Question

我缺少什么？

我正在尝试使用像这样的TransactionScope读取NOLOCK：

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查询中（查看SQL Profiler以及自定义DbCommandInterceptor - 但它不在那里......

更新：经过一些研究，我想知道所选的光标是否正在被使用，只是没有NOLOCK“提示”（特定于SQL Server - 并且也只针对一个表），我找到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Entity Framework确实尊重IsolationLevel，只是它没有使用NOLOCK提示（可能是因为它太具有数据库特性）而这就是我对EF的主要抱怨 - 它不是非常适合不同的数据库类型，另一个例子是新标识将AspNetUsers的GUID主键保存为字符串（再次缺乏优化）除此之外（以及其他一些事情）EF非常棒！

我无法在任何地方找到问题的解决方案，我绝对不想让我的所有查询都使用NOLOCK - 只是未提交的，所以我最终结合了两个解决方案（有一些更改）：

NoLockInterceptor - 用于动态添加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)");
        }
    }
}

TransactionWrapper - 调用NoLockInterceptor行为，对重复使用事务（http://haacked.com/archive/2009/08/18/simpler-transactions.aspx/）也很有用：

/// <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：

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中。

Answer 2

您无法获取实体框架来呈现NOLOCK提示。如果要读取未提交的数据，则必须执行与将TransactionScope与IsolationLevel.ReadUncommited添加到TransactionOptions时所做的不同的操作。

编写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

我已经尝试了事务范围，然后分析了对数据库的调用。 EF开始和结束事务，但永远不会从Read Committed更改隔离级别。

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

为什么Entity Framework忽略TransactionScope（不添加NOLOCK）？

3 个答案: