Windows Forms Documentation Updates : Data Binding

Maintaining position in a child table

cherylws — Tue, 06 Feb 2007 18:36:00 GMT

I've recently written a document covering some changes to the CurrencyManager in .NET Framework version 2.0. I'll summarize the document here, as it will not be published for a few months, and this subtle change in behavior could be causing problems in .NET Framework 1.1 applications.

Starting with the .NET Framework version 2.0, when you display data in a parent/child view you might have to take extra steps to make sure that the currently selected row in the child table is not reset to the first row of the table. In order to do this, you will have to cache the child table position and reset it after the parent table changes. Typically the child reset occurs the first time a field in a row of the parent table changes. Following are the steps to cache and reset the child position:

First cache the child position:

1. Declare an integer variable to store the child list position and a Boolean variable to store whether to cache the child position.

private int cachedPosition = -1;

private bool cacheChildPosition = true;

2. Handle the CurrencyManager.ListChanged event for the child binding's CurrencyManager and check for a ListChangedType of ListChangedType.Reset.

3. Check the current position of the CurrencyManager. If it is greater than first entry in the list (typically 0), save it to a variable.

void relatedCM_ListChanged(object sender, ListChangedEventArgs e)

{

// Check to see if this is a caching situation.

if (cacheChildPosition)

{

// If so, check to see if it is a reset situation,

// and the current position is greater than zero.

CurrencyManager relatedCM = sender as CurrencyManager;

if (e.ListChangedType == ListChangedType.Reset &&

relatedCM.Position > 0)

// If so, cache the position of the child table.

cachedPosition = relatedCM.Position;

}

4. Handle the parent list's CurrentChanged event for the parent currency manager. In the handler, set the Boolean value to indicate it is not a caching scenario. If the CurrentChanged occurs, the change to the parent is a list position change and not an item value change.

void bindingSource1_CurrentChanged(object sender, EventArgs e)

{

// If the CurrentChanged event occurs, this is not a caching

// situation.

cacheChildPosition = false;

}

Then reset the child position, if necessary:

1. Handle the PositionChanged event for the child binding's CurrencyManager.

2. Reset the child table position to the cached position saved in the previous procedure.

void relatedCM_PositionChanged(object sender, EventArgs e)

{

// Check to see if this is a caching situation.

if (cacheChildPosition)

{

CurrencyManager relatedCM = sender as CurrencyManager;

// If so, check to see if the current position is

// not equal to the cached position and the cached

// position is not out of bounds.

if (relatedCM.Position != cachedPosition && cachedPosition

> 0 && cachedPosition < relatedCM.Count)

{

relatedCM.Position = cachedPosition;

cachedPosition = -1;

}

Following is the code for a sample application that demonstrates the reset behavior and the cache and reset of the child position.

To run this example in Visual Studio, you'll need to paste it into a blank project, or you can compile it on the command line.

When you run the sample, click the Clear parent field button to cause a change in a field of the parent table. Notice that the selected row in the child table does not change. Then, close and run the example again. (You need to do this because the reset behavior occurs only on the first change in a parent row.) Clear the Cache and reset position check box and click the Clear parent field button. Notice that the selected row in the child table changes to the first row.

using System;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Text;

using System.Windows.Forms;

namespace BT2

{

public class Form1 : Form

{

public Form1()

{

InitializeControlsAndDataSource();

}

// Declare the controls to be used.

private BindingSource bindingSource1;

private DataGridView dataGridView1;

private Button button1;

private DataGridView dataGridView2;

private CheckBox cachePositionCheckBox;

public DataSet set1;

private void InitializeControlsAndDataSource()

{

// Initialize the controls and set location, size and

// other basic properties.

this.dataGridView1 = new DataGridView();

this.bindingSource1 = new BindingSource();

this.button1 = new Button();

this.dataGridView2 = new DataGridView();

this.cachePositionCheckBox =

new System.Windows.Forms.CheckBox();

this.dataGridView1.ColumnHeadersHeightSizeMode =

DataGridViewColumnHeadersHeightSizeMode.AutoSize;

this.dataGridView1.Dock = DockStyle.Top;

this.dataGridView1.Location = new Point(0, 20);

this.dataGridView1.Size = new Size(292, 170);

this.button1.Location =

new System.Drawing.Point(18, 175);

this.button1.Size = new System.Drawing.Size(125, 23);

button1.Text = "Clear Parent Field";

this.button1.Click +=

new System.EventHandler(this.button1_Click);

this.dataGridView2.ColumnHeadersHeightSizeMode =

DataGridViewColumnHeadersHeightSizeMode.AutoSize;

