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”?

Answer 2

Struct has value type semantics. So any modification you make to the struct instance wouldn't affect the original instance. The C# compiler is trying to warn you of this.

Answer 3

C# doesn't iterate structs by reference in the "foreach (MyStruct ms...)" so ms in that context is immutable.

Replace MyStruct with a class instead.

QuickWatch can manipulate value types on the stack.

Answer 4

this is because struct is valuetype and not a reference type. if MyStruct was a class it would have compiled without issues. check this thread for details.

Answer 5

You can't change what an iteration variable references: that is, you can't point the variable to a different instance (to find out why that is, see Why is The Iteration Variable in a C# foreach statement read-only? ).

'Modifying' a struct (a value type) creates a new instance of the type, so the statement ms.MyBool = false is meaningless.

The call to MyFunction(ms) compiles because it operates on a copy of ms (though it still won't do what you might expect).