using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    class Document

    {

        public int Id { get; set; }

    }

    class DocumentReference

    { 

        public int Id { get; set; }

        public Document Document { get; set; }

    }

    public static void Main()

    {

        var documentReferences = new[] { 1, 2, 3, 4, 5 }.Select(i => new DocumentReference { Id = i });

​

        var documents = new Dictionary<int, Document>

        {

            { 1, new Document { Id = 1 } },

            { 4, new Document { Id = 4 } }

        };

​

        foreach (var docRef in documentReferences)

        {

            if (!documents.TryGetValue(docRef.Id, out Document doc))

                continue;

​

            docRef.Document = doc;

        }

​

        Console.WriteLine($"Matched document references: {documentReferences.Count(d => d.Document != null)}");

    }

}