BeginExecuteReader(CommandBehavior) Method

In This Topic

Starts an asynchronous invocation of an System.Data.Common.DbCommand.ExecuteReader method.

Syntax

Visual Basic
C#


'Declaration Public Overloads Function BeginExecuteReader( _ ByVal behavior As CommandBehavior _ ) As IAsyncResult


public IAsyncResult BeginExecuteReader( CommandBehavior behavior )

Parameters

behavior: One of the System.Data.CommandBehavior values.

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 PostgreSQL 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(CommandBehavior.Default); 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(); } }

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(CommandBehavior.Default);
  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(CommandBehavior.Default) 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

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(CommandBehavior.Default)
  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

Reference

DbCommandBase Class
DbCommandBase Members
Overload List

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