为什么实体框架会忽略 TransactionScope（不使用 NOLOCK 添加）？

Question

我错过了什么？

我正在尝试使用这样的 TransactionScope 使用 NOLOCK 进行读取：

var scopeOptions = new TransactionOptions { IsolationLevel = IsolationLevel.ReadUncommitted };
using (var scope = new TransactionScope(TransactionScopeOption.Required, scopeOptions))
{
   using (var db = new MyDbContext(ConnectionStringEntities))
   {
      // Simple read with a try catch block...
   }
   scope.Complete();
}

我希望看到 NOLOCK 添加到 SQL 查询（查看 SQL Profiler 和自定义 DbCommandInterceptor - 但它不存在......

更新：经过更多研究，我想知道是否最终使用了选定的游标，只是没有 NOLOCK“提示”（特定于 SQL Server - 也特定于一个表），我发现了一些获取当前事务的代码，它似乎显示了正确选择的事务隔离（ReadUncommitted / Serializable 等）我仍然想测试它，但如果您有任何想法，请告诉我

Get current .net TransactionScope IsolationLevel

Transaction trans = Transaction.Current;
System.Transactions.IsolationLevel level = trans.IsolationLevel;
LogService.Instance.Debug($"Transaction IsolationLevel = {level.ToString()}");

Answer 1

所以看起来实体框架确实尊重 IsolationLevel，只是它不使用 NOLOCK 提示（可能是因为它太特定于数据库），顺便说一下，我对 EF 的主要抱怨 - 它没有针对不同的数据库类型，另一个例子是新身份将 AspNetUsers 的 GUID 主键保存为字符串（再次因为缺乏优化），除此之外（以及其他一些事情）EF 很棒！

我在任何地方都找不到解决问题的方法，我绝对不想让我的所有查询都使用 NOLOCK - 只是未提交的查询，所以我最终结合了两个解决方案（进行了一些更改）：

1. NoLockInterceptor - 用于即时添加 NOLOCK (Entity Framework with NOLOCK)：

   /// <summary>
   /// Add "WITH (NOLOCK)" hint to SQL queries, SQL Server specifc - may break queries on different databases.
   /// (conditionally turn off with NoLockInterceptor.AddNoLockHintToSqlQueries = false to change on runtime)
   /// <para>
   /// https://stackoverflow.com/questions/926656/entity-framework-with-nolock
   /// </para>
   /// </summary>
   public class NoLockInterceptor : DbCommandInterceptor
   {
       private static readonly Regex TableAliasRegex = new Regex(
           @"(?<tableAlias>AS \[Extent\d+\](?! WITH \(NOLOCK\)))",
           RegexOptions.Multiline | RegexOptions.IgnoreCase);
   
       /// <summary>
       /// Add "WITH (NOLOCK)" hint to SQL queries - unique to each thread 
       /// (set to true only when needed and then back to false)
       /// </summary>
       [ThreadStatic]
       public static bool AddNoLockHintToSqlQueries;
   
       public NoLockInterceptor()
       {
           // Do not use by default for all queries
           AddNoLockHintToSqlQueries = false;
       }
   
       public override void ScalarExecuting(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
       {
           if (AddNoLockHintToSqlQueries)
           {
               command.CommandText = TableAliasRegex.Replace(command.CommandText, "${tableAlias} WITH (NOLOCK)");
           }
       }
   
       public override void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
       {
           if (AddNoLockHintToSqlQueries)
           {
               command.CommandText = TableAliasRegex.Replace(command.CommandText, "${tableAlias} WITH (NOLOCK)");
           }
       }
   }
   

2. TransactionWrapper - 调用 NoLockInterceptor 行为，也可用于重复使用事务 (http://haacked.com/archive/2009/08/18/simpler-transactions.aspx/)：

   /// <summary>
   /// Transaction wrapper for setting pre-defined transaction scopes
   /// <para>
   /// http://haacked.com/archive/2009/08/18/simpler-transactions.aspx/
   /// </para>
   /// </summary>
   public static class TransactionWrapper
   {
       /// <summary>
       /// Set transaction scope and using NoLockInterceptor for adding SQL Server specific "WITH (NOLOCK)" 
       /// to ReadUncommitted isolation level transactions (not supported by Entity Framework)
       /// </summary>
       /// <param name="isolationLevel"></param>
       /// <param name="transactionScopeOption"></param>
       /// <param name="timeout"></param>
       /// <param name="action"></param>
       public static void SetScope(IsolationLevel isolationLevel, TransactionScopeOption transactionScopeOption,
           TimeSpan timeout, Action action)
       {
           var transactionOptions = new TransactionOptions { IsolationLevel = isolationLevel, Timeout = timeout };
           using (var transactionScope = new TransactionScope(transactionScopeOption, transactionOptions))
           {
               if (isolationLevel == IsolationLevel.ReadUncommitted)
                   NoLockInterceptor.AddNoLockHintToSqlQueries = true;
   
               action();
               transactionScope.Complete();
   
               if (isolationLevel == IsolationLevel.ReadUncommitted)
                   NoLockInterceptor.AddNoLockHintToSqlQueries = false;
           }
       }
   }
   

像这样使用它：

var timeout = TimeSpan.FromSeconds(ConfigVariables.Instance.Timeout_Transaction_Default_In_Seconds);
TransactionWrapper.SetScope(IsolationLevel.ReadUncommitted, TransactionScopeOption.Required, timeout, () =>
{
    using (var db = new MyDbContext(MyDbContextConnectionStringEntities))
    {
       // Do stuff...
    }
});

NOLOCK 现在仅添加到具有 ReadUncommitted 事务隔离级别范围的查询。

Answer 2

您无法让实体框架呈现 NOLOCK 提示。如果你想读取未提交的数据，你必须做一些不同的事情，比如将带有 IsolationLevel.ReadUncommited 的 TransactionScope 添加到 TransactionOptions。

编写您自己的命令拦截器或您自己的 EF 提供程序也可以。

https://msdn.microsoft.com/en-us/data/dn469464.aspx

Answer 3

我已经尝试过事务范围，然后它会分析对数据库的调用。 EF 开始和结束事务，但从不更改已提交读的隔离级别。

            using (var scope = new TransactionScope(
            TransactionScopeOption.Required,
            new TransactionOptions()
            {
                IsolationLevel = System.Transactions.IsolationLevel.ReadUncommitted
            }))
        {
            List<T> toReturn = query.ToList();
            scope.Complete();
            return toReturn;
        }