Best way to combine two or more byte arrays in C#

For primitive types (including bytes), use System.Buffer.BlockCopy instead of System.Array.Copy. It's faster.

I timed each of the suggested methods in a loop executed 1 million times using 3 arrays of 10 bytes each. Here are the results:

1. New Byte Array using System.Array.Copy - 0.2187556 seconds
2. New Byte Array using System.Buffer.BlockCopy - 0.1406286 seconds
3. IEnumerable<byte> using C# yield operator - 0.0781270 seconds
4. IEnumerable<byte> using LINQ's Concat<> - 0.0781270 seconds

I increased the size of each array to 100 elements and re-ran the test:

1. New Byte Array using System.Array.Copy - 0.2812554 seconds
2. New Byte Array using System.Buffer.BlockCopy - 0.2500048 seconds
3. IEnumerable<byte> using C# yield operator - 0.0625012 seconds
4. IEnumerable<byte> using LINQ's Concat<> - 0.0781265 seconds

I increased the size of each array to 1000 elements and re-ran the test:

1. New Byte Array using System.Array.Copy - 1.0781457 seconds
2. New Byte Array using System.Buffer.BlockCopy - 1.0156445 seconds
3. IEnumerable<byte> using C# yield operator - 0.0625012 seconds
4. IEnumerable<byte> using LINQ's Concat<> - 0.0781265 seconds

Finally, I increased the size of each array to 1 million elements and re-ran the test, executing each loop only 4000 times:

1. New Byte Array using System.Array.Copy - 13.4533833 seconds
2. New Byte Array using System.Buffer.BlockCopy - 13.1096267 seconds
3. IEnumerable<byte> using C# yield operator - 0 seconds
4. IEnumerable<byte> using LINQ's Concat<> - 0 seconds

So, if you need a new byte array, use

byte[] rv = new byte[a1.Length + a2.Length + a3.Length];
System.Buffer.BlockCopy(a1, 0, rv, 0, a1.Length);
System.Buffer.BlockCopy(a2, 0, rv, a1.Length, a2.Length);
System.Buffer.BlockCopy(a3, 0, rv, a1.Length + a2.Length, a3.Length);

But, if you can use an IEnumerable<byte>, DEFINITELY prefer LINQ's Concat<> method. It's only slightly slower than the C# yield operator, but is more concise and more elegant.

IEnumerable<byte> rv = a1.Concat(a2).Concat(a3);

If you have an arbitrary number of arrays and are using .NET 3.5, you can make the System.Buffer.BlockCopy solution more generic like this:

private byte[] Combine(params byte[][] arrays)
{
    byte[] rv = new byte[arrays.Sum(a => a.Length)];
    int offset = 0;
    foreach (byte[] array in arrays) {
        System.Buffer.BlockCopy(array, 0, rv, offset, array.Length);
        offset += array.Length;
    }
    return rv;
}

*Note: The above block requires you adding the following namespace at the the top for it to work.

using System.Linq;

To Jon Skeet's point regarding iteration of the subsequent data structures (byte array vs. IEnumerable<byte>), I re-ran the last timing test (1 million elements, 4000 iterations), adding a loop that iterates over the full array with each pass:

1. New Byte Array using System.Array.Copy - 78.20550510 seconds
2. New Byte Array using System.Buffer.BlockCopy - 77.89261900 seconds
3. IEnumerable<byte> using C# yield operator - 551.7150161 seconds
4. IEnumerable<byte> using LINQ's Concat<> - 448.1804799 seconds

The point is, it is VERY important to understand the efficiency of both the creation and the usage of the resulting data structure. Simply focusing on the efficiency of the creation may overlook the inefficiency associated with the usage. Kudos, Jon.

Many of the answers seem to me to be ignoring the stated requirements:

• The result should be a byte array
• It should be as efficient as possible

These two together rule out a LINQ sequence of bytes - anything with yield is going to make it impossible to get the final size without iterating through the whole sequence.

