JoinMapper.cs

public static class QueryableExtensions 
{
    public static IQueryable<JoinMapper<TA, TB>> Join<TA, TB, TKey>(
        this IQueryable<TA> outer, 
        IQueryable<TB> inner,
        Expression<Func<TA, TKey>> outerSelector,
        Expression<Func<TB, TKey>> innerSelector
    )
    {
        return outer.Join(
            inner,
            outerSelector,
            innerSelector,
            (a, b) => new JoinMapper<TA, TB>
            {
                A = a,
                B = b
            }
        );
    }

    public static IQueryable<JoinMapper<TA, TB, TC>> Join<TA, TB, TC, TKey>(
        this IQueryable<JoinMapper<TA, TB>> outer, 
        IQueryable<TC> inner,
        Expression<Func<JoinMapper<TA, TB>, TKey>> outerSelector,
        Expression<Func<TC, TKey>> innerSelector
    )
    {
        return outer.Join(
            inner,
            outerSelector,
            innerSelector,
            (ab, c) => new JoinMapper<TA, TB, TC>
            {
                A = ab.A,
                B = ab.B,
                C = c
            }
        );
    }

    public static IQueryable<JoinMapper<TA, TB, TC, TD>> Join<TA, TB, TC, TD, TKey>(
        this IQueryable<JoinMapper<TA, TB, TC>> outer, 
        IQueryable<TD> inner,
        Expression<Func<JoinMapper<TA, TB, TC>, TKey>> outerSelector,
        Expression<Func<TD, TKey>> innerSelector
    )
    {
        return outer.Join(
            inner,
            outerSelector,
            innerSelector,
            (abc, d) => new JoinMapper<TA, TB, TC, TD>
            {
                A = abc.A,
                B = abc.B,
                C = abc.C,
                D = d
            }
        );
    }
}

public class JoinMapper<TA, TB> 
{
   public required TA A { get; init; }

   public required TB B { get; init; } 
}

public class JoinMapper<TA, TB, TC> 
{
   public required TA A { get; init; }

   public required TB B { get; init; }

   public required TC C { get; init; } 
}

public class JoinMapper<TA, TB, TC, TD> 
{
   public required TA A { get; init; }

   public required TB B { get; init; }

   public required TC C { get; init; }

   public required TD D { get; init; } 
}