Mutating Readonly Structs

Mutating Readonly Structs

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Consider this program which attempts to mutate a readonly mutable struct. What happens?

struct Mutable {
    private int x;
    public int Mutate() {
        this.x = this.x + 1;
        return this.x;

class Test {
    public readonly Mutable m = new Mutable();
    static void Main(string[] args) {
        Test t = new Test();

There are a number of things this program could do. Does it:

1) Print 1, 2, 3 -- because m is readonly, but the "readonly" only applies to m, not to its contents.
2) Print 0, 0, 0 -- because m is readonly, x cannot be changed. It always has its default value of zero.
3) Throw an exception at runtime, when the attempt is made to mutate the contents of a readonly field.
4) Do something else


People are frequently surprised to learn that the answer is (4). In fact, this prints 1, 1, 1. 


Because, remember, accessing a value type gives you a COPY of the value. When you say t.m, you get a copy of whatever is presently stored in m. m is immutable, but the copy is not. The copy is then mutated, and the value of x in the copy is returned. But m remains untouched.

The relevant section of the specification is 7.5.4, which states that when resolving "E.I" where E is an object and I is a field...

...if the field is readonly and the reference occurs outside an instance constructor of the class in which the field is declared, then the result is a value, namely the value of the field I in the object referenced by E.

The important word here is that the result is the value of the field, not the variable associated with the field. Readonly fields are not variables outside of the constructor. (The initializer here is considered to be inside the constructor; see my earlier post on that subject.)

Great. What about that second dot, as in ".Mutate()"?  We look at section 7.4.4 to find out how to invoke E.M():

If E is not classified as a variable, then a temporary local variable of E's type is created and the value of E is assigned to that variable. E is then reclassified as a reference to that temporary local variable. The temporary variable is accessible as this within M, but not in any other way. Thus, only when E is a true variable is it possible for the caller to observe the changes that M makes to this.

And there you go. Value semantics are tricky!

This is yet another reason why mutable value types are evil. Try to always make value types immutable.

  • The C# designers once again ignored the principle of least surprise :(

    I'd expect a compiler error - something like "Mutable structs can't be readonly." or "Methods muting the value can't be called on readonly values." (though I also strongly encourage everyone to make all value types immutable).

    What does the rest of the community expect?

  • There is a spectrum, from "clearly desirable behaviour", to "possibly dodgy behaviour that still makes some sense", to "clearly undesirable behaviour".  We try to make the latter into warnings or, better, errors.  But stuff that is in the middle category you don't want to restrict unless there is a clear way to work around it.

    For example, something like:  mydictionary[123].blah = 456; is a compiler error if the indexer returns a mutable struct.  The result of an indexer is not a variable, and therefore, the write to blah will be on the copy returned by the indexer, not the contents of the dictionary. That's clearly not at all what the author intended.  They meant temp = mydictionary[123]; temp.blah = 456; mydictionary[123] = temp;.  Since the code is clearly bogus and there is a simple workaround, we give an error.

    In your example, how would the user work around the problem?  They might not own the struct definition, so they cannot make it immutable.  If you tell them to make their field mutable instead of readonly, then you're exacerbating the problem -- the mutability is now spreading from type to type!

  • I would rather say, in this case, that structs are the evil.

  • Thanks for the article!


  • Another interesting article. Could FxCop be made to check for mutable value types? I've not really explored the customization side of that tool yet.

  • @Thomas D: How is the compiler meant to know that the struct is mutable? Or that the method mutates things? That ends up prying into the guts of an object more than I'd be happy with. (It would be nice for a type to be able to say that it's immutable, but that's certainly not available at the moment.) Any feature where the compiler has to care about the implementation (as opposed to the interface) of something it's calling into sounds suspect to me.

    @Thomas E: I don't think structs are evil per se. I think there's a lot of misunderstanding around them, but there are times when they're handy.

    @Eric: Wow, that's an evil one. I can absolutely see why the language is specified that way, but I get worried when I can't predict the answer to a language question. (I didn't even try to guess, because they all sounded wrong.)

  • @Eric:

    You can work around it the same way the C# compiler does:

    Mutable temp = t.m;


    I, for one, find it extremely irritating that there is a special treatment of that special case in the docs. Why the hell does the C# compiler _explicitly_ support such a "category". IMHO it's far a way form any "middle category". Whenever a user wants to do the same as your code-example, why not force him to do the workaround himself? Extra support from the C# compiler just makes things more complicated (in terms of readability of the code and pollution of the C# spec). That's one of the decisions I definitely don't understand (though I'm trying very hard - most of the time).

    Ok, it's reasonable why "Mutable structs can't be readonly." is a bad idea, "Methods muting the value can't be called on readonly values." may be a bad idea either (thinking of versioning problems). But what do the CLI specs they to such a scenario? I.e. what would happen if the C# compiler wouldn't generate a local variable but call the method on the readonly field directly?


    The C# compiler is well aware about a method muting its object. He even has to be aware of it to be able to compile the code correctly. (Think of the AST the compiler generates.)

