Advanced C# Tips: Using Array Segments Instead of Copying Arrays

Even though it is not a very popular C# tool, the ArraySegment<T> is quite old and it was introduced in .NET Framework 2.0, which was released in 2005. It is available for developers to utilize in their applications to improve performance when dealing with partial arrays.

Technically, an ArraySegment is a struct, in other words, it is a value type in C#. When you instantiate an ArraySegment<T>, you are creating a lightweight object that doesn’t have the overhead of a class, such as a reference type’s heap allocation etc. The ArraySegment<T> struct internally holds three pieces of data: a reference to the original array (_array), the position where the segment starts (_offset), and the number of elements in the segment (_count). It’s this minimalistic design that allows ArraySegment<T> to be both fast and memory-efficient. It is not he same but its working principle can be compared to pointers in C/C++. A variable holds the address for the other variable.

When you use ArraySegment<T>, you do not create a new array. Instead, you’re simply pointing to an existing one while defining a specific segment into that array. This approach avoids the need for additional memory allocation, which in turn minimizes the work the garbage collector has to do, leading to better performance, particularly in memory-intensive applications. Furthermore, because ArraySegment<T> is a struct, passing it to methods doesn’t create copies of the array segment. Instead, you pass a copy of the struct itself and that is a very lightweight operation due to its small size. This makes ArraySegment<T> not only efficient for memory usage but also for passing data around within your application.

Here’s an example of how you might use ArraySegment<T>:

int[] numbers = new int[10] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

// Let's say we're interested in the segment of the array from 
// index 2 to index 5.
ArraySegment<int> segment = new ArraySegment<int>(numbers, 2, 4);

// We can now work with the segment.
for (int i = segment.Offset; i < segment.Offset + segment.Count; i++)
{
    Console.WriteLine(numbers[i]); // This will print numbers 2, 3, 4, 5
}

In this piece of code, instead of creating a new array for the subset, we created an ArraySegment pointing to the portion of the numbers array that we’re interested in. This is more efficient because no new array is created, and no data is copied.

Suleyman Cabir Ataman, PhD

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