How to: Create an Owner-Drawn List Box

Artigo
12/13/2006

You can create an owner-drawn list box with the .NET Compact Framework, although not as easily as with the full .NET Framework. The .NET Compact Framework does not support the DrawMode, DrawItem, and other drawing members for list box and other controls, but you can program that functionality. This example provides a custom control class to create an owner-drawn list box, and implements the control as a list box for selecting fonts.

This sample contains a base class, OwnerDrawnListBox, from which the FontListBox is derived. The OwnerDrawnListBox core functions include the following:

Provides the vertical scroll bar, an array for containing the items, and a bitmap for constructing the list box.
Provides an item height property and determines how many items can be drawn given the height of the control.
Provides a SelectedIndex property, which determines the selected item by current scroll bar, item height, and mouse pointer coordinate values.
Provides a SelectedIndexChanged event.
Ensures the visibility of items.
Removes the scroll bar, if not needed.
Provides keyboard operation with the up and down navigation keys.

The FontListBox class inherits from OwnerDrawnListBox. The implementations and techniques include the following:

Determines the proper item height.
Draws the items, with the selected item drawn as highlighted, using an overridden OnPaint method. To prevent flicker, this method does not display the control until all its graphical features are drawn.
Provides the SelectedFaceName property, containing the name of the selected font.

The form, DrawFontList, does the following:

Creates a new instance of FontListBox, and adds it to the controls collection.
Adds an event handler for the SelectedIndexChanged event.
Populates the FontListBox with a list of fonts.
Provides labels for displaying the sample of the selected font.

To create an owner drawn list box

Add the OwnerDrawnListBox custom control class to your project.

' Base custom control for DrawFontList
Class OwnerDrawnListBox
    Inherits Control
    Private Const SCROLL_WIDTH As Integer = 20
    Private itemH As Integer = -1
    Private selIndex As Integer = -1

    Private offScreenBitmap As Bitmap
    Private vs As VScrollBar
    Private itemsAList As ArrayList


    Public Sub New()
        Me.vs = New VScrollBar
        Me.vs.Parent = Me
        Me.vs.Visible = False
        Me.vs.SmallChange = 1
        AddHandler Me.vs.ValueChanged, AddressOf Me.ScrollValueChanged

        Me.itemsAList = New ArrayList
    End Sub


    Public ReadOnly Property Items() As ArrayList
        Get
            Return Me.itemsAList
        End Get
    End Property


    Protected ReadOnly Property OffScreen() As Bitmap
        Get
            Return Me.offScreenBitmap
        End Get
    End Property


    Protected ReadOnly Property VScrollBar() As VScrollBar
        Get
            Return Me.vs
        End Get
    End Property

    Friend Event SelectedIndexChanged As EventHandler


    Protected Overridable Sub OnSelectedIndexChanged(ByVal e As EventArgs)
        RaiseEvent SelectedIndexChanged(Me, e)
    End Sub

    Protected Overrides Sub OnMouseDown(ByVal e As MouseEventArgs)
        Me.SelectedIndex = Me.vs.Value + e.Y / Me.ItemHeight

        ' Invalidate the control so we can draw the item as selected.
        Me.Refresh()
    End Sub

    ' Get or set index of selected item.
    Public Property SelectedIndex() As Integer
        Get
            Return Me.selIndex
        End Get

        Set(ByVal Value As Integer)
            Me.selIndex = Value

            RaiseEvent SelectedIndexChanged(Me, Nothing)
        End Set
    End Property


    Protected Sub ScrollValueChanged(ByVal o As Object, ByVal e As EventArgs)
        Me.Refresh()
    End Sub


    Protected Overridable Property ItemHeight() As Integer
        Get
            Return Me.itemH
        End Get

        Set(ByVal Value As Integer)
            Me.itemH = Value
        End Set
    End Property


    ' If the requested index is before the first visible index then set the
    ' first item to be the requested index. If it is after the last visible
    ' index, then set the last visible index to be the requested index.
    Public Sub EnsureVisible(ByVal index As Integer)
        If index < Me.vs.Value Then
            Me.vs.Value = index
            Me.Refresh()
        ElseIf index >= Me.vs.Value + Me.DrawCount Then
            Me.vs.Value = index - Me.DrawCount + 1
            Me.Refresh()
        End If
    End Sub


    ' Need to set focus to the control when it 
    ' is clicked so that keyboard events occur.
    Protected Overrides Sub OnClick(ByVal e As EventArgs)
        Me.Focus()
        MyBase.OnClick(e)
    End Sub

