Struct implementing IEnumerable yields unexpected error with LINQ

Question

I've implemented a Maybe<T> class which implements IEnumerable in C# inspired by Mark Seemann and it works great. I've got a suite of helper extension methods that make common operations Maybe-friendly and they all work great too. I had the idea to change Maybe<T> from a class to a struct to prevent it from being nullable and it seems to work great with one problem:

var inputs = new Maybe<string>[] { /* from somewhere */ };

// This line works with class Maybe and struct Maybe
var results1 = inputs.SelectMany(x => ParseMaybe.ToInt32(x));

// This line works with class Maybe but with struct Maybe it yields a compiler error: 
// Error CS0407 'Maybe<int> ParseMaybe.ToInt32(string)' has the wrong return type
var results2 = inputs.SelectMany(ParseMaybe.ToInt32);

Note: Because Maybe implements IEnumerable here, I'm using SelectMany instead of Select.

This code works with the class version but gets the compiler error as a struct. Why?

Would you like to show us the struct and class implementation? — BendEg
– BendEg, Commented Dec 16, 2015 at 21:48
SelectMany is used to flatten nested collections. You should use Select here, otherwise you will get an enumeration of chars. — Olivier Jacot-Descombes
– Olivier Jacot-Descombes, Commented Dec 16, 2015 at 22:18
Youre right about the chars. In simplifying my example I lost some context. I fixed it to make it more clear. — Charles Josephs
– Charles Josephs, Commented Dec 16, 2015 at 22:31
gist.github.com/darkmyst/ecbc00e2f2f6c5f42ded is the class implementation. The struct version differs only in removing the default constructor and avoiding null refs on _value — Charles Josephs
– Charles Josephs, Commented Dec 16, 2015 at 22:38
@Charles struct are not a non nullable version of class. They are passed as value and not as reference and are boxed-unboxed when used. That can cause strange issue when use in foreach or during enumeration, i suggest you to avoid your idea to use it as non nullable version of class. If non nullable values is a must for you, try to create a static helper methd dat initialize properties (or field) of its target. — Skary
– Skary, Commented Dec 16, 2015 at 22:40

Lee · Accepted Answer · 2015-12-16 23:07:43Z

3

This is because struct type parameters are invariant. Even though Func<A, B> is covariant in its return type this only applies if B is a reference type. This means you cannot assign a Func<string, Maybe<int>> to a Func<string, IEnumerable<int>> even though Maybe<T> implements IEnumerable<T> e.g.

Func<string, Maybe<int>> f = ParseMaybe.ToInt32;
Func<string, IEnumerable<int>> g = f; //won't compile

In contrast the following will compile since int[] is a reference type:

Func<string, int[]> f = ???
Func<string, IEnumerable<int>> g = f;

edited Dec 16, 2015 at 23:07

answered Dec 16, 2015 at 22:39

Lee

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