SqlCommand.BeginExecuteReader Method (CommandBehavior)
Assembly: System.Data (in System.Data.dll)
[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>] member BeginExecuteReader : behavior:CommandBehavior -> IAsyncResult
Any error that occurred while executing the command text.
A timeout occurred during a streaming operation. For more information about streaming, see SqlClient Streaming Support.
The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.
The BeginExecuteReader method starts the process of asynchronously executing a Transact-SQL statement or stored procedure that returns rows, so that other tasks can run concurrently while the statement is executing. When the statement has completed, developers must call the EndExecuteReader method to finish the operation and retrieve the SqlDataReader returned by the command. The BeginExecuteReader method returns immediately, but until the code executes the corresponding EndExecuteReader method call, it must not execute any other calls that start a synchronous or asynchronous execution against the same SqlCommand object. Calling the EndExecuteReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.
The behavior parameter lets you specify options that control the behavior of the command and its connection. These values can be combined together (using the programming language's OR operator); generally, developers use the CommandBehavior.CloseConnection value to make sure that the connection is closed by the runtime when the SqlDataReader is closed.
Note that the command text and parameters are sent to the server synchronously. If a large command or many parameters are sent, this method may block during writes. After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous. Although command execution is asynchronous, value fetching is still synchronous. This means that calls to Read may block if more data is required and the underlying network's read operation blocks.
Because this overload does not support a callback procedure, developers must either poll to determine whether the command has completed, using the IsCompleted property of the IAsyncResult returned by the BeginExecuteNonQuery method; or wait for the completion of one or more commands using the AsyncWaitHandle property of the returned IAsyncResult.
If you use ExecuteReader or BeginExecuteReader to access XML data, SQL Server returns any XML results greater than 2,033 characters in length in multiple rows of 2,033 characters each. To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries. For more information, see article Q310378, "PRB: XML Data Is Truncated When You Use SqlDataReader," in the Microsoft Knowledge Base at http://support.microsoft.com.
The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.
The following console application starts the process of retrieving a data reader asynchronously. While waiting for the results, this simple application sits in a loop, investigating the IsCompleted property value. Once the process has completed, the code retrieves the SqlDataReader and displays its contents.
This example also passes the CommandBehavior.CloseConnection and CommandBehavior.SingleRow values in the behavior parameter, causing the connection to be closed with the returned SqlDataReader is closed, and to optimize for a single row result.