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How does C# handle arrays of struct - do I need to allocate each array element (as if it was an array of class objects)?

Example:

public struct RGBA { public byte red, green, blue, alpha; }

public RGBA [] colorBuffer = new RGBA [1024*1024];

Now is colorBuffer an array of pointers to RGBA objects, and do I have to allocate them, e.g. like this:

for (int i = 0; i < colorBuffer.Length; i++)
    colorBuffer [i] = new RGBA ();

or does colorBuffer point to a memory chunk of 4 MB in size, containing 1 MB RGBA structs?

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  • 1
    I had thought of that, but I decided to ask instead. No need to waste your time on questions you don't want to answer. ;) Commented Aug 16, 2011 at 13:43
  • Do not use arrays of structs, do you know what is boxing/unboxing? Use List<T> Commented Aug 16, 2011 at 14:23
  • @sll, you don't know what boxing/unboxing is. There will be no boxing/unboxing with a strongly typed array (as in the question)! Commented Mar 25, 2012 at 1:15

4 Answers 4

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The colorBuffer variable value will be a reference to the array object. The array object itself will be a single object, 4MB in size (4 bytes * 1024 * 1024). The array is a reference type, but each element is a value type. The element value is the RGBA value - it's not a pointer.

The array will be initialized to default(RGBA) for each element automatically, do you don't need to perform your own initialization.

If you do:

RGBA[] other = colorBuffer;

that's just copying a reference - the two variables now refer to the same array.

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3 Comments

Thanks for the insight. That leads me to the next question: I am having a const array of struct, and the structs contained the array need to be initialized to various values. The only way I found to write this down was to allocate each struct in the array:
@karx11erx: There's really no such thing as a "const array" - but if you mean you've got a static readonly variable, I suggest you write a method to create the array... that'll be easier to manage. LINQ may work for you too.
Actually I am just trying to port a simple lookup table from C++ ... btw, yeah, 4 MB memory with 1 MB elements.
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Structs are always value-type. If you create array of 1024*1024 items, the required memory is (1024*1024)*(1+1+1+1) + some_overhead_of_array.

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Arrays in C# are reference types, even though it's underlying type my be value type.

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Your heading says this. "Array of struct - array elements are reference or value type?" Is that not also a question?
You said "arrays are reference types", but I asked about the "array elements". That's quite a difference, and I made sure to express myself properly in the question heading.
@karx11erx, yeah true. Sorry I get ahead of myself sometimes. Esp when Mr Skeet is awake. Oh btw if you get a chance read C# in Depth, it goes through Reference and Values types very nicely. Have to give the man rep! ;)
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The array elements in this case are value types. They do not need to be created using new. For example, the following code will compile and execute fine:

void Main()
{
    var colorBuffer = new RGBA [10];
    for (int i = 0; i < colorBuffer.Length; i++)
    {
        colorBuffer[i].red = (byte)i;
        Console.WriteLine(colorBuffer[i].red);
    }
}

struct RGBA { public byte red, green, blue, alpha; }
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