FromAsyncPattern和SQLDataReader

本文介绍了FromAsyncPattern和SQLDataReader的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 您好 我正在尝试使用上面的模式来包装sqldatareader。下面的最后一行导致问题使用cmd = connection.CreateCommand cmd.CommandType = CommandType.Text cmd.CommandText = script.ToSql（New SqlServerWriter） connection.Open（） Dim asyncReader = Observable.FromAsyncPattern（Of SqlDataReader）（cmd.BeginExecuteReader，cmd.EndExecuteReader）因为VB编译器抱怨EndExecuteReader需要参数asyncResult的参数。我已经检查了几个C＃示例，这些示例似乎没有这样的参数。  一定是显而易见的我缺少的东西。如果有人可以在这里指出我正确的VB实现，我将不胜感激。 很多thx S 解决方案   > VB编译器抱怨EndExecuteReader需要参数asyncResult的参数 b 根据文档，这里是 EndExecuteReader 的签名： http：// msdn。 microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.endexecutereader.aspx public SqlDataReader EndExecuteReader（IAsyncResult asyncResult）显然，有一个 IAsyncResult 参数。  （我检查了.NET Framework的所有版本以确保，因为您没有指定版本。） 所以问题必须是 FromAsyncPattern重载决议。 唯一的另一个参数是 BeginExecuteReader ，所以我们来看看。  每个版本的.NET  Framework都有4个。重载&NBSP;以下是根据文档签名： http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.beginexecutereader.aspx IAsyncResult BeginExecuteReader（） IAsyncResult BeginExecuteReader（CommandBehavior） IAsyncResult BeginExecuteReader（AsyncCallback，Object） IAsyncResult BeginExecuteReader（AsyncCallback，Object，CommandBehavior）后两者符合APM的要求。  或许他们对 FromAsyncPattern 的重载解析不明确，或者编译器只是选择错了。 那么让我们看看 FromAsyncPattern 的重载是候选者...... 你指定了一个泛型类型参数，所以这里是签名（根据文档， FromAsyncPattern 具有单一泛型类型参数的退货类型） http://msdn.microsoft.com/en-us/library/hh229919(v=vs.103).aspx\">http://msdn.microsoft.com/en-us/library/hh229919(v=vs。 103）.aspx FromAsyncPattern< TResult>（Func< AsyncCallback，Object，IAsyncResult>，Func< IAsyncResult，TResult>） FromAsyncPattern< ; T1>（Func< T1，AsyncCallback，Object，IAsyncResult>，Action< IAsyncResult>）查看参数的数量和顺序，似乎只有第一个签名是有效的匹配，它只能对应的第三个签名 BeginExecuteReader ： IAsyncResult BeginExecuteReader（AsyncCallback，Object）请注意，第二个签名与的第四个签名 BeginExecuteReader 因为 CommandBehavior 位于错误的位置，这会将 对象放在需要 AsyncCallback 的位置，尽管它具有正确的数量泛型类型参数。 所以看起来我们已经获胜了。 但令我惊讶的是，它还匹配 EndExecuteReader 的签名。 IAsyncResult BeginExecuteReader（AsyncCallback，Object） SqlDataReader EndExecuteReader（IAsyncResult asyncResult） FromAsyncPattern< TResult>（Func< AsyncCallback，Object，IAsyncResult>，Func< IAsyncResult，TResult>）显然，我没有不要在Visual Studio中尝试这个。 如果将鼠标光标放在 FromAsyncPattern 上，那么IntelliSense显示编译器正在选择哪个重载？ - Dave Hi I'm trying to use the above pattern to wrap the sqldatareader. The last line below is causing a problemUsing cmd = connection.CreateCommand cmd.CommandType = CommandType.Text cmd.CommandText = script.ToSql(New SqlServerWriter) connection.Open() Dim asyncReader = Observable.FromAsyncPattern(Of SqlDataReader)(cmd.BeginExecuteReader, cmd.EndExecuteReader)since the VB compiler is complaining that EndExecuteReader needs an argument for parameter asyncResult. I have checked several C# examples which don't seem to have such a parameter. Must be something obviously I'm missing. If someone could point me to a correct VB implementation here, I'd be grateful.Many thxS 解决方案 Hi, > the VB compiler is complaining that EndExecuteReader needs an argument for parameter asyncResultHere's the signature of EndExecuteReader according to the documentation:http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.endexecutereader.aspxpublic SqlDataReader EndExecuteReader(IAsyncResult asyncResult)Clearly there's an IAsyncResult parameter.  (I checked all versions of the .NET Framework to be sure, since you didn't specify a version.)So the problem must be with overload resolution of FromAsyncPattern.The only other argument is BeginExecuteReader, so let's have a look at that.  Each version of the .NET Framework has 4 overloads.  Here are the signatures according to the documentation:http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.beginexecutereader.aspxIAsyncResult BeginExecuteReader()IAsyncResult BeginExecuteReader(CommandBehavior)IAsyncResult BeginExecuteReader(AsyncCallback, Object)IAsyncResult BeginExecuteReader(AsyncCallback, Object, CommandBehavior)The latter two meet the requirements for the APM.  Perhaps they're ambiguous for overload resolution ofFromAsyncPattern, or the compiler is simply choosing the wrong one.  So let's see what overloads ofFromAsyncPattern are candidates...You're specifying a single generic type argument, so here are the signatures (excluding return types) ofFromAsyncPattern that have a single generic type argument according to the documentation:http://msdn.microsoft.com/en-us/library/hh229919(v=vs.103).aspxFromAsyncPattern<TResult>(Func<AsyncCallback, Object, IAsyncResult>, Func<IAsyncResult, TResult>)FromAsyncPattern<T1>(Func<T1, AsyncCallback, Object, IAsyncResult>, Action<IAsyncResult>)Looking at the number and order of the parameters, it seems that only the first signature is a valid match and it can only correspond to the third signature ofBeginExecuteReader: IAsyncResult BeginExecuteReader(AsyncCallback, Object)Note that the second signature doesn't match the fourth signature ofBeginExecuteReader because CommandBehavior is in the wrong place, which would putObject where AsyncCallback is expected, though it has the correct number of generic type arguments.So it looks like we've got our winner.  But to my surprise, it also matches the signature ofEndExecuteReader.IAsyncResult BeginExecuteReader(AsyncCallback, Object)SqlDataReader EndExecuteReader(IAsyncResult asyncResult)FromAsyncPattern<TResult>(Func<AsyncCallback, Object, IAsyncResult>, Func<IAsyncResult, TResult>)Obviously, I didn't try this in Visual Studio.  If you place the mouse cursor overFromAsyncPattern, which overload does IntelliSense show that the compiler is choosing?- Dave 这篇关于FromAsyncPattern和SQLDataReader的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！