struct - what is it for?

Question

I know something about struct type. But I can\'t understand: what is it for? when have I use it? Classes, simple value-types and enums - that\'s all that I need

Answer 1

You choose a struct if you want value-type semantics. You choose a class if you want reference-type semantics. All other concerns are secondary to this one.

Answer 2

If you don't know why you need it, you probably don't.

struct is a value type rather than a reference type. If you don't know what that means, then you probably aren't going to need it.

Answer 3

In reality, I think struct is an legacy from C. I do not think we MUST use it in any condition. Perhaps sometime you feel that leaving something on stack rather than on heap is more efficient; but as Java/C# never takes efficient as its first stand, so just neglect it:) That's my opinion.

Answer 4

Things that should be a struct (because they are values):

• struct Color
• struct Point
• struct Rectangle
• struct GLVertex (contains location, color, normal and texcoord)
• struct DateTime

Things that should be a class (because they are things to which you refer):

• class RandomGenerator
• class Socket
• class Thread
• class Window

Why? Consider the following code.

class Button
{
    public Point Location { get; set; }
}

class Program
{
    public static void Main()
    {
        var button = Util.GetButtonFromSomewhere();
        var location = button.Location;
        Util.DrawText("one", location);
        location.Y += 50;
        Util.DrawText("two", location);
        location.Y += 50;
        Util.DrawText("three", location);
    }
}

This will draw 3 text labels, vertically aligned. But if Point is a class, this will also move the button, which is really unexpected: var location = button.Location feels like it should copy a value, and not a reference! In other words, we think of Point as a value type and not a reference type. "value" here is used in the mathematical sense of value. Consider the number 5, it's an abstract object "somewhere out there", you just "refer" to it. Similarly, a Point simply is. It doesn't reside anywhere, we can't change it. Therefore we choose to make it a struct, so it has the semantics users expect.

On the other hand, we could have class Button { public Window Parent { get; set; } }. Here, Parent is an entity, so we represent it with a reference type - Window. It may make sense to use code like myButton.Parent.Redraw();. So Window should be a class.

So far so good. But all this probably sounds too vague to you. How do you really decide if something "feels" like a reference or a value? My rule of thumb is simple:

What should Foo a = b; a.Mutate(); do?

• If it seems like it should leave b unchanged, make Foo a struct.
• Otherwise make it a class.

Use the principle of least surprise here.

Answer 5

Example: Say you want a data type to represent Coordinates at X Y Z. You don't really need any functionality, only three variables. A struct would be good for this, a class may be overkill.

Answer 6

MSDN provdies a guide : Choosing Between Classes and Structures:

Consider defining a structure instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

Do not define a structure unless the type has all of the following characteristics:

• It logically represents a single value, similar to primitive types (integer, double, > and so on).
• It has an instance size smaller than 16 bytes.
• It is immutable.
• It will not have to be boxed frequently.