    ' Selected item moves when you use the keyboard up/down keys.
    Protected Overrides Sub OnKeyDown(ByVal e As KeyEventArgs)
        Select Case e.KeyCode
            Case Keys.Down
                If Me.SelectedIndex < Me.vs.Maximum Then
                    Me.SelectedIndex = Me.SelectedIndex + 1
                    EnsureVisible(Me.SelectedIndex + 1)
                    Me.Refresh()
                End If
            Case Keys.Up
                If Me.SelectedIndex > Me.vs.Minimum Then
                    Me.SelectedIndex = Me.SelectedIndex - 1
                    EnsureVisible(Me.SelectedIndex - 1)
                    Me.Refresh()
                End If
            Case Keys.PageDown
                Me.SelectedIndex = Math.Min(Me.vs.Maximum, Me.SelectedIndex + Me.DrawCount)
                EnsureVisible(Me.SelectedIndex)
                Me.Refresh()
            Case Keys.PageUp
                Me.SelectedIndex = Math.Max(Me.vs.Minimum, Me.SelectedIndex - Me.DrawCount)
                EnsureVisible(Me.SelectedIndex)
                Me.Refresh()
            Case Keys.Home
                Me.SelectedIndex = 0
                EnsureVisible(Me.SelectedIndex)
                Me.Refresh()
            Case Keys.End
                Me.SelectedIndex = Me.itemsAList.Count - 1
                EnsureVisible(Me.SelectedIndex)
                Me.Refresh()
        End Select

        MyBase.OnKeyDown(e)
    End Sub

    ' Calculate how many items we can draw given the height of the control.
    Protected ReadOnly Property DrawCount() As Integer
        Get
            If Me.vs.Value + Me.vs.LargeChange > Me.vs.Maximum Then
                Return Me.vs.Maximum - Me.vs.Value + 1
            Else
                Return Me.vs.LargeChange
            End If
        End Get
    End Property

    Protected Overrides Sub OnResize(ByVal e As EventArgs)
        Dim viewableItemCount As Integer = Me.ClientSize.Height / Me.ItemHeight

        Me.vs.Bounds = New Rectangle(Me.ClientSize.Width - SCROLL_WIDTH, 0, SCROLL_WIDTH, Me.ClientSize.Height)


        ' Determine if scrollbars are needed
        If Me.itemsAList.Count > viewableItemCount Then
            Me.vs.Visible = True
            Me.vs.LargeChange = viewableItemCount
            Me.offScreenBitmap = New Bitmap(Me.ClientSize.Width - SCROLL_WIDTH - 1, Me.ClientSize.Height - 2)
        Else
            Me.vs.Visible = False
            Me.vs.LargeChange = Me.itemsAList.Count
            Me.offScreenBitmap = New Bitmap(Me.ClientSize.Width - 1, Me.ClientSize.Height - 2)
        End If

        Me.vs.Maximum = Me.itemsAList.Count - 1
    End Sub
End Class

// Base custom control for DrawFontList
class OwnerDrawnListBox : Control
{
    const int SCROLL_WIDTH = 20;
    int itemHeight         = -1;
    int selectedIndex      = -1;

    Bitmap offScreen;
    VScrollBar vs;
    ArrayList items;

    public OwnerDrawnListBox()
    {
        this.vs = new VScrollBar();
        this.vs.Parent      = this;
        this.vs.Visible     = false;
        this.vs.SmallChange = 1;
        this.vs.ValueChanged += new EventHandler(this.ScrollValueChanged);

        this.items = new ArrayList();
    }

    public ArrayList Items
    {
        get
        {
            return this.items;
        }
    }

    protected Bitmap OffScreen
    {
        get
        {
            return this.offScreen;
        }
    }

    protected VScrollBar VScrollBar
    {
        get
        {
            return this.vs;
        }
    }

    public event EventHandler SelectedIndexChanged;

    // Raise the SelectedIndexChanged event
    protected virtual void OnSelectedIndexChanged(EventArgs e)
    {
        if(this.SelectedIndexChanged != null)
            this.SelectedIndexChanged(this, e);
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        this.SelectedIndex = this.vs.Value + (e.Y / this.ItemHeight);

        // Invalidate the control so we can draw the item as selected.
        this.Refresh();
    }

    // Get or set index of selected item.
    public int SelectedIndex
    {
        get
        {
            return this.selectedIndex;
        }

        set
        {
            this.selectedIndex = value;

            if (this.SelectedIndexChanged != null)
                this.SelectedIndexChanged(this, EventArgs.Empty);

