Why are they Structs
Value Semantics
There is no essential difference between two identical instances of these values. Any Point with coordinates, [2,3] is equal to any other point with the same coordinates, much like any two ints with similar value are equal.
This is in conformance with the design guideline:
It logically represents a single value, similar to primitive types
(integer, double, and so on).
Performance
Value types are cheaper to allocate and deallocate.
There is often requirement to create many instances of these values. Structs cost less to create, and if they are local values, they will be created on the stack, relieving pressure from the GC.
Size
Let's consider the size of these values:
Point : 8 bytes
Size: 8 bytes
Rectangle: 16 bytes
For Point and Size, their size is the same as a reference to a class instance would be in a 64-bit
system.
Quotes taken from Microsoft's guidelines: Choosing Between Classes and Structures
Why are they Mutable
These structs are fully mutable. This is done (against the guidelines) for the sake of performance, as it avoids the need to create new values for modification operations.
Regarding the OP's code example in the comments:
Point[] points = new Point[] { new Point(0,0), new Point(1,1), new Point(2,2) };
foreach (Point p in points)
{
p.X += 1;
}
The only reason this foreach fails, is because p is boxed to object in order to provide iteration, and you Cannot modify the result of an unboxing conversion, (thanks Rajeev) the iterator returns the data by value, and you would only be making changes to the copy of the value.
This works fine:
for (int i = 0; i < points.Length; i++)
{
points[i].X += 1;
}
