C# Combine Arrays: List, Array.Copy and Buffer.BlockCopy

Combine arrays. Two arrays can be merged.

We have two arrays with the same types of elements. For further processing in your program, we must combine these two arrays. This allows us to use both parts of the data in a single collection.

AddRange. This code is unlikely to fail due to off-by-one programming errors. When you have two arrays, you can use several ways to combine them. You can allocate a new array and loop to copy, or use Array.Copy or Buffer.BlockCopy.

However: These approaches use loop indexes and can cause subtle bugs. We use the List type, AddRange and ToArray to get the combined array.

AddRangeToArray

C# program that combines two arrays

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
	// ::: Declare two integer arrays with five elements each
	int[] array1 = { 1, 2, 3, 4, 5 };
	int[] array2 = { 6, 7, 8, 9, 10 };

	// ::: Loop through the two arrays and print them
	foreach (int element in array1)
	{
	    Console.WriteLine(element);
	}
	foreach (int element in array2)
	{
	    Console.WriteLine(element);
	}

	// ::: Create new List of integers and call AddRange twice
	var list = new List<int>();
	list.AddRange(array1);
	list.AddRange(array2);

	// ::: Call ToArray to convert List to array
	int[] array3 = list.ToArray();

	// ::: Loop through array elements of combined array and print them
	foreach (int element in array3)
	{
	    Console.WriteLine(element);
	}
    }
}

Output

1       (array1 start) (array3 start)
2
3
4
5
6       (array2 start)
7
8
9
10

In Main, we see two array declarations for array1 and array2 at the start of the method body. The two arrays both contain five integers. The code here could be changed to use string arrays or any other type.

Next, the program uses the List type to provide a level of abstraction over the actual copying to the new array. The AddRange method invocations will actually loop through the array1 and array2 elements and copy them into new memory.

List

Internals. Here we discuss the internal implementation of the AddRange method. The AddRange method internally calls InsertRange. This validates the arguments before proceeding. It uses the ICollection generic interface to access the elements.

And: It calls the fast Array.Copy method to do a bitwise copy. If you call Array.Copy manually, you could improve performance.

Performance. The List technique has overhead related to casting and parameter validation. You can directly call Array.Copy for good performance, and sometimes even call Buffer.BlockCopy to perform a lightning-fast bitwise copy of the element values.

Array.Resize

Array.Copy. This example program shows how to use Array.Copy to combine arrays. This is more efficient than the List approach. It only requires a new array. The code becomes more complex with multiple arrays.

Array.Copy

Note: This pattern of code is the same as that for the Buffer.BlockCopy method shown next: it just uses element counts, not byte counts.

C# program that uses Array.Copy

using System;

class Program
{
    static void Main()
    {
	int[] values1 = { 4, 4, 4 };
	int[] values2 = { 5, 5 };

	int[] all = new int[values1.Length + values2.Length];
	Array.Copy(values1, all, values1.Length);
	Array.Copy(values2, 0, all, values1.Length, values2.Length);

	foreach (int value in all)
	{
	    Console.WriteLine(value);
	}
    }
}

Output

4
4
4
5
5

BlockCopy. Next we use Buffer.BlockCopy to merge two int arrays. This method acts upon bytes, not elements (which are four bytes here). We must multiply the element counts by the sizeof(int) to get correct units.

Buffer

Here: We merge two 3-element int arrays. We BlockCopy the first into the "final" array, and then BlockCopy the second.

Tip: This version would be the fastest one according to my previous benchmarks of BlockCopy.

C# program that uses BlockCopy

using System;

class Program
{
    static void Main()
    {
	// ... Two input arrays.
	int[] array = { 1, 2, 3 };
	int[] array2 = { 4, 5, 6 };

	// ... Destination array.
	int[] final = new int[array.Length + array2.Length];

	// ... Copy first array.
	Buffer.BlockCopy(array,
	    0,
	    final,
	    0,
	    array.Length * sizeof(int));

	// ... Copy second.
	//     Note the starting offset.
	Buffer.BlockCopy(array2,
	    0,
	    final,
	    array.Length * sizeof(int),
	    array2.Length * sizeof(int));

	// ... Display.
	foreach (int value in final)
	{
	    Console.WriteLine(value);
	}
    }
}

Output

1
2
3
4
5
6

Summary. We combined two arrays using a useful method (AddRange) from the List type. This method will also work with more than two arrays. The arrays must all have the same type of elements. We also saw two alternative methods.

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