kitlabcode

public static IEnumerable<TResult> LeftJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source,
                                             IEnumerable<TInner> inner,
                                             Func<TSource, TKey> primaryKey,
                                             Func<TInner, TKey> foreignKey,
                                             Func<TSource, TInner, TResult> resultCollection)
    {
        var result = from s in source
            join i in inner
                on primaryKey(s) equals foreignKey(i) into joinData
            from left in joinData.DefaultIfEmpty()

kitlabcode

public static class LinqExtension
{
    public static IEnumerable<TResult> LeftJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source,
                                             IEnumerable<TInner> inner,
                                             Func<TSource, TKey> primaryKey,
                                             Func<TInner, TKey> foreignKey,
                                             Func<TSource, TInner, TResult> resultCollection)
    {
        var result = from s in source
            join i in inner