Foreach struct weird compile error in C#

Question

namespace MyNamespace
{
    public struct MyStruct
    {
        public string MyString;
        public int MyInt;
        public bool MyBool;
    }

    public class MyClass
    {
        private List<MyStruct> MyPrivateVariable;

        public List<MyStruct> MyVariable
        {
            get
            {
                if (MyPrivateVariable == null)
                {
                    MyPrivateVariable = new List<MyStruct>();

                    MyPrivateVariable.Add(new MyStruct());
                    MyPrivateVariable.Add(new MyStruct());
                }

                return MyPrivateVariable;
            }
        }

        public void MyLoop()
        {
            foreach (MyStruct ms in MyVariable)
            {
                // Doesn't compile, but it works if you execute it through the Immediate window, or in Quickwatch
                ms.MyBool = false;

                // Compiles, works
                MyFunction(ms);
            }
        }

        public void MyFunction(MyStruct ms)
        {
            ms.MyBool = false;
        }
    }
}

Any reasonable explanations for this?

The compiler returns:

Error: Cannot modify members of 'ms' because it is 'foreach iteration variable'

EDIT:

Extra question:

I just tried changing a string from MyFunction, and it doesn't actually update ms. BUT: If I go to quickwatch and assign the same value there, it does update ms. Why does this happen if it shouldn't even be compiling in the first place, shouldn't quickwatch throw an exception?

EDIT2:

Ok, quick watch also works on a copy of ms, so that's why I can edit it's value, it doesn't actually alter the contents of MyPrivateVariable.

Answer 1

You're using them as mutable structs. Avoid doing that:

Why are mutable structs “evil”?