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_Note: this was tested in a .NET Framework 4.6.2 project._

[This answer](https://stackoverflow.com/a/18782867/2780791) provides an extension method that greatly simplifies (and makes it more intuitive) the written code:

    public static IQueryable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
        this IQueryable<TOuter> outer,
        IQueryable<TInner> inner,
        Expression<Func<TOuter, TKey>> outerKeySelector,
        Expression<Func<TInner, TKey>> innerKeySelector,
        Expression<Func<TOuter, TInner, TResult>> resultSelector)
    {
        MethodInfo groupJoin = typeof (Queryable).GetMethods()
                                                 .Single(m => m.ToString() == "System.Linq.IQueryable`1[TResult] GroupJoin[TOuter,TInner,TKey,TResult](System.Linq.IQueryable`1[TOuter], System.Collections.Generic.IEnumerable`1[TInner], System.Linq.Expressions.Expression`1[System.Func`2[TOuter,TKey]], System.Linq.Expressions.Expression`1[System.Func`2[TInner,TKey]], System.Linq.Expressions.Expression`1[System.Func`3[TOuter,System.Collections.Generic.IEnumerable`1[TInner],TResult]])")
                                                 .MakeGenericMethod(typeof (TOuter), typeof (TInner), typeof (TKey), typeof (LeftJoinIntermediate<TOuter, TInner>));
        MethodInfo selectMany = typeof (Queryable).GetMethods()
                                                  .Single(m => m.ToString() == "System.Linq.IQueryable`1[TResult] SelectMany[TSource,TCollection,TResult](System.Linq.IQueryable`1[TSource], System.Linq.Expressions.Expression`1[System.Func`2[TSource,System.Collections.Generic.IEnumerable`1[TCollection]]], System.Linq.Expressions.Expression`1[System.Func`3[TSource,TCollection,TResult]])")
                                                  .MakeGenericMethod(typeof (LeftJoinIntermediate<TOuter, TInner>), typeof (TInner), typeof (TResult));

        var groupJoinResultSelector = (Expression<Func<TOuter, IEnumerable<TInner>, LeftJoinIntermediate<TOuter, TInner>>>)
                                      ((oneOuter, manyInners) => new LeftJoinIntermediate<TOuter, TInner> {OneOuter = oneOuter, ManyInners = manyInners});

        MethodCallExpression exprGroupJoin = Expression.Call(groupJoin, outer.Expression, inner.Expression, outerKeySelector, innerKeySelector, groupJoinResultSelector);

        var selectManyCollectionSelector = (Expression<Func<LeftJoinIntermediate<TOuter, TInner>, IEnumerable<TInner>>>)
                                           (t => t.ManyInners.DefaultIfEmpty());

        ParameterExpression paramUser = resultSelector.Parameters.First();

        ParameterExpression paramNew = Expression.Parameter(typeof (LeftJoinIntermediate<TOuter, TInner>), "t");
        MemberExpression propExpr = Expression.Property(paramNew, "OneOuter");

        LambdaExpression selectManyResultSelector = Expression.Lambda(new Replacer(paramUser, propExpr).Visit(resultSelector.Body), paramNew, resultSelector.Parameters.Skip(1).First());

        MethodCallExpression exprSelectMany = Expression.Call(selectMany, exprGroupJoin, selectManyCollectionSelector, selectManyResultSelector);

        return outer.Provider.CreateQuery<TResult>(exprSelectMany);
    }

    private class LeftJoinIntermediate<TOuter, TInner>
    {
        public TOuter OneOuter { get; set; }
        public IEnumerable<TInner> ManyInners { get; set; }
    }

    private class Replacer : ExpressionVisitor
    {
        private readonly ParameterExpression _oldParam;
        private readonly Expression _replacement;

        public Replacer(ParameterExpression oldParam, Expression replacement)
        {
            _oldParam = oldParam;
            _replacement = replacement;
        }

        public override Expression Visit(Expression exp)
        {
            if (exp == _oldParam)
            {
                return _replacement;
            }

            return base.Visit(exp);
        }
    }

This allows replacing GroupJoin with LeftJoin which is a slight improvement.