using System;

using System.Diagnostics;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var sw = new System.Diagnostics.Stopwatch();

        sw.Start();

        var forms = GetForms();

        var models = GetModels();

        sw.Stop();

        Console.WriteLine($"instantiate operation took => {sw.Elapsed}");

        sw.Restart();

        models = models.LeftOuterJoin(forms, (outter) => outter.ClientId, (inner) => inner.ClientId,

                         (outter, inner) => {

                             outter.referralForm = inner;

                             return outter;

                         });

        sw.Stop();

        Console.WriteLine($"query operation took => {sw.Elapsed}{Environment.NewLine}");

        Console.WriteLine($"Models Count {models.Count()}");

        Console.WriteLine($"Models with null ReferralForm {models.Count(m => m.referralForm == null)}");

        Console.WriteLine($"Models with ReferralForm instances {models.Count(m => !(m.referralForm == null))}");

    }

    static IEnumerable<ReferralModel> GetModels() => GetForms().Select(f => new ReferralModel { ClientId = f.ClientId })

        .Concat(Enumerable.Range(0, 500).Select((idx) => new ReferralModel { ClientId = Guid.NewGuid() })).OrderBy(item => item.ClientId)

        .ToHashSet(); 

    static IEnumerable<ReferralForm> _forms = Enumerable.Empty<ReferralForm>();

    static IEnumerable<ReferralForm> GetForms() 

    {

        if ((_forms?.Count() ?? 0) == 0) 

        {

            _forms = Enumerable.Range(0, 500).Select((idx) => new ReferralForm { ClientId = Guid.NewGuid() }).ToHashSet();

        }

        return _forms;

    }

}

​

public static class LinqExt

{

    public static IEnumerable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TKey> leftKey, Func<TRight, TKey> rightKey,

        Func<TLeft, TRight, TResult> result)

    {

        return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })

             .SelectMany(o => o.r.DefaultIfEmpty(), (l, r) => new { lft = l.l, rght = r })

             .Select(o => result.Invoke(o.lft, o.rght));

    }

}

​

public class ReferralModel

{

    public string ClientName { get; set; }

    public Guid? ClientId { get; set; }

    public string ClientNumber { get; set; }

    public string ClientDOB { get; set; }

    public string DateSubmitted { get; set; } = default;

    public DateTime? ModifiedDateSubmitted { get; set; }

    public string ReportStatus { get; set; }

    public string ReferralNotes { get; set; }

    public ReferralForm referralForm { get; set; }

}

​

public class ReferralForm

{

    public Guid FileId { get; set; }

    public Guid ClientId { get; set; }

    public string Name { get; set; }

    public string FileName { get; set; }

    public string ContentType { get; set; }

    public string FileExtension { get; set; }

    public byte[] FileContent { get; set; }

    public DateTime CreatedDate { get; set; }

}