Update Struct in foreach loop in C#

Question

I have this code (C#):

using System.Collections.Generic;

namespace ConsoleApplication1
{
    public struct Thing
    {
        public string Name;
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<Thing> things = new List<Thing>();
            foreach (Thing t in things) //  for each file
            {
                t.Name = "xxx";
            }
        }
    }
}

It won't compile.
The error is:

Cannot modify members of 't' because it is a 'foreach iteration variable'

If I change Thing to a class rather than a struct, however, it does compile.

Please can someone explain what's going on?

Related question stackoverflow.com/questions/1538301/…

Brian Rasmussen
– Brian Rasmussen

2009-10-23 10:42:24 +00:00
Commented Oct 23, 2009 at 10:42 — Brian Rasmussen
– Brian Rasmussen, Commented Oct 23, 2009 at 10:42

Henk Holterman · Accepted Answer · 2009-10-23 10:33:51Z

12

More or less what it says, the compiler won't let you change (parts of) the looping var in a foreach.

Simply use:

for(int i = 0; i < things.Count; i+= 1) //  for each file
{
    things[i].Name = "xxx";
}

And it works when Thing is a class because then your looping var is a reference, and you only make changes to the referenced object, not to the reference itself.

answered Oct 23, 2009 at 10:33

Henk Holterman

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Comments

Johannes Rudolph · Accepted Answer · 2009-10-23 10:32:56Z

8

A struct is no reference type but a value type.

If you would have a class instead of a struct for Thing, the foreach loop would create a reference variable for you, that would point to the correct element in you list. But since it is a value type, it only operates on a copy of your Thing, which is in this case the iteration variable.

answered Oct 23, 2009 at 10:32

Johannes Rudolph

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2 Comments

Robin Day Over a year ago

Yes, it is relevant. Because it is a value type, the entire struct is the looping variable and therefore no part of it can be modified.

AJ. Over a year ago

Thanks for explaining the differene, Johannes. Much appreciated. +1

Neutrino · Accepted Answer · 2024-05-07 14:40:56Z

4

An alternate syntax that I prefer to @Henk's solution is this.

DateTime[] dates = new DateTime[10];

foreach(int index in Enumerable.Range(0, dates.Length))
{
   ref DateTime date = ref dates[index];

   // Do stuff with date.
   // ...
}

If you are doing a reasonable amount of work in the loop then not having to repeat the indexing everywhere is easier on the eye imo.

P.S. DateTime is actually a really poor example as it doesn't have any properties you can set, but you get the picture.

UPDATE

Works with arrays for me.

struct A       
{
   public string Name;
}

var list = new A[]
{
   new A { Name = "Bob" },
   new A { Name = "Bob" }
};

foreach (int index in Enumerable.Range(0, list.Length))
{
   ref A itemRef = ref list[index];
   itemRef.Name = "Bill";
}

edited May 7, 2024 at 14:40

answered Nov 29, 2017 at 15:53

Neutrino

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1 Comment

Soylent Graham Over a year ago

This doesn't seem to work with a list; Indexer access returns temporary value. 'ref' argument must be an assignable variable, field or an array element

armannvg · Accepted Answer · 2009-10-23 10:34:47Z

1

A struct is a value type but a class is a reference type. That's why it compiles when This is a class but not when it is a struct

See more: http://www.albahari.com/valuevsreftypes.aspx

answered Oct 23, 2009 at 10:34

armannvg

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