EndExecuteReader method

Finishes asynchronous execution of a SQL statement, returning the requested ULDataReader.

Syntax

Visual Basic

Public Function EndExecuteReader( _
   ByVal asyncResult As IAsyncResult _
) As ULDataReader

C#

public ULDataReader EndExecuteReader(
   IAsyncResult asyncResult
);

Parameters

asyncResult The System.IAsyncResult returned by the call to BeginExecuteReader.

Return value

An ULDataReader object that can be used to retrieve the requested rows (the same behavior as ExecuteReader).

Remarks

You must call EndExecuteReader once for every call to BeginExecuteReader. The call must be after BeginExecuteReader has returned. ADO.NET is not thread safe; it is your responsibility to ensure that BeginExecuteReader has returned. The System.IAsyncResult passed to EndExecuteReader must be the same as the one returned from the BeginExecuteReader call that is being completed. It is an error to call EndExecuteReader to end a call to BeginExecuteNonQuery, and vice versa.

If an error occurs while executing the command, the exception is thrown when EndExecuteReader is called.

There are four ways to wait for execution to complete:

Call EndExecuteReader Calling EndExecuteReader blocks until the command completes. For example:

' Visual Basic
Dim cmd As ULCommand = new ULCommand( _
    "SELECT * FROM Departments", conn _
  )
Dim res As IAsyncResult res = _
  cmd.BeginExecuteReader()
' perform other work
' this will block until the command completes
Dim reader As ULDataReader = _
  cmd.EndExecuteReader( res )

// C#
ULCommand cmd = new ULCommand(
    "SELECT * FROM Departments", conn
  );
IAsyncResult res = cmd.BeginExecuteReader();
// perform other work
// this will block until the command completes
ULDataReader reader = cmd.EndExecuteReader( res );

Poll the IsCompleted property of the IAsyncResult You can poll the IsCompleted property of the IAsyncResult. For example:

' Visual Basic
Dim cmd As ULCommand = new ULCommand( _
    "SELECT * FROM Departments", conn _
  )
Dim res As IAsyncResult res = _
  cmd.BeginExecuteReader()
While( !res.IsCompleted )
  ' do other work
End While
' this will block until the command completes
Dim reader As ULDataReader = _
  cmd.EndExecuteReader( res )

// C#
ULCommand cmd = new ULCommand(
    "SELECT * FROM Departments", conn
  );
IAsyncResult res = cmd.BeginExecuteReader();
while( !res.IsCompleted ) {
  // do other work
}
// this will block until the command completes
ULDataReader reader = cmd.EndExecuteReader( res );

Use the IAsyncResult.AsyncWaitHandle property to get a synchronization object You can use the IAsyncResult.AsyncWaitHandle property to get a synchronization object, and wait on that. For example:

' Visual Basic
Dim cmd As ULCommand = new ULCommand( _
    "SELECT * FROM Departments", conn _
  )
Dim res As IAsyncResult res = _
  cmd.BeginExecuteReader()
' perform other work
Dim wh As WaitHandle = res.AsyncWaitHandle
wh.WaitOne()
' this will not block because the command is finished
Dim reader As ULDataReader = _
  cmd.EndExecuteReader( res )

// C#
ULCommand cmd = new ULCommand(
    "SELECT * FROM Departments", conn
  );
IAsyncResult res = cmd.BeginExecuteReader();
// perform other work
WaitHandle wh = res.AsyncWaitHandle;
wh.WaitOne();
// this will not block because the command is finished
ULDataReader reader = cmd.EndExecuteReader( res );

Specify a callback function when calling BeginExecuteReader You can specify a callback function when calling BeginExecuteReader. For example:

' Visual Basic
Private Sub callbackFunction(ByVal ar As IAsyncResult)
  Dim cmd As ULCommand = _
    CType(ar.AsyncState, ULCommand)
  ' this won’t block since the command has completed
  Dim reader As ULDataReader = cmd.EndExecuteReader()
End Sub

' elsewhere in the code
Private Sub DoStuff() 
  Dim cmd As ULCommand = new ULCommand( _
      "SELECT * FROM Departments", conn _
    )
  Dim res As IAsyncResult = _
    cmd.BeginExecuteReader( _
        callbackFunction, cmd _
      )
  ' perform other work.  The callback function 
  ' will be called when the command completes
End Sub

// C#
private void callbackFunction( IAsyncResult ar )
{
  ULCommand cmd = (ULCommand) ar.AsyncState;
  // this won’t block since the command has completed
  ULDataReader reader = cmd.EndExecuteReader();
}

// elsewhere in the code
private void DoStuff() 
{
  ULCommand cmd = new ULCommand(
      "SELECT * FROM Departments", conn
    );
  IAsyncResult res = cmd.BeginExecuteReader(
      callbackFunction, cmd
    );
  // perform other work.  The callback function 
  // will be called when the command completes
}

The callback function executes in a separate thread, so the usual caveats related to updating the user interface in a threaded program apply.