Tài liệu Filling a DataSet Asynchronously pdf

Solution Create a background thread and use it to run a query to fill a DataSet.. The sample code contains one event handler and two methods: Go Button.Click Checks whether there is an

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Recipe 9.1 Filling a DataSet Asynchronously

Problem

Given some database queries that return large result sets and cause the calling application

to be unresponsive, you need to make the application more responsive during the fill

Solution

Create a background thread and use it to run a query to fill a DataSet You can also fill a DataGrid object from a background thread

The sample code contains one event handler and two methods:

Go Button.Click

Checks whether there is an existing background thread loading the DataSet If the DataSet is not being loaded, a new thread is created invoking the

AsyncFillDataSet( ) method to fill a DataSet Otherwise, a message is displayed stating that the DataSet is currently being filled

AsyncFillDataSet( )

This method loads a DataSet with the Orders and Order Details tables from the Northwind database The BindDataSetToDataGrid( ) method is called

asynchronously on the form's thread to display the results Finally, messages are displayed to indicate that the AsyncFillDataSet( ) method has started and when it has completed

BindDataSetToDataGrid( )

This method binds the default view of the Orders table in the DataSet to the data grid on the form

The C# code is shown in Example 9-1

Example 9-1 File: AsynchronousFillForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Threading;

using System.Runtime.Remoting.Messaging;

using System.Data;

using System.Data.SqlClient;

private delegate void BindDataSetToDataGridDelegate(DataSet ds);

// Table name constants

private const String ORDERS_TABLE = "Orders";

private const String ORDERDETAILS_TABLE = "OrderDetails";

// Relation name constants

private const String ORDERS_ORDERDETAILS_RELATION = "Orders_OrderDetails_Relation";

// Field name constants

private const String ORDERID_FIELD = "OrderID";

private const String ORDERDATE_FIELD = "OrderDate";

private bool isWorking = false;

//

private delegate void BindDataSetToDataGridDelegate(DataSet ds);

private void goButton_Click(object sender, System.EventArgs e)

{

// Check if the DataSet is already being filled

if (!isWorking)

{

isWorking = true;

// Clear the data grid

resultDataGrid.DataSource = null;

// Create and start a new thread to fill the DataSet

Thread thread = new Thread(new ThreadStart(AsyncFillDataSet)); thread.Start( );

}

else

{

// DataSet already being filled Display a message

statusTextBox.Text += "DataSet still filling " +

Environment.NewLine;

}

}

private void AsyncFillDataSet( )

{

statusTextBox.Text = "Filling DataSet " + Environment.NewLine;

DataSet ds = new DataSet("Source");

SqlDataAdapter da;

// Fill the Order table and add it to the DataSet

da = new SqlDataAdapter("SELECT * FROM Orders",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable orderTable = new DataTable(ORDERS_TABLE);

da.FillSchema(orderTable, SchemaType.Source);

da.Fill(orderTable);

ds.Tables.Add(orderTable);

// Fill the OrderDetails table and add it to the DataSet

da = new SqlDataAdapter("SELECT * FROM [Order Details]",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable orderDetailTable = new DataTable(ORDERDETAILS_TABLE); da.FillSchema(orderDetailTable, SchemaType.Source);

da.Fill(orderDetailTable);

ds.Tables.Add(orderDetailTable);

// Create a relation between the tables

ds.Relations.Add(ORDERS_ORDERDETAILS_RELATION,

ds.Tables[ORDERS_TABLE].Columns[ORDERID_FIELD],

ds.Tables[ORDERDETAILS_TABLE].Columns[ORDERID_FIELD], true);

statusTextBox.Text += "DataSet Fill complete." + Environment.NewLine;

// Call the BindDataSetToDataGrid method asynchronously

// on the Form's thread

this.BeginInvoke(

new BindDataSetToDataGridDelegate(BindDataSetToDataGrid),

new object[] {ds});

// Set flag indicating that the async fill is complete

isWorking = false;

}

private void BindDataSetToDataGrid(DataSet ds)

{

// Bind the default view of the Orders table to the data grid

resultDataGrid.DataSource = ds.Tables[ORDERS_TABLE].DefaultView;

}

Discussion

When a synchronous call is made, the caller thread is blocked until the call completes An asynchronous call returns immediately, freeing the calling thread to continue with its work while a new thread is created to run the method in parallel

A new instance of a Thread is initialized using a constructor that takes a ThreadStart delegate argument, which references the method to be executed when the Thread starts executing The Start( ) method of the Thread changes the state of the Thread to

ThreadState.Running allowing the operating system to schedule it for execution Once it begins executing, the ThreadStart delegate supplied in the Thread constructor invokes its method

Windows Form or control methods, such as a DataGrid, cannot be called on any thread other than the one that created the form or control because they are based on a single-threaded apartment (STA) model Method calls from other threads must be marshaled to the creation thread This can be done asynchronously by calling the BeginInvoke( )

method of the form, forcing the method to be executed on the thread that created the form

or control

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