In C#, are the values in a List<struct> boxed?

Question

Suppose I declare a generic List containing values of a struct type:

struct MyStruct {     public MyStruct(int val1, decimal val2) : this() {         Val1 = val1;         Val2 = val2;     }     public int Val1 {get; private set;}     public decimal Val2 {get; private set;} }  List<MyStruct> list; 

Does the List<> store each individual value as a boxed struct, allocated individually on the heap? Or is it smarter than that?

Answer 1

It isn't boxed, but the data will be a copy of the original, and every time you get the data out, it will copy again. This tends to make it easy to lose changes. As such, you should aim to not write mutable structs. Unless MyStruct actually represents a unit-of-measure (or similar), make it a class. And if it is a measure, make it immutable (no public settable members).

And either way, don't expose fields! Use properties ;-p

For example:

struct MyStruct {     public MyStruct(int val1, decimal val2) {         this.val1 = val1;         this.val2 = val2;     }     private readonly int val1;     private readonly decimal val2;     public int Val1 {get {return val1;}}     public decimal Val2 {get {return val2;}} } 

Or alternatively (C# 3.0):

struct MyStruct {     public MyStruct(int val1, decimal val2) : this() {         Val1 = val1;         Val2 = val2;     }     public int Val1 {get; private set;}     public decimal Val2 {get; private set;} } 

Answer 2

There is no boxing.

"No, there will be no boxing. That was one of the main design goals of Generics."

http://social.msdn.microsoft.com/Forums/en-US/csharplanguage/thread/359cf58a-f53d-478e-bc31-1507e77c9454/

"If a value type is used for type T, the compiler generates an implementation of the List<T> class specifically for that value type. That means a list element of a List<T> object does not have to be boxed before the element can be used, and after about 500 list elements are created the memory saved not boxing list elements is greater than the memory used to generate the class implementation."

http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx