How to quickly zero out an array?

Try Array.Clear():

Sets a range of elements in the Array to zero, to false, or to null (Nothing in Visual Basic), depending on the element type.

• C++: memset(array, 0, array_length_in_bytes);

• C++11: array.fill(0);

• C#: Array.Clear(array, startingIndex, length);

• Java: Arrays.fill(array, value);

UPDATE

Based on the benchmark regarding Array.Clear() and array[x] = default(T) performance, we can state that there are two major cases to be considered when zeroing an array:

A) There is an array that is 1..76 items long;

B) There is an array that is 77 or more items long.

So the orange line on the plot represents Array.Clear() approach.

The blue line on the plot represents array[x] = default(T) approach (iteration over the array and setting its values to default(T)).

You can write once a Helper to do this job, like this:

public static class ArrayHelper
{
    // Performance-oriented algorithm selection
    public static void SelfSetToDefaults<T>(this T[] sourceArray)
    {
        if (sourceArray.Length <= 76)
        {
            for (int i = 0; i < sourceArray.Length; i++)
            {
                sourceArray[i] = default(T);
            }
        }
        else { // 77+
             Array.Clear(
                 array: sourceArray,
                 index: 0,
                 length: sourceArray.Length);
        }
    }
}

Usage:

someArray.SelfSetToDefaults();

Array.Clear(integerArray, 0, integerArray.Length);