        }
    }

    protected void ScrollValueChanged(object o, EventArgs e)
    {
        this.Refresh();
    }

    protected virtual int ItemHeight
    {
        get
        {
            return this.itemHeight;
        }

        set
        {
            this.itemHeight = value;
        }
    }

    // If the requested index is before the first visible index then set the
    // first item to be the requested index. If it is after the last visible
    // index, then set the last visible index to be the requested index.
    public void EnsureVisible(int index)
    {
        if(index < this.vs.Value)
        {
            this.vs.Value = index;
            this.Refresh();
        }
        else if(index >= this.vs.Value + this.DrawCount)
        {
            this.vs.Value = index - this.DrawCount + 1;
            this.Refresh();
        }
    }


    // Need to set focus to the control when it
    // is clicked so that keyboard events occur.
    protected override void OnClick(EventArgs e)
    {
        this.Focus();
        base.OnClick(e);
    }

    // Selected item moves when you use the keyboard up/down keys.
    protected override void OnKeyDown(KeyEventArgs e)
    {
        switch(e.KeyCode)
        {
            case Keys.Down:
                if(this.SelectedIndex < this.vs.Maximum)
                {
                    EnsureVisible(++this.SelectedIndex);
                    this.Refresh();
                }
                break;
            case Keys.Up:
                if(this.SelectedIndex > this.vs.Minimum)
                {
                    EnsureVisible(--this.SelectedIndex);
                    this.Refresh();
                }
                break;
            case Keys.PageDown:
                this.SelectedIndex = Math.Min(this.vs.Maximum, this.SelectedIndex + this.DrawCount);
                EnsureVisible(this.SelectedIndex);
                this.Refresh();
                break;
            case Keys.PageUp:
                this.SelectedIndex = Math.Max(this.vs.Minimum, this.SelectedIndex - this.DrawCount);
                EnsureVisible(this.SelectedIndex);
                this.Refresh();
                break;
            case Keys.Home:
                this.SelectedIndex = 0;
                EnsureVisible(this.SelectedIndex);
                this.Refresh();
                break;
            case Keys.End:
                this.SelectedIndex = this.items.Count - 1;
                EnsureVisible(this.SelectedIndex);
                this.Refresh();
                break;
        }

        base.OnKeyDown(e);
    }

    // Calculate how many items we can draw given the height of the control.
    protected int DrawCount
    {
        get
        {
            if(this.vs.Value + this.vs.LargeChange > this.vs.Maximum)
                return this.vs.Maximum - this.vs.Value + 1;
            else
                return this.vs.LargeChange;
        }
    }

    protected override void OnResize(EventArgs e)
    {
        int viewableItemCount = this.ClientSize.Height / this.ItemHeight;

        this.vs.Bounds = new Rectangle(this.ClientSize.Width - SCROLL_WIDTH,
            0,
            SCROLL_WIDTH,
            this.ClientSize.Height);


        // Determine if scrollbars are needed
        if(this.items.Count > viewableItemCount)
        {
            this.vs.Visible = true;
            this.vs.LargeChange = viewableItemCount;
            this.offScreen = new Bitmap(this.ClientSize.Width - SCROLL_WIDTH - 1, this.ClientSize.Height - 2);
        }
        else
        {
            this.vs.Visible = false;
            this.vs.LargeChange = this.items.Count;
            this.offScreen = new Bitmap(this.ClientSize.Width - 1, this.ClientSize.Height - 2);
        }

        this.vs.Maximum = this.items.Count - 1;
    }
}

Add the FontListBox class (which is an implementation of OwnerDrawnListBox) to your project.

' Derive an implementation of the
' OwnerDrawnListBox class
Class FontListBox
    Inherits OwnerDrawnListBox
    Private Const FONT_SIZE As Integer = 10
    Private Const DRAW_OFFSET As Integer = 5


    Public Sub New()

        ' Determine what the item height should be
        ' by adding 30% padding after measuring
        ' the letter A with the selected font.
        Dim g As Graphics = Me.CreateGraphics()
        Me.ItemHeight = Fix(g.MeasureString("A", Me.Font).Height * 1.3)
        g.Dispose()
    End Sub

    ' Return the name of the selected font.
    Public ReadOnly Property SelectedFaceName() As String
        Get
            Return CStr(Me.Items(Me.SelectedIndex))
        End Get
    End Property


