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c#structinitializationimmutability
schulmaster
schulmaster

Reputation: 443

C# struct best practices, possible contradiction?

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.

Upvotes: 1

Views: 1667

Answers (2)

Flydog57
Flydog57

Reputation: 7111

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.

Upvotes: 1

Zohar Peled
Zohar Peled

Reputation: 82474

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.

Upvotes: 1

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