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I know there are approximately 1,000 questions on the C# struct. I want to iterate I understand the value semantics, the performance benefits of stackallocs, etc. My specific question stems from this msdn article on when to use a struct over a class. MSDN struct vs class C#

First, they speak to the benefit of inline data for containers, specifically arrays. One allocation, and the un-user-definable default initialization of all structs in the array. However, they also emphasize that structs should be immutable.

If I declare a mystuct[] s = new mystruct[16];, I have 16 mystructs with default values inline. If I have created a 'kosher' struct with no external mutability as recommended, how is the construct of any use to me? I doubt I intend to have an array of 0-integrals and nulls exclusively.

Do they mean immutable when functioning as a record or a property return only, ie singular data transport?

If the specific conflict between default array initialization, and recommended immutability has been broached before, please mark as a duplicate. Thanks for any insight.

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The immutability being talked about is via the struct's methods, not the value of the struct itself. An instance of a struct can always be mutated through assignment - think of the integer loop variable in a for loop; it gets changed on each pass through the loop.

But, it wouldn't make sense for the framework's System.Int32 struct (i.e., int) to have a DoubleIt method that causes the underlying integer to have its value doubled without some obvious assignment operation (like how ++i; is actually i=i+1;).

Consider the difference between string.Replace and StringBuilder.Replace. The System.String class is immutable. The Replace method on a string returns a new string instance that represents the original string after the replacement operation. StringBuilder's Replace does the replacement in place, mutating the internal object.

So, if i create an integer array:

var ar = new int[] {1, 2, 3, 4};

I can alway mutate the contents of that array (a System.Array instance, a reference type) by array assignment:

 ar[2] = 200;

What would make no sense would be to call some mutating method on that (integer) array element.

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There is no actual contradiction between the two:

after you do var s = new mystruct[16];, it's true that you have and array of 16 instances of mystruct initialized to the default value.

However, doing s[0] = new mystruct(<arguments list here>) does not mutate the struct at the first cell of the array, it replaces it with the new struct.

So the struct it self is still immutable.

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If the intended default initialization is supposed to be either for benefit of performance or convenience, but then I have to loop over every default initialized element, overriding all fields with values based on a user defined constructor, why not let me specify the values of default initialization? I can't see the benefit of two member-wise passes over the structure, even if only one allocation took place.
but you can initialize an array with your user defined data: var x = new mystruct[] { new mystruct(<arguments>), new mystruct(<arguments>), new mystruct(<arguments>) }; will create an array with 3 fully initialized instances of mystruct....
If I have a 256 element array, and I have one int in my struct I want to be -1 instead of 0, an initialization list isn't a practical solution.
@schulmaster how would you initialize a new int array?
@schulmaster no, it wouldn't. but a loop will do nicely. If you are asking what is the benefit of initializing the array "cells" when initializing the array, I can't answer that of the top of my head, I'll have to do some reading and it's approaching midnight here, so not today. I'm pretty sure there's a good reason behind it - c# was designed by some smart people.
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