SingleOrDefault() throws an exception on more than one element

Question

I'm getting an exception whenever I fetch like this

Feature f = o.Features.SingleOrDefault(e => e.LinkName == PageLink);

because this can return one or more than one element. What is the alternative approach that I can use to solve this issue?

Answer 1

Single and SingleOrDefault are designed to throw if more that one match exists in the sequence. A consequence of this is that the entire sequence must be iterated prior to completion. It does not sound like this is what you want. Try FirstOrDefault instead:

Feature f = o.Features
    .FirstOrDefault(e => e.vcr_LinkName == PageLink && e.bit_Activate == true);

This will (generally) perform better because it completes as soon as a match is found.

Of course, if you actually want to retain more than one element, a Where clause would be more appropriate:

IEnumerable<Feature> fs = o.Features
    .Where(e => e.vcr_LinkName == PageLink && e.bit_Activate == true);

Answer 2

Alternatively, if you only want the item when there is exactly one match and do not want to throw when there are more than one, this can be easily accomplished. I've created an extension method for this in my project:

public static class QueryableExtensions
{
    public static TSource SingleWhenOnly<TSource>(this IQueryable<TSource> source)
    {
        if (source == null)
            throw new ArgumentNullException("source");

        var results = source.Take(2).ToArray();

        return results.Length == 1 ? results[0] : default(TSource);
    }

    public static TSource SingleWhenOnly<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
    {
        if (source == null)
            throw new ArgumentNullException("source");
        if (predicate == null)
            throw new ArgumentNullException("predicate");

        var results = source.Where(predicate).Take(2).ToArray();

        return results.Length == 1 ? results[0] : default(TSource);
    }
}