this.dataGridView2.Location =

new System.Drawing.Point(0, 225);

this.dataGridView2.Size =

new System.Drawing.Size(309, 130);

this.cachePositionCheckBox.AutoSize = true;

this.cachePositionCheckBox.Checked = true;

this.cachePositionCheckBox.Location =

new System.Drawing.Point(150, 175);

this.cachePositionCheckBox.Name = "radioButton1";

this.cachePositionCheckBox.Size =

new System.Drawing.Size(151, 17);

this.cachePositionCheckBox.Text =

"Cache and restore position";

this.ClientSize = new System.Drawing.Size(325, 420);

this.Controls.Add(this.dataGridView1);

this.Controls.Add(this.cachePositionCheckBox);

this.Controls.Add(this.dataGridView2);

this.Controls.Add(this.button1);

// Initialize the data.

set1 = InitializeDataSet();

// Set the data source to the DataSet.

bindingSource1.DataSource = set1;

//Set the DataMember to the Menu table.

bindingSource1.DataMember = "Customers";

// Add the control data bindings.

dataGridView1.DataSource = bindingSource1;

// Set the data source and member for the second

// DataGridView.

dataGridView2.DataSource = bindingSource1;

dataGridView2.DataMember = "custOrders";

// Get the currency manager for the customer orders

// binding.

CurrencyManager relatedCM =

bindingSource1.GetRelatedCurrencyManager(

"custOrders");

// Set the position in the child table for demonstration

// purposes.

relatedCM.Position = 3;

// Handle the current changed event. This event occurs

// when the current item is changed, but not when a

// field of the current item is changed.

bindingSource1.CurrentChanged +=

new EventHandler(bindingSource1_CurrentChanged);

// Handle the two events for caching and resetting the

// position.

relatedCM.ListChanged += new

ListChangedEventHandler(relatedCM_ListChanged);

relatedCM.PositionChanged

+= new EventHandler(relatedCM_PositionChanged);

}

// Establish the data set with two tables and a relationship

// between them.

private DataSet InitializeDataSet()

{

set1 = new DataSet();

// Declare the DataSet and add a table and column.

set1.Tables.Add("Customers");

set1.Tables[0].Columns.Add("CustomerID");

set1.Tables[0].Columns.Add("Customer Name");

set1.Tables[0].Columns.Add("Contact Name");

// Add some rows to the table.

set1.Tables["Customers"].Rows.Add("c1",

"Fabrikam, Inc.", "Ellen Adams");

set1.Tables[0].Rows.Add("c2", "Lucerne Publishing",

"Don Hall");

set1.Tables[0].Rows.Add("c3", "Northwind Traders",

"Lori Penor");

set1.Tables[0].Rows.Add("c4", "Tailspin Toys",

"Michael Patten");

set1.Tables[0].Rows.Add("c5", "Woodgrove Bank",

"Jyothi Pai");

// Declare the DataSet and add a table and column.

set1.Tables.Add("Orders");

set1.Tables[1].Columns.Add("CustomerID");

set1.Tables[1].Columns.Add("OrderNo");

set1.Tables[1].Columns.Add("OrderDate");

// Add some rows to the table.

set1.Tables[1].Rows.Add("c1", "119", "10/04/2006");

set1.Tables[1].Rows.Add("c1", "149", "10/10/2006");

set1.Tables[1].Rows.Add("c1", "159", "10/12/2006");

set1.Tables[1].Rows.Add("c2", "169", "10/10/2006");

set1.Tables[1].Rows.Add("c2", "179", "10/10/2006");

set1.Tables[1].Rows.Add("c2", "189", "10/12/2006");

set1.Tables[1].Rows.Add("c3", "122", "10/04/2006");

set1.Tables[1].Rows.Add("c4", "130", "10/10/2006");

set1.Tables[1].Rows.Add("c5", "1.29", "10/14/2006");

DataRelation dr = new DataRelation("custOrders",

set1.Tables["Customers"].Columns["CustomerID"],

set1.Tables["Orders"].Columns["CustomerID"]);

set1.Relations.Add(dr);

return set1;

}

private int cachedPosition = -1;

private bool cacheChildPosition = true;

void relatedCM_ListChanged(object sender,

ListChangedEventArgs e)

{

// Check to see if this is a caching situation.

if (cacheChildPosition && cachePositionCheckBox.Checked)

{

// If so, check to see if it is a reset situation,

//and the current position is greater than zero.