If those aren't the real requirements of course, LINQ could be a perfectly good solution (or the IList<T> implementation). However, I'll assume that Superdumbell knows what he wants.

(EDIT: I've just had another thought. There's a big semantic difference between making a copy of the arrays and reading them lazily. Consider what happens if you change the data in one of the "source" arrays after calling the Combine (or whatever) method but before using the result - with lazy evaluation, that change will be visible. With an immediate copy, it won't. Different situations will call for different behaviour - just something to be aware of.)

Here are my proposed methods - which are very similar to those contained in some of the other answers, certainly :)

public static byte[] Combine(byte[] first, byte[] second)
{
    byte[] ret = new byte[first.Length + second.Length];
    Buffer.BlockCopy(first, 0, ret, 0, first.Length);
    Buffer.BlockCopy(second, 0, ret, first.Length, second.Length);
    return ret;
}

public static byte[] Combine(byte[] first, byte[] second, byte[] third)
{
    byte[] ret = new byte[first.Length + second.Length + third.Length];
    Buffer.BlockCopy(first, 0, ret, 0, first.Length);
    Buffer.BlockCopy(second, 0, ret, first.Length, second.Length);
    Buffer.BlockCopy(third, 0, ret, first.Length + second.Length,
                     third.Length);
    return ret;
}

public static byte[] Combine(params byte[][] arrays)
{
    byte[] ret = new byte[arrays.Sum(x => x.Length)];
    int offset = 0;
    foreach (byte[] data in arrays)
    {
        Buffer.BlockCopy(data, 0, ret, offset, data.Length);
        offset += data.Length;
    }
    return ret;
}

Of course the "params" version requires creating an array of the byte arrays first, which introduces extra inefficiency.

If you simply need a new byte array, then use the following:

byte[] Combine(byte[] a1, byte[] a2, byte[] a3)
{
    byte[] ret = new byte[a1.Length + a2.Length + a3.Length];
    Array.Copy(a1, 0, ret, 0, a1.Length);
    Array.Copy(a2, 0, ret, a1.Length, a2.Length);
    Array.Copy(a3, 0, ret, a1.Length + a2.Length, a3.Length);
    return ret;
}

Alternatively, if you just need a single IEnumerable, consider using the C# 2.0 yield operator:

IEnumerable<byte> Combine(byte[] a1, byte[] a2, byte[] a3)
{
    foreach (byte b in a1)
        yield return b;
    foreach (byte b in a2)
        yield return b;
    foreach (byte b in a3)
        yield return b;
}

Can use generics to combine arrays. Following code can easily be expanded to three arrays. This way you never need to duplicate code for different type of arrays. Some of the above answers seem overly complex to me.

private static T[] CombineTwoArrays<T>(T[] a1, T[] a2)
    {
        T[] arrayCombined = new T[a1.Length + a2.Length];
        Array.Copy(a1, 0, arrayCombined, 0, a1.Length);
        Array.Copy(a2, 0, arrayCombined, a1.Length, a2.Length);
        return arrayCombined;
    }

    /// <summary>
    /// Combine two Arrays with offset and count
    /// </summary>
    /// <param name="src1"></param>
    /// <param name="offset1"></param>
    /// <param name="count1"></param>
    /// <param name="src2"></param>
    /// <param name="offset2"></param>
    /// <param name="count2"></param>
    /// <returns></returns>
    public static T[] Combine<T>(this T[] src1, int offset1, int count1, T[] src2, int offset2, int count2) 
        => Enumerable.Range(0, count1 + count2).Select(a => (a < count1) ? src1[offset1 + a] : src2[offset2 + a - count1]).ToArray();

Concat is the right answer, but for some reason a handrolled thing is getting the most votes. If you like that answer, perhaps you'd like this more general solution even more:

    IEnumerable<byte> Combine(params byte[][] arrays)
    {
        foreach (byte[] a in arrays)
            foreach (byte b in a)
                yield return b;
    }

which would let you do things like:

    byte[] c = Combine(new byte[] { 0, 1, 2 }, new byte[] { 3, 4, 5 }).ToArray();