Filter Linq EXCEPT on properties

Question

This may seem silly, but all the examples I\'ve found for using Except in linq use two lists or arrays of only strings or integers and filters them based on the

Answer 1

Construct a List<AppMeta> from the excluded List and use the Except Linq operator.

var ex = excludedAppIds.Select(x => new AppMeta{Id = x}).ToList();                           
var result = ex.Except(unfilteredApps).ToList();

Answer 2

I use an extension method for Except, that allows you to compare Apples with Oranges as long as they both have something common that can be used to compare them, like an Id or Key.

public static class ExtensionMethods
{
    public static IEnumerable<TA> Except<TA, TB, TK>(
        this IEnumerable<TA> a,
        IEnumerable<TB> b,
        Func<TA, TK> selectKeyA,
        Func<TB, TK> selectKeyB, 
        IEqualityComparer<TK> comparer = null)
    {
        return a.Where(aItem => !b.Select(bItem => selectKeyB(bItem)).Contains(selectKeyA(aItem), comparer));
    }
}

then use it something like this:

var filteredApps = unfilteredApps.Except(excludedAppIds, a => a.Id, b => b);

the extension is very similar to ColinE 's answer, it's just packaged up into a neat extension that can be reused without to much mental overhead.