CurrencyManager relatedCM = sender as

CurrencyManager;

if (e.ListChangedType == ListChangedType.Reset &&

relatedCM.Position > 0)

// If so, cache the position of the child table.

cachedPosition = relatedCM.Position;

}

void bindingSource1_CurrentChanged(object sender,

EventArgs e)

{

// If the CurrentChanged event occurs, this is not a

// caching situation.

cacheChildPosition = false;

}

void relatedCM_PositionChanged(object sender, EventArgs e)

{

// Check to see if this is a caching situation.

if (cacheChildPosition && cachePositionCheckBox.Checked)

{

CurrencyManager relatedCM =

sender as CurrencyManager;

// If so, check to see if the current position is

// not equal to the cached position and the cached

// position is not out of bounds.

if (relatedCM.Position != cachedPosition &&

cachedPosition > 0 && cachedPosition

< relatedCM.Count)

{

relatedCM.Position = cachedPosition;

cachedPosition = -1;

}

private void button1_Click(object sender, EventArgs e)

{

// For demo purposes--modifies a value in the first row

// of the parent table.

DataRow row1 = set1.Tables[0].Rows[0];

row1[1] = DBNull.Value;

}

[STAThread]

static void Main()

{

Application.EnableVisualStyles();

Application.SetCompatibleTextRenderingDefault(false);

Application.Run(new Form1());

}

Sharing data across forms with a BindingSource

cherylws — Tue, 17 Oct 2006 23:24:00 GMT

I've seen a lot of questions posted in forums regarding how to use a BindingSource component to share data between forms and keep bound controls synchronized. We created a couple of topics on how to do this, post-VS2005, so you may not have discovered them yet. Check out a topic on sharing a BindingSource here, and a topic on keeping controls synchronized here.

New White Paper: Windows Forms 2.0 Data Binding Improvements

Jay A. Allen — Fri, 04 Aug 2006 20:46:00 GMT

Yes - another white paper! We've been quite the busy beavers here in User Education. Cheryl Simmons has a great two-parter up detailing improvements to the data binding architecture in Windows Forms 2.0. Check it out:

Behind the Scenes: Improvements to Windows Forms Data Binding in the .NET Framework 2.0, Part 1

Behind the Scenes: Improvements to Windows Forms Data Binding in the .NET Framework 2.0, Part 2

If you have any feedback, questions or corrections for Cheryl, feel free to post them here.

New White Paper: DataGridView Drop-Down Filter Lists

Karl Erickson — Wed, 02 Aug 2006 01:57:00 GMT

Are you spoiled by the AutoFilter feature in Excel? Do you want the same functionality in the DataGridView control? Then my new whitepaper and sample are for you. See:

Building a Drop-Down Filter List for a DataGridView Column Header Cell

The DataGridViewAutoFilter library provides a custom DataGridViewColumnHeaderCell with a drop-down filter list that displays each unique value in a column, and updates the filter of the bound data source when a value is selected from the list. A custom DataGridViewColumn is also provided to make using the AutoFilter header cell easier, particularly in the Windows Forms Designer.

Customizing DataGridView column header cells is not as easy as customizing regular cells. For one thing, column header cells were never meant to be edited, so they cannot take advantage of the editing-control infrastructure provided for regular cells. This paper describes some of the issues that you must address when creating a control-hosting DataGridView cell from scratch.

SQL Server Express, Windows Forms, and ClickOnce

Jay A. Allen — Fri, 14 Jul 2006 02:32:00 GMT

I'm working on a project that will be included in a white paper, and needed to use SQL Server Express 2005 as a database engine. Being the "ClickOnce guy" on the Windows Forms doc team, I immediately wondered: How will I deploy this once it's ready?

Fortunately, Robert Walters already answered that question for me with his article, Embedding SQL Server Express into Custom Applications. Go give it a read if you're facing a similar need with your Windows Forms app.

SQL Server Everywhere

Jay A. Allen — Mon, 10 Jul 2006 20:17:00 GMT

Steve Lasker has a stream of information about this new technology. Start with his Info post and work your way up.

SQL Server Everywhere is essentially SQL Server Mobile with the licensing restrictions removed, so that it can be deployed by client applications on Windows desktops. Thye're delaying release to incorporate a mechnsim for servicing Everywhere on client machines, so that the application can both be ClickOnce deployed and serviced if a security issue is discovered.

Exciting stuff.

How to Data Bind to an RSS Feed (Blog Posts) in Windows Forms

Jay A. Allen — Thu, 15 Jun 2006 00:48:00 GMT

RSS feeds are XML files that represent the current posts on a syndicated content channel, such as a blog or news feed. Since RSS feeds are XML, they can be loaded into an ADO.NET DataSet and bound to the elements of a form.

