EndExecuteReader( IAsyncResult ) method

Visual Basic syntax

Public Function EndExecuteReader (ByVal asyncResult As IAsyncResult ) As SADataReader

C# syntax

public SADataReader EndExecuteReader ( IAsyncResult asyncResult)

Parameters

• asyncResult – The IAsyncResult returned by the call to SACommand.BeginExecuteReader.

Returns

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

Exceptions

• ArgumentException – The asyncResult parameter is null (Nothing in Microsoft Visual Basic)
• InvalidOperationException – The SACommand.EndExecuteReader(IAsyncResult) was called more than once for a single command execution, or the method was mismatched against its execution method.

Usage

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 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:

(1) Call EndExecuteReader.

Calling EndExecuteReader blocks until the command completes. For example:

                 
                    SAConnection conn = new SAConnection("DSN=SQL Anywhere 16 Demo");
conn.Open();
SACommand cmd = new SACommand( "SELECT * FROM Departments", conn );
IAsyncResult res = cmd.BeginExecuteReader();
// perform other work
// this blocks until the command completes
SADataReader reader = cmd.EndExecuteReader( res );

(2) Poll the IsCompleted property of the IAsyncResult.

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

                 
                    SAConnection conn = new SAConnection("DSN=SQL Anywhere 16 Demo");
conn.Open();
SACommand cmd = new SACommand( "SELECT * FROM Departments", conn );
IAsyncResult res = cmd.BeginExecuteReader();
while( !res.IsCompleted ) {
    // do other work
}
// this does not block because the command is finished
SADataReader reader = cmd.EndExecuteReader( res );

(3) 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:

                 
                    SAConnection conn = new SAConnection("DSN=SQL Anywhere 16 Demo");
conn.Open();
SACommand cmd = new SACommand( "SELECT * FROM Departments", conn );
IAsyncResult res = cmd.BeginExecuteReader();
// perform other work
WaitHandle wh = res.AsyncWaitHandle;
wh.WaitOne();
// this does not block because the command is finished
SADataReader reader = cmd.EndExecuteReader( res );

(4) Specify a callback function when calling BeginExecuteReader

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

                 private void callbackFunction( IAsyncResult ar ) {
    SACommand cmd = (SACommand) ar.AsyncState;
    // this does not block since the command has completed
    SADataReader reader = cmd.EndExecuteReader();
}
              


                  // elsewhere in the code
private void DoStuff() {
    SAConnection conn = new SAConnection("DSN=SQL Anywhere 16 Demo");
    conn.Open();
    SACommand cmd = new SACommand( "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.