Fake implementation of Entity Framework's DbSet for fast unit testing

FakeDbSet.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.SqlServer;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;
using NeinLinq;

namespace EntityFakes
{
    public class FakeDbSet<TEntity> : DbSet<TEntity>, IQueryable<TEntity>, IDbAsyncEnumerable<TEntity>
        where TEntity : class
    {
        private readonly IEnumerable<PropertyInfo> keys;
        private readonly ICollection<TEntity> items;
        private readonly IQueryable<TEntity> query;

        public FakeDbSet()
        {
            keys = typeof(TEntity).GetProperties()
                                  .Where(p => Attribute.IsDefined(p, typeof(KeyAttribute))
                                           || "Id".Equals(p.Name, StringComparison.Ordinal))
                                  .ToList();

            items = new List<TEntity>();
            query = items.AsQueryable()
                         .ToSubstitution(typeof(SqlFunctions), typeof(FakeDbFunctions))
                         .ToNullsafe();
        }

        public override TEntity Add(TEntity entity)
        {
            if (entity == null)
                throw new ArgumentNullException("entity");
            if (keys.Any(k => k.GetValue(entity) == null))
                throw new ArgumentOutOfRangeException("entity");

            items.Add(entity);
            return entity;
        }

        public override IEnumerable<TEntity> AddRange(IEnumerable<TEntity> entities)
        {
            if (entities == null)
                throw new ArgumentNullException("entities");

            return entities.Select(Add).ToList();
        }

        public override TEntity Attach(TEntity entity)
        {
            if (entity == null)
                throw new ArgumentNullException("entity");
            if (keys.Any(k => k.GetValue(entity) == null))
                throw new ArgumentOutOfRangeException("entity");

            var item = items.SingleOrDefault(i =>
                keys.All(k => k.GetValue(entity).Equals(k.GetValue(i)))
            );

            if (item == null)
                items.Add(entity);
            return item ?? entity;
        }

        public override TDerivedEntity Create<TDerivedEntity>()
        {
            return Activator.CreateInstance<TDerivedEntity>();
        }

        public override TEntity Create()
        {
            return Activator.CreateInstance<TEntity>();
        }

        public override TEntity Find(params object[] keyValues)
        {
            if (keyValues == null)
                throw new ArgumentNullException("keyValues");
            if (keyValues.Any(k => k == null))
                throw new ArgumentOutOfRangeException("keyValues");
            if (keyValues.Length != keys.Count())
                throw new ArgumentOutOfRangeException("keyValues");

            return items.SingleOrDefault(i =>
                keys.Zip(keyValues, (k, v) => v.Equals(k.GetValue(i)))
                    .All(r => r)
            );
        }

        public override Task<TEntity> FindAsync(params object[] keyValues)
        {
            return Task.FromResult(Find(keyValues));
        }

        public override Task<TEntity> FindAsync(CancellationToken cancellationToken, params object[] keyValues)
        {
            return Task.FromResult(Find(keyValues));
        }

        public override TEntity Remove(TEntity entity)
        {
            if (entity == null)
                throw new ArgumentNullException("entity");
            if (keys.Any(k => k.GetValue(entity) == null))
                throw new ArgumentOutOfRangeException("entity");

            var item = items.SingleOrDefault(i =>
                keys.All(k => k.GetValue(entity).Equals(k.GetValue(i)))
            );

            if (item != null)
                items.Remove(item);
            return item;
        }

        public override IEnumerable<TEntity> RemoveRange(IEnumerable<TEntity> entities)
        {
            if (entities == null)
                throw new ArgumentNullException("entities");

            return entities.Select(Remove).ToList();
        }

        public Type ElementType
        {
            get { return query.ElementType; }
        }

        public Expression Expression
        {
            get { return query.Expression; }
        }

        public IQueryProvider Provider
        {
            get { return query.Provider; }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return query.GetEnumerator();
        }

        public IEnumerator<TEntity> GetEnumerator()
        {
            return query.GetEnumerator();
        }

        IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
        {
            return ((IDbAsyncEnumerable)query).GetAsyncEnumerator();
        }

        public IDbAsyncEnumerator<TEntity> GetAsyncEnumerator()
        {
            return ((IDbAsyncEnumerable<TEntity>)query).GetAsyncEnumerator();
        }
    }

    public static class FakeDbFunctions
    {
        public static int? CharIndex(string toSearch, string target)
        {
            return CharIndex(toSearch, target, null);
        }

        public static int? CharIndex(string toSearch, string target, int? startLocation)
        {
            if (toSearch == null)
                return null;
            if (target == null)
                return null;

            return target.IndexOf(toSearch, (startLocation ?? 1) - 1, StringComparison.CurrentCultureIgnoreCase) + 1;
        }

        public static int? PatIndex(string textPattern, string target)
        {
            if (textPattern == null)
                return null;
            if (target == null)
                return null;

            var pattern = Regex.Escape(textPattern)
                               .Replace("_", ".")
                               .Replace("\\[.]", "_")
                               .Replace("%", ".*")
                               .Replace("\\[.*]", "%")
                               .Replace("\\[\\[]", "\\[");

            var match = Regex.Match(target, "(?i)^" + pattern + "$");
            return match.Success ? match.Index + 1 : 0;
        }

        // TODO: implement more SQL functions
    }
}

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