    ' Determine what the text color should be
    ' for the selected item drawn as highlighted
    Function CalcTextColor(ByVal backgroundColor As Color) As Color
        If backgroundColor.Equals(Color.Empty) Then
            Return Color.Black
        End If
        Dim sum As Integer = backgroundColor.R + backgroundColor.G + backgroundColor.B

        If sum > 256 Then
            Return Color.Black
        Else
            Return Color.White
        End If
    End Function

    Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)
        Dim font As Font
        Dim fontColor As Color

        ' The base class contains a bitmap, offScreen, for constructing
        ' the control and is rendered when all items are populated.
        ' This technique prevents flicker.
        Dim gOffScreen As Graphics = Graphics.FromImage(Me.OffScreen)
        gOffScreen.FillRectangle(New SolidBrush(Me.BackColor), Me.ClientRectangle)

        Dim itemTop As Integer = 0

        ' Draw the fonts in the list.
        Dim n As Integer
        For n = Me.VScrollBar.Value To (Me.VScrollBar.Value + DrawCount) - 1
            ' If the font name contains "dings" it needs to be displayed
            ' in the list box with a readable font with the default font.
            If CStr(Me.Items(n)).ToLower().IndexOf("dings") <> -1 Then
                font = New Font(Me.Font.Name, FONT_SIZE, FontStyle.Regular)
            Else
                font = New Font(CStr(Me.Items(n)), FONT_SIZE, FontStyle.Regular)
            End If
            ' Draw the selected item to appear highlighted
            If n = Me.SelectedIndex Then
                gOffScreen.FillRectangle(New SolidBrush(SystemColors.Highlight), 1, itemTop + 1, Me.ClientSize.Width - IIf(Me.VScrollBar.Visible, Me.VScrollBar.Width, 2), Me.ItemHeight)
                ' If the scroll bar is visible, subtract the scrollbar width
                ' otherwise subtract 2 for the width of the rectangle
                fontColor = CalcTextColor(SystemColors.Highlight)
            Else
                fontColor = Me.ForeColor
            End If
            ' Draw the item
            gOffScreen.DrawString(CStr(Me.Items(n)), font, New SolidBrush(fontColor), DRAW_OFFSET, itemTop)
            itemTop += Me.ItemHeight
        Next n

        ' Draw the list box
        e.Graphics.DrawImage(Me.OffScreen, 1, 1)

        gOffScreen.Dispose()
    End Sub


    ' Draws the external border around the control.
    Protected Overrides Sub OnPaintBackground(ByVal e As PaintEventArgs)
        e.Graphics.DrawRectangle(New Pen(Color.Black), 0, 0, Me.ClientSize.Width - 1, Me.ClientSize.Height - 1)
    End Sub
End Class

// Derive an implementation of the
// OwnerDrawnListBox class
class FontListBox : OwnerDrawnListBox
{
    const int FONT_SIZE    = 10;
    const int DRAW_OFFSET  = 5;

    public FontListBox()
    {

        // Determine what the item height should be
        // by adding 30% padding after measuring
        // the letter A with the selected font.
        Graphics g = this.CreateGraphics();
        this.ItemHeight = (int)(g.MeasureString("A", this.Font).Height * 1.3);
        g.Dispose();
    }

    // Return the name of the selected font.
    public string SelectedFaceName
    {
        get
        {
            return (string)this.Items[this.SelectedIndex];
        }
    }

    // Determine what the text color should be
    // for the selected item drawn as highlighted
    Color CalcTextColor(Color backgroundColor)
    {
        if(backgroundColor.Equals(Color.Empty))
            return Color.Black;

        int sum = backgroundColor.R + backgroundColor.G + backgroundColor.B;

        if(sum > 256)
            return Color.Black;
        else
            return Color.White;
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        Font font;
        Color fontColor;

        // The base class contains a bitmap, offScreen, for constructing
        // the control and is rendered when all items are populated.
        // This technique prevents flicker.
        Graphics gOffScreen = Graphics.FromImage(this.OffScreen);
        gOffScreen.FillRectangle(new SolidBrush(this.BackColor), this.ClientRectangle);

        int itemTop = 0;

        // Draw the fonts in the list.
        for(int n = this.VScrollBar.Value; n < this.VScrollBar.Value + DrawCount; n++)
        {
            // If the font name contains "dings" it needs to be displayed
            // in the list box with a readable font with the default font.
            if(((string)this.Items[n]).ToLower().IndexOf("dings") != -1)
                font = new Font(this.Font.Name, FONT_SIZE, FontStyle.Regular);
            else
                font = new Font((string)this.Items[n], FONT_SIZE, FontStyle.Regular);

