common language runtime (CLR) is unable to allocate enough memory that would be necessary to perform the current operation.

We’ll spend this article seeing exactly where the System.OutOfMemoryException resides within the .NET exception hierarchy, while also examining a trio of possible causes that could present a System.OutOfMemoryException in your own code. Let the adventure begin!

The Technical Rundown

When Should You Use It?

In spite of the name, the most likely cause of a System.OutOfMemoryException is not technically due to a lack of memory. Instead, a System.OutOfMemoryException can occur when attempting to increase the length of an instance of the StringBuilder class, beyond what is specified by its current MaxCapacity property.

To illustrate, here we have some simple code that generates a new StringBuilder instance called builder:

As indicated by the comments, we’re using a particular override of StringBuilder, in this case the StringBuilder(Int32, Int32) override, which defines the capacity and MaxCapacity property during initialization. In this case, both are set to 3, the length of our firstName string.

We then .Append that initial value to our builder, after which we attempt to .Insert our second value at the end of the existing string index. However, because we’ve already set the MaxCapacity value to 3, and we’ve appended 3 characters, we’ve used up all allocated memory for our StringBuilder instance. Thus, our .Insert attempt throws a System.OutOfMemoryException:

In this case, the issue is that we’ve told the CLR how much memory to allocate using the MaxCapacity property, which was assigned by using the StringBUilder(Int32, Int32) override. The simplest solution is to use a different override, one that doesn’t assign the MaxCapacity property. This will cause the default value to be set, which is Int32.MaxValue (i.e. roughly 2.15 billion).

Another potential cause of a System.OutOfMemoryException is, of course, actually running out of memory during execution. This could be due to repeatedly concatenating large strings, executing as a 32-bit process (which can only allocate a maximum of 2GB of memory), or attempting to retain massive data sets in memory during execution. We’ll use the latter issue in our example snippet below:

This code serves no real functional purpose, but instead just illustrates one possible way of manipulating a huge data set within memory, without using any form of chunking to reduce the allocated memory footprint of the application. In this case, we’re just looping some 200 million times and adding a random number to our list of Doubles every time. Every 10 million loops we also output our current total.

The result is that, eventually, the system cannot handle the amount of memory being used, so a System.OutOfMemoryException is thrown:

The final snippet we’ll look at today is taken from the official documentation. However, this code isn’t producing a System.OutOfMemoryException due to a memory issue, as with our other examples. Instead, this snippet illustrates how System.OutOfMemoryExceptions should be properly handled:

Since a System.OutOfMemoryException indicates a catastrophic error within the system, it’s recommended that anywhere a potential System.OutOfMemoryException could occur be passed to the Environment.FailFast method, which terminates the process and writes a message to the Windows Log. Sure enough, executing the snippet above generates a log entry in the Windows Log, which we can see using the Event Viewer application:

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