Add functionality to verify mapping similar to FluentNHibernate's PersistenceSpecification class.

DataContractExtensions.cs

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Transactions;
using FluentAssertions;

// ReSharper disable CheckNamespace
namespace System.Data.Linq
// ReSharper restore CheckNamespace
{
    public static class DataContextExtensions
    {
        public static void VerifyTheMapping<T>(this DataContext dataContext, T first, Expression<Func<T, bool>> identity) where T : class
        {
            using (var transaction = new TransactionScope(TransactionScopeOption.RequiresNew))
            {
                var table = dataContext.GetTable<T>();
                table.InsertOnSubmit(first);
                dataContext.SubmitChanges();
    
                dataContext.Refresh(RefreshMode.OverwriteCurrentValues, first);
                var second = table.First(identity);
    
                second.ShouldHave().AllProperties().EqualTo(first);
                
                transaction.Dispose();
            }
        }
    }
}

WidgetFacts.cs

using System;
using System.Collections.Generic;
using System.Data.Linq;
using System.Linq;
using FizzWare.NBuilder;
using MyApp.Domain;
using Xunit;

namespace MyApp.IntegrationTests.Persistence
{
    public class WidgetFacts : IUseFixture<DatabaseFixture>
    {
        public void SetFixture(DatabaseFixture data) { }

        [Fact]
        public void CanPersistWidget()
        {
            using (var dataContext = new AppDataContext(DatabaseFixture.ConnectionString))
            {
                var entity = Builder<Widget>.CreateNew().Build();

                dataContext.VerifyTheMapping(entity, widget => widget.Id == entity.Id);
            }
            
        }
    }
}