  • > The C# compiler is well aware about a method muting its object. . He even has to be aware of it to be able to compile the code correctly.

    What if the compiler is not compiling the method?  The method might be external.

  • > You can work around it the same way the C# compiler does:

    Now you're answering a question I didn't ask.  Let me re-state.

    Your position is that marking a field of a mutable struct type as "readonly" ought to be a compiler error.  My question is: suppose the user WANTS a field which is readonly and of a mutable struct type. In your compiler, that's an error.  What do they do to get their program to work?  Do they (a) make the struct type immutable?  They might not own the struct type. And they are almost certainly making the field readonly precisely BECAUSE they want this copy of the struct to be immutable.  So this seems like a bad idea.  Their only recourse in this case is (b) make the field no longer readonly.  Which means that they've just made an immutable field into an immutable field.  Suppose the current type is itself a struct.  Now they've just been forced to turn an immutable struct into a mutable struct, and the taint is spreading.  All the users of THIS struct will now no longer be able to mark their fields of this type as readonly...

  • > But what do the CLI specs they to such a scenario?

    You know, you could look it up yourself rather than asking me.

    But because I am in a charitable mood, and have the spec open already, I'll just tell you that Partition II section 16.1.2 states "These fields shall only be mutated inside a constructor. If the field is a static field, then it shall be mutated only inside the type initializer of the type in which it was declared. If it is an instance field, then it shall be mutated only in one of the instance constructors of the type in which it was defined. It shall not be mutated in any other method or in any other constructor, including constructors of derived classes."

    So, in short, if we generated the code the way you suggest, that would be illegal code. Specifically, it would be unverifiable; unverifiable code will not load at all in any environment with restricted security. In a fully trusted environment, I do not know whether that code would crash the process, throw a managed exception, silently fail, or silently succeed -- or, for that matter, erase your hard disk.  Unverifiable code is not guaranteed to do anything in particular; hence the name.

    Obviously we do not wish to generate unverifiable code.

  • > I, for one, find it extremely irritating that there is a special treatment of that special case in the docs

    What's the special treatment?  There are only two rules here:

    1) Readonly fields are not variables.

    2) Accessing anything on a value type which is not a variable acts on a copy of the value type.

    That the interaction of these two simple rules have complex consequences is interesting.  And that some of those consequences are sufficiently interesting that the spec explicitly calls your attention to them is us being nice to you and calling your attention to something you might have otherwise missed.

    Now, perhaps you don't like one of these rules. I'm not sure which one of them you don't like, or how you'd propose replacing it with something you do like that doesn't make the whole situation much worse. (Like, causing the compiler to generate unverifiable code.)  But frankly, they seem like sensible rules to me.

  • "Could FxCop be made to check for mutable value types?"

    Try this:

  • "This is yet another reason why mutable value types are evil."

    How about the evil immutable reference types which pretend to be mutable? System.String for starters. Many have fallen in to the trap of doing s [ 0 ] = 'a' only to find that it won't compile.

    Just yesterday, I found a gem in a function written by an interviewee which reversed a string in-place. [Evil grin]

  • Sorry if I sounded a bit offending, I just didn't see the dilemma.

    It's just feeling strange that "Accessing anything on a value type which is not a variable acts on a copy of the value type."

    It's really sensible if you want to do something like integers[13].ToString(), or obj.value.any_pure_function but it opens a can of worms if the called method is not a pure function but mutating the struct.

    As you pointed out in this blog post, statement 2 doesn't play well with mutable structs. That's quite a fundamental problem and not at all easy to solve.

    One solution might be to not allow accessing anything on a value type which is not a variable, but it's far too late to do such an enormous breaking change.

    Detecting muting operations is also difficult (though you could just be pessimistic on external code - which is very rare anyway) and it introduces versioning issues too.

    Don't get me wrong, the C# team did a really great job. C# is my favorite language for most problems I've to solve and I really enjoy to use that language. But still, there's also a lot of room for improvements.

    Let's just stick with it that mutable structs are bad.

  • A question about "...mutable value types are evil. Try to always make value types immutable." (and also some comments that suggests also suggests that creating mutalbe structs are pure evil) :

    I think there are really good reasons to make structs mutable, for example:

    struct Vector3D


     public double X;

     public double Y;

     public double Z;


    struct Triangle


     public Vector3D A;

     public Vector3D B;

     public Vector3D C;



    var triangle = new Triangle()


     A = new Vector3D() { X = 0.0, Y = 0.0, Z = 0.0 },

     B = new Vector3D() { X = 0.0, Y = 1.0, Z = 0.0 },

     C  = new Vector3D() { X = 0.0, Y = 0.0, Z = 1.0 },


    triangle.A.X = -1.0;

    If Triangle was immutable I would have to write something that was far more complicated...

    I mean, the triangle gets copied whenever it is used somewhere else; so the only way to modify the triangle is inside the acutal function (if not initialized and accessed through interface of course).

    So my question is: Making the triangle mutable just makes my code more succinct and I see no reason to make it mutable - you think this is a special case where mutable structs are ok or would you also make anything immutalbe?

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