检查列表中的任何项是否与另一个列表中的任何项匹配

Question

A coleague asked me to write a one-liner to replace the following method:

public static bool IsResourceAvailableToUser(IEnumerable<string> resourceRoles, IEnumerable<string> userRoles)
{
    foreach (var userRole in userRoles)
        foreach (var resourceRole in resourceRoles)
            if (resourceRole == userRole)
                return true;
    return false;
}

Resharper and I came up with this:

public static bool IsResourceAvailableToUser(IEnumerable<string> resourceRoles, IEnumerable<string> userRoles)
{
    return userRoles.Where(resourceRoles.Contains).Count() > 0;
}

Is there a better way?

Answer 1

Given LINQ, yes:

return userRoles.Intersect(resourceRoles).Any();

Note that aside from the use of Intersect to turn it into O(m) + O(n) instead O(m * n), using Any is more efficient than using Count() > 0 - you know the answer as soon as you've found the first match.

Answer 2

You could write a generic extension method that would handle many cases. The meat of the function itself is one line.

/// <summary>
/// Compares both lists to see if any item in the enumerable 
/// equals any item in the other enumerable. 
/// </summary>
public static bool AnyItem<T>(this IEnumerable<T> source, IEnumerable<T> other, IEqualityComparer<T> comparer = null)
{
    return (comparer == null ? source.Intersect(other) : source.Intersect(other, comparer)).Any();
}

Older, Less efficient answer

public static bool AnyItem<T>(this IEnumerable<T> source, IEnumerable<T> other)
{
    return source.Any(s => other.Any(o => EqualityComparer<T>.Default.Equals(s, o)));
}

I think this is also more efficient than the current answer (It's not). I would have to check if getting the EqualityComparer is expensive, but I'm willing to doubt it.

---

You could also extend this function to accept an Expression that would evaluate what properties to compare for enumerables that contain objects.

public static bool AnyItem<T, TResult>(
        this IEnumerable<T> source, 
        IEnumerable<T> other, 
        Expression<Func<T, TResult>> compareProperty = null)
{
    if (compareProperty == null)
    {
        return source.Any(s => other.Any(o => EqualityComparer<T>.Default.Equals(s, o)));
    }

    return source.Any(s => other.Any(o => 
                    EqualityComparer<TResult>.Default.Equals(
                    s.GetPropertyValue(compareProperty),
                    o.GetPropertyValue(compareProperty))));
}

public static TValue GetPropertyValue<TTarget, TValue>(
    this TTarget target, Expression<Func<TTarget, TValue>> memberLamda)
{
    var memberSelectorExpression = memberLamda.Body as MemberExpression;
    var property = memberSelectorExpression?.Member as PropertyInfo;
    return (TValue)property?.GetValue(target);
}