The Dark Side of Covariance

What’s wrong with the following code?

var names = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
if (names.Contains(sqlCommand.ExecuteScalar())

This  code is intended to check whether the result of a SQL query is contained in a case-insensitive collection of names.  However, if you run this code, the resulting check will be case-sensitive.  Why?

As you may have guessed from the title, this is caused by covariance.  In fact, this code will not compile at all against .Net 3.5. 

The problem is that ExecuteScalar() returns object, not string.  Therefore, it doesn’t call HashSet<string>.Contains(string), which is what it’s intending to call (and which uses the HashSet’s comparer).  Instead, on .Net 4.0, this calls the  Enumerable.Contains<object>(IEnumerable<object>, string) extension method, using the covariant conversion from IEnumerable<string> to IEnumerable<object>.  Covariance allows us to pass object to the Contains method of any strongly-typed collection (of reference types).

Still, why is it case-sensitive?  As Jon Skeet points out, the LINQ Contains() method is supposed to call any built-in Contains() method from ICollection<T>, so it should still use the HashSet’s case-insensitive Contains().

The reason is that although HashSet<String> implements ICollection<string>, it does not implement ICollection<object>.  Since we’re calling Enumerable.Contains<object>, it checks whether the sequence implements ICollection<object>, which it doesn’t.  (ICollection<T> is not covariant, since it allows write access)

Fortunately, there’s a simple fix: just cast the return value back to string (and add a comment explaining the problem).  This allows the compiler to call HashSet<string>.Contains(string), as was originally intended.

//Call HashSet<string>.Contains(string), not the
//covariant Enumerable.Contains(IEnumerable<object>, object)
if (names.Contains((string)sqlCommand.ExecuteScalar())
(I discovered this issue in my StringListConstraint for ASP.Net MVC)


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