            // Draw the selected item to appear highlighted
            if(n == this.SelectedIndex)
            {
                gOffScreen.FillRectangle(new SolidBrush(SystemColors.Highlight),
                    1,
                    itemTop + 1,
                    // If the scroll bar is visible, subtract the scrollbar width
                    // otherwise subtract 2 for the width of the rectangle
                    this.ClientSize.Width - (this.VScrollBar.Visible ? this.VScrollBar.Width : 2),
                    this.ItemHeight);
                fontColor = CalcTextColor(SystemColors.Highlight);
            }
            else
                fontColor = this.ForeColor;

            // Draw the item
            gOffScreen.DrawString((string)this.Items[n], font, new SolidBrush(fontColor), DRAW_OFFSET, itemTop);
            itemTop += this.ItemHeight;
        }

        // Draw the list box
        e.Graphics.DrawImage(this.OffScreen, 1, 1);

        gOffScreen.Dispose();
    }

    // Draws the external border around the control.

    protected override void OnPaintBackground(PaintEventArgs e)
    {
        e.Graphics.DrawRectangle(new Pen(Color.Black), 0, 0, this.ClientSize.Width - 1, this.ClientSize.Height - 1);
    }
}

In the form, add two Label controls and name them descLabel and sampleLabel; the latter will show a font example selected from FontListBox.
Create an instance of FontListBox for the form.

Add the following code to the constructor of the Load event for form.

' Create a new instance of FontListBox.
myListBox.Parent = Me

' Configure the event handler for the SelectedIndexChanged event. 
AddHandler myListBox.SelectedIndexChanged, AddressOf Me.SelectedIndexChanged

' Draw the bounds of the FontListBox.
myListBox.Bounds = New Rectangle(5, 5, 150, 100)

' Add fonts to list, repeat the list 4 times
' so that the scroll bar can be demonstrated.
Dim n As Integer
For n = 0 To 3
    myListBox.Items.Add("Bookdings")
    myListBox.Items.Add("Courier New")
    myListBox.Items.Add("Frutiger Linotype")
    myListBox.Items.Add("Tahoma")
Next n


' Labels to show the selected font from the list box. Assumes
' instances of these labels have been declared for the form.
descLabel.Parent = Me
descLabel.Text = "Font Sample:"
descLabel.Bounds = New Rectangle(10, myListBox.Bottom + 20, Me.ClientSize.Width - 10, 30)

' Assumes an instance sampleLabel is declared for the form.
sampleLabel.Parent = Me
sampleLabel.Bounds = New Rectangle(10, descLabel.Bottom, Me.ClientSize.Width - 10, 30)
sampleLabel.Text = "AaBbCc 123"

// Create a new instance of FontListBox.
box = new FontListBox();
box.Parent = this;

// Configure the event handler for the SelectedIndexChanged event.
box.SelectedIndexChanged += new EventHandler(this.SelectedIndexChanged);

// Draw the bounds of the FontListBox.
box.Bounds = new Rectangle(5, 5, 150, 100);

// Add fonts to list, repeat the list 4 times
// so that the scroll bar can be demonstrated.
for(int n = 0; n < 4; n++)
{
    box.Items.Add("Bookdings");
    box.Items.Add("Courier New");
    box.Items.Add("Frutiger Linotype");
    box.Items.Add("Tahoma");
}


// Labels to show the selected font and a sample.
Label desc = new Label();
desc.Parent = this;
desc.Text = "Font Sample:";
desc.Bounds = new Rectangle(10, box.Bottom + 20, this.ClientSize.Width - 10, 30);

sample = new Label();
sample.Parent = this;
sample.Bounds = new Rectangle(10, desc.Bottom, this.ClientSize.Width - 10, 30);
sample.Text = "AaBbCc 123";

Add code for the SelectedChangedEvent event of the FontListBox control, which displays the selected font in the sampleLabel control.

' Event handler for SelectedIndexChanged to
' display a sample of the selected font.

' Assumes that an instace of myListBox has been declared for the form.
Friend Sub SelectedIndexChanged(ByVal o As Object, ByVal e As EventArgs)
    sampleLabel.Font = New Font(myListBox.SelectedFaceName, 12, FontStyle.Regular)
End Sub

// Event handler for SelectedIndexChanged to
// display a sample of the selected font.
private void SelectedIndexChanged(object o, EventArgs e)
{
    sample.Font = new Font(box.SelectedFaceName, 12, FontStyle.Regular);
}

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How to: Create an Owner-Drawn List Box

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