C# Struct instance behavior changes when captured in lambda

Question

I've got a work around for this issue, but I'm trying to figure out why it works . Basically, I'm looping through a list of structs using foreach. If I include a LINQ statement that references the current struct before I call a method of the struct, the method is unable to modify the members of the struct. This happens regardless of whether the LINQ statement is even called. I was able to work around this by assigning the value I was looking for to a variable and using that in the LINQ, but I would like to know what is causing this. Here's an example I created.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WeirdnessExample
{
    public struct RawData
    {
        private int id;

        public int ID
        {
            get{ return id;}
            set { id = value; }
        }

        public void AssignID(int newID)
        {
            id = newID;
        }
    }

    public class ProcessedData
    {
        public int ID { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<ProcessedData> processedRecords = new List<ProcessedData>();
            processedRecords.Add(new ProcessedData()
            {
                ID = 1
            });


            List<RawData> rawRecords = new List<RawData>();
            rawRecords.Add(new RawData()
            {
                ID = 2
            });


            int i = 0;
            foreach (RawData rawRec in rawRecords)
            {
                int id = rawRec.ID;
                if (i < 0 || i > 20)
                {
                    List<ProcessedData> matchingRecs = processedRecords.FindAll(mr => mr.ID == rawRec.ID);
                }

                Console.Write(String.Format("With LINQ: ID Before Assignment = {0}, ", rawRec.ID)); //2
                rawRec.AssignID(id + 8);
                Console.WriteLine(String.Format("ID After Assignment = {0}", rawRec.ID)); //2
                i++;
            }

            rawRecords = new List<RawData>();
            rawRecords.Add(new RawData()
            {
                ID = 2
            });

            i = 0;
            foreach (RawData rawRec in rawRecords)
            {
                int id = rawRec.ID;
                if (i < 0)
                {
                    List<ProcessedData> matchingRecs = processedRecords.FindAll(mr => mr.ID == id);
                }
                Console.Write(String.Format("With LINQ: ID Before Assignment = {0}, ", rawRec.ID)); //2
                rawRec.AssignID(id + 8);
                Console.WriteLine(String.Format("ID After Assignment = {0}", rawRec.ID)); //10
                i++;
            }

            Console.ReadLine();
        }
    }
}

Answer 1

Okay, I've managed to reproduce this with a rather simpler test program, as shown below, and I now understand it. Admittedly understanding it doesn't make me feel any less nauseous, but hey... Explanation after code.

using System;
using System.Collections.Generic;

struct MutableStruct
{
    public int Value { get; set; }

    public void AssignValue(int newValue)
    {
        Value = newValue;
    }
}

class Test
{
    static void Main()
    {
        var list = new List<MutableStruct>()
        {
            new MutableStruct { Value = 10 }
        };

        Console.WriteLine("Without loop variable capture");
        foreach (MutableStruct item in list)
        {
            Console.WriteLine("Before: {0}", item.Value); // 10
            item.AssignValue(30);
            Console.WriteLine("After: {0}", item.Value);  // 30
        }
        // Reset...
        list[0] = new MutableStruct { Value = 10 };

        Console.WriteLine("With loop variable capture");
        foreach (MutableStruct item in list)
        {
            Action capture = () => Console.WriteLine(item.Value);
            Console.WriteLine("Before: {0}", item.Value);  // 10
            item.AssignValue(30);
            Console.WriteLine("After: {0}", item.Value);   // Still 10!
        }
    }
}

The difference between the two loops is that in the second one, the loop variable is captured by a lambda expression. The second loop is effectively turned into something like this:

// Nested class, would actually have an unspeakable name
class CaptureHelper
{
    public MutableStruct item;

    public void Execute()
    {
        Console.WriteLine(item.Value);
    }
}

...
// Second loop in main method
foreach (MutableStruct item in list)
{
    CaptureHelper helper = new CaptureHelper();
    helper.item = item;
    Action capture = helper.Execute;

    MutableStruct tmp = helper.item;
    Console.WriteLine("Before: {0}", tmp.Value);

    tmp = helper.item;
    tmp.AssignValue(30);

    tmp = helper.item;
    Console.WriteLine("After: {0}", tmp.Value);
}

Now of course each time we copy the variable out of helper we get a fresh copy of the struct. This should normally be fine - the iteration variable is read-only, so we'd expect it not to change. However, you have a method which changes the contents of the struct, causing the unexpected behaviour.

Note that if you tried to change the property, you'd get a compile-time error:

Test.cs(37,13): error CS1654: Cannot modify members of 'item' because it is a
    'foreach iteration variable'

Lessons:

• Mutable structs are evil
• Structs which are mutated by methods are doubly evil
• Mutating a struct via a method call on an iteration variable which has been captured is triply evil to the extent of breakage

It's not 100% clear to me whether the C# compiler is behaving as per the spec here. I suspect it is. Even if it's not, I wouldn't want to suggest the team should put any effort into fixing it. Code like this is just begging to be broken in subtle ways.