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 r

Answer 1

I've found I need the behavior of returning a default value if there is not exactly one element (i.e. zero, two, or more) more often than I need the normal SingleOrDefault behavior, so here's my adapted version of Pieter van Ginkel's answer:

public static class LinqExtensions
{
    /// 

    /// Returns the only element of a sequence, or a default value if the sequence is empty or contains more than one element.
    /// 
    public static TSource SingleOrDefaultIfMultiple(this IEnumerable source)
    {
        var elements = source.Take(2).ToArray();

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

    /// 

    /// Returns the only element of a sequence, or a default value if the sequence is empty or contains more than one element.
    /// 
    public static TSource SingleOrDefaultIfMultiple(this IEnumerable source, Func predicate)
    {
        return source.Where(predicate).SingleOrDefaultIfMultiple();
    }

    /// 

    /// Returns the only element of a sequence, or a default value if the sequence is empty or contains more than one element.
    /// 
    public static TSource SingleOrDefaultIfMultiple(this IQueryable source)
    {
        var elements = source.Take(2).ToArray();

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

    /// 

    /// Returns the only element of a sequence, or a default value if the sequence is empty or contains more than one element.
    /// 
    public static TSource SingleOrDefaultIfMultiple(this IQueryable source, Expression> predicate)
    {
        return source.Where(predicate).SingleOrDefaultIfMultiple();
    }
}

I've omitted the null argument checks because I'm OK with relying on the Take and Where calls to throw exceptions when the arguments are null, but you might feel otherwise.