Left Join, Right Join and Full Outer Join

public static IEnumerable<TResult> LeftJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source,
                                         IEnumerable<TInner> inner,
                                         Func<TSource, TKey> pk,
                                         Func<TInner, TKey> fk,
                                         Func<TSource, TInner, TResult> result)
{
    IEnumerable<TResult> _result = Enumerable.Empty<TResult>();

    _result = from s in source
              join i in inner
              on pk(s) equals fk(i) into joinData
              from left in joinData.DefaultIfEmpty()
              select result(s, left);

    return _result;
}

public static IEnumerable<TResult> RightJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source,
                                          IEnumerable<TInner> inner,
                                          Func<TSource, TKey> pk,
                                          Func<TInner, TKey> fk,
                                          Func<TSource, TInner, TResult> result)
{
    IEnumerable<TResult> _result = Enumerable.Empty<TResult>();

    _result = from i in inner
              join s in source
              on fk(i) equals pk(s) into joinData
              from right in joinData.DefaultIfEmpty()
              select result(right, i);

    return _result;
}

public static IEnumerable<TResult> FullOuterJoinJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source,
                                                  IEnumerable<TInner> inner,
                                                  Func<TSource, TKey> pk,
                                                  Func<TInner, TKey> fk,
                                                  Func<TSource, TInner, TResult> result)
{

    var left = source.LeftJoin(inner, pk, fk, result).ToList();
    var right = source.RightJoin(inner, pk, fk, result).ToList();

    return left.Union(right);
}