是否可以通过在 C# 中使用 foreach 的引用来迭代 int 数组

Question

Is it possible to iterate an int array by reference in a C# foreach loop?

I mean something like this:

 int[] tab = new int[5];
 foreach (ref int i in tab)
 {
     i=5;
 }

thanks

Answer 1

Is it possible to iterate an int array by reference in a C# foreach loop?

No, foreach loop in C# is not designed to make changes to the collection it iterates. It uses a readonly local variable that cannot be used as an assignment target.

Still you can use for loop to do it :

var list = new List<MyClass>();

for(var i = 0; i < list.Count; i++)
{
   list[i] = new MyClass();
}

Or use LINQ :

list = list.Select(e => new MyClass()).ToList(); // note that this will create a copy of list

Answer 2

Since C# 7.3, it works, but you need to cast the array as a Span.

int[] tab = new int[5];

foreach (ref int i in tab.AsSpan()) {
    i = 5;
}

Console.WriteLine(tab.Sum()); // 25

Tested on Net Core 3.1 and Net 5.

Answer 3

No, a foreach loop will not allow the loop variable to reference another variable. It will always copy the value out of the sequence and into a new variable. To do what you're looking to do you'd need to use some structure other than a foreach loop. While technically you could write your own ForEach method accepting a lambda where the parameter is a reference to an item in an array, the result...probably isn't worth it.

public delegate void ReferenceAction<T>(ref T param);
public static void ForEach<T>(T[] source, ReferenceAction<T> action)
{
    for (int i = 0; i < source.Length; i++)
        action(ref source[i]);
}

---

int[] tab = new int[5];
ForEach(tab, (ref int n) => n = 5);

There are almost certainly going to be better ways of accomplishing what you're trying to do without actually having a loop where the loop variable is a reference.

Answer 4

Yes, that is possible in the exact same syntax as proposed in the question.

From Microsoft docs: Beginning with C# 7.3, if the enumerator's Current property returns a reference return value (ref T where T is the type of the collection element), you can declare the iteration variable with the ref or ref readonly modifier.

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/foreach-in

Example:

int val = 0;
int[] tab = new int[5];

foreach (ref int i in tab) {
    i = val++;
}

foreach (ref readonly var j in tab) {
    Console.WriteLine(j);
}

Output:

0
1
2
3
4