How to quickly zero out an array?

Question

I am currently doing it in a for loop, and I know in C there is the ZeroMemory API, however that doesn\'t seem to be available in C#. Nor does the somewhat equivalent Array.

Answer 1

Several people have posted answers, then deleted them, saying that in any language a for loop will be equally performant as a memset or FillMemory or whatever.

For example, a compiler might chunk it into 64-bit aligned pieces to take advantage of a 64bit zero assignment instruction, if available. It will take alignment and stuff into consideration. Memset's implementation is certainly not trivial.

one memset.asm. Also see memset-is-faster-than-simple-loop.html.

Never underestimate the infinite deviousness of compiler and standard library writers.

Answer 2

Calling the method by using dll import.Its fast and easy to use :)

 [DllImport("msvcrt.dll", EntryPoint = "memset", CallingConvention = CallingConvention.Cdecl, SetLastError = false)]
 public static extern IntPtr MemSet(IntPtr dest, int c, int byteCount);

c is the value you want to set in the memory

OR

[DllImport("kernel32.dll", EntryPoint="RtlZeroMemory")]
public unsafe static extern bool ZeroMemory(byte* destination, int length);

this only sets the given array to zero

Answer 3

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();

Answer 4

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

• C++11: array.fill(0);

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

• Java: Arrays.fill(array, value);

Answer 5

I believe this is what you're looking for I wrote this in visual basic however I'm sure you can convert it.

Imports System.Runtime.InteropServices

Module Module1

    'import Kernel32 so we can use the ZeroMemory Windows API function
    <DllImport("kernel32.dll")>
    Public Sub ZeroMemory(ByVal addr As IntPtr, ByVal size As IntPtr)

    End Sub

    Private Sub ZeroArray(array As ArrayList)
        'Iterate from 0 to the lenght of the array zeroing each item at that index
        For i As Integer = 0 To array.Count - 1
            'Pin the array item in memory
            Dim gch As GCHandle = GCHandle.Alloc((array(i)), GCHandleType.Pinned)
            'Get the memory address of the object pinned
            Dim arrayAddress As IntPtr = gch.AddrOfPinnedObject()
            'Get size of the array
            Dim arraySize As IntPtr = CType(array.Count, IntPtr)
            'Zero memory at the current index address in memory
            ZeroMemory(arrayAddress, arraySize)
            gch.Free()
        Next

    End Sub


    Sub Main()
        'Initialize ArrayList with items
        Dim strArray As New ArrayList From {
            "example one",
            "example two",
            "example three"
        }

        'Pass array as parameter to a function which will iterate through the arraylist zeroing each item in memory
        ZeroArray(strArray)

        Console.ReadLine()
    End Sub

End Module

Answer 6

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.