Picture 1: Simple RSS Feed Reader

The following C# code shows the basics of wiring up a feed to a form so that the title element is bound to a Label and the description (blog post content) is bound to the DocumentText property of the WebBrowser control. The method GetBlogPost() is called in my code from a list of entries contained on a ToolStrip control; I use the Padding property of ToolStripItem to make the ToolStrip list appear as a categorized hierarchy of max one depth level. The feed URI is saved in the Tag property of the ToolStripButton, which is what the user clicks to call GetBlogPost(). The button controls at the bottom of the form allow you to flip back and forward through the currently available posts in the latest version of the feed.

The form for this sample requires these controls:

postTitleLink: LinkLabel
webBrowser1: WebBrowser
firstbutton, lastButton, previousButton, nextButton: Button

The trickiest part of this code occurs partway down the GetBlogPosts() method, where I need to obtain the list of item elements in the RSS feed to which I data bind. The item elements are consider children of the channel element. When I load the XML file into an ADO.NET DataSet, ADO.NET makes the item elements available as a DataRelation whose name is a conjunction of the channel and item elements. In other words, to get at the item elements, I have to use this line:

           bindingTable = dt.ChildRelations["channel_item"].ChildTable;

- where dt is the name of my DataTable containing the RSS data.

Here's the full sample. (NOTE: This is a preview of a full sample I'll be publishing as the subject of a white paper in July. The final sample will be more fleshed out than this skeleton sample.)

        private void GetBlogPost(string feed)
        {
            DataSet ds = new DataSet();
            XmlReader reader = null;

            try
            {
                reader = XmlReader.Create(feed);
            }
            catch (FileNotFoundException fileEx)
            {
                MessageBox.Show("Cannot file blog feed. Error returned: " + fileEx.Message);
                return;
            }
            catch (SecurityException secEx)
            {
                MessageBox.Show("You do not have permission to view this feed. Error returned: " + secEx.Message);
                return;
            }

            // Cruise forward to the <item> elements.
            reader.ReadToDescendant("channel");

            ds.ReadXml(reader);
            DataTable dt = ds.Tables[0];

            // Set up data binding.
            bindingTable = dt.ChildRelations["channel_item"].ChildTable;
            postTitleLink.DataBindings.Add(new Binding("Text", bindingTable, "title"));
            //dateLabel.DataBindings.Add(new Binding("Text", bindingTable, "dc:date"));
            webBrowser1.DataBindings.Add(new Binding("DocumentText", bindingTable, "description"));

            // Set the Previous and Next buttons as applicable.
            previousButton.Enabled = false;
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            if (myContext.Count == 1)
            {
                nextButton.Enabled = false;
                lastButton.Enabled = false;
            }
            else
            {
                nextButton.Enabled = true;
                lastButton.Enabled = true;
            }

            firstButton.Enabled = false;

            recordLabel.Text = (myContext.Position + 1).ToString();

            myContext.CurrentChanged += new EventHandler(myContext_CurrentChanged);
        }

        void myContext_CurrentChanged(object sender, EventArgs e)
        {
            recordLabel.Text = (this.BindingContext[bindingTable].Position + 1).ToString();
        }

        private void nextButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position += 1;
            previousButton.Enabled = true;
            if (myContext.Position + 1 == myContext.Count)
            {
                nextButton.Enabled = false;
                lastButton.Enabled = false;
            }
            if (myContext.Position == 1)
            {
                firstButton.Enabled = true;
            }
        }

        private void previousButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position -= 1;
            nextButton.Enabled = true;
            if (myContext.Position == 0)
            {
                previousButton.Enabled = false;
                firstButton.Enabled = false;
            }
            if (myContext.Position + 2 == myContext.Count)
            {
                lastButton.Enabled = true;
            }
        }

        private void firstButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position = 0;
            previousButton.Enabled = false;
            firstButton.Enabled = false;
            if (lastButton.Enabled == false)
            {
                lastButton.Enabled = true;
                nextButton.Enabled = true;
            }
        }

        private void lastButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position = myContext.Count - 1;
            nextButton.Enabled = false;
            lastButton.Enabled = false;
            if (firstButton.Enabled == false)
            {
                firstButton.Enabled = true;
                previousButton.Enabled = true;
            }
        }

        private void postTitleLink_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            DataRowView rowView = (DataRowView)myContext.Current;
            string permalink = rowView["link"].ToString();
            Process.Start(permalink);
        }