BeginExecuteReader(AsyncCallback,Object) Method

In This Topic

Starts an asynchronous invocation of an System.Data.Common.DbCommand.ExecuteReader method with information for callback function.

Syntax

Visual Basic
C#

'Declaration
 
Public Overloads Function BeginExecuteReader( _
   ByVal callback As AsyncCallback, _
   ByVal stateObject As Object _
) As IAsyncResult

public IAsyncResult BeginExecuteReader( 
   AsyncCallback callback,
   object stateObject
)

Parameters

callback: The delegate to call when the asynchronous invoke is complete. If callback is a null reference (Nothing in Visual Basic), the delegate is not called.
stateObject: State information that is passed on to the delegate.

Return Value

An System.IAsyncResult interface that represents the asynchronous operation started by calling this method.

Remarks

BeginExecuteReader method enables you to execute a query on a server without having current thread blocked. In other words, your program can continue execution while the query runs on SQL Server so you do not have to wait for it.

Refer to "Asynchronous Query Execution" article for detailed information.

To start running a query, you have to call BeginExecuteReader method, which in turn invokes appropriate actions in another thread. Return value of this method must be assigned to an System.IAsyncResult object. After executing this method, the program flow continues.

When you are ready to accept query results, call EndExecuteReader. If at the moment you call this function the query execution has not yet been finished, application stops and waits till the function returns. Then you can treat a DbDataReaderBase in a common way.

Example

In the example below the easiest way of executing an asynchronous operation is shown. First, all components are set up and a connection is opened. Second, a data retrieval operation is started. The operation state (done or in progress) is checked and a report is written to the console. At last query results are sent to DbDataReaderBase and rendered to the console.

csharp

public void Async_Exec(DbConnectionBase myConnection)
{
  DbCommandBase myCommand = (DbCommandBase)myConnection.CreateCommand();
  myCommand.CommandText = "SELECT DeptNo, DName, Loc FROM Test.Dept";
  Console.WriteLine("Starting asynchronous retrieval of data...");
  myConnection.Open();
  IAsyncResult cres = myCommand.BeginExecuteReader(null, null);
  if (cres.IsCompleted)
    Console.WriteLine("Completed.");
  else
    Console.WriteLine("Have to wait for operation to complete...");
  DbDataReader myReader = myCommand.EndExecuteReader(cres);
  try
  {
    while (myReader.Read())
    {
      Console.WriteLine(myReader.GetInt32(0).ToString() + " " + myReader.GetString(1) + " " + myReader.GetString(2));
    }
  }
  finally
  {
    myReader.Close();
    myConnection.Close();
  }
}

Visual Basic

Public Sub Async_Exec(ByVal myConnection As DbConnectionBase)
  Dim myCommand As DbCommandBase = myConnection.CreateCommand()
  myCommand.CommandText = "SELECT DeptNo, DName, Loc FROM Test.Dept"
  Console.WriteLine("Starting asynchronous retrieval of data...")
  myConnection.Open()
  Dim cres As IAsyncResult = myCommand.BeginExecuteReader(Nothing, Nothing)
  If cres.IsCompleted Then
    Console.WriteLine("Completed.")
  Else
    Console.WriteLine("Have to wait for operation to complete...")
  End If
  Dim myReader As DbDataReader = myCommand.EndExecuteReader(cres)
  Try
    While myReader.Read()
      Console.WriteLine(String.Concat(myReader.GetInt32(0).ToString(), " ", myReader.GetString(1), " ", myReader.GetString(2)))
    End While
  Finally
    myReader.Close()
    myConnection.Close()
  End Try
End Sub

Requirements

Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference

DbCommandBase Class
DbCommandBase Members
Overload List

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