RasterThumbnailBrowser Class


Derives from the RasterImageList control to provide functionality to automatically generate thumbnails for images in a disk folder.


public class RasterThumbnailBrowser : RasterImageList 
public ref class RasterThumbnailBrowser : public RasterImageList, System.ComponentModel.IComponent, System.ComponentModel.ISynchronizeInvoke, System.IDisposable, System.Windows.Forms.IBindableComponent, System.Windows.Forms.IDropTarget, System.Windows.Forms.IWin32Window   
   ref class RasterThumbnailBrowser : RasterImageList 


The RasterThumbnailBrowser lets you quickly and easily create thumbnails for images that reside on any file folder in the system. Since the RasterThumbnailBrowser derives from the RasterImageList control, you get all the visual styles available to the image list (different view styles, control the item size, color, etc.) as well as the same interface to manually add/remove/edit items.

You call the LoadThumbnails(String,String,RasterThumbnailBrowserLoadFlags) method passing it the path of the folder for which you want to create thumbnails, a search pattern and a boolean value indicating whether you want the control to block until all thumbnails are created or to return immediately and continue loading the thumbnails in a background thread.

The control will first read the folder and create the items you need. These items will initially have a default "loading" thumbnail set in their RasterImageListItem.Image that you can access and change through the LoadingThumbnail property. Once the item image file is loaded and its thumbnail extracted, the control will replace the item Image property with the correct thumbnail. This gives the user a visual feedback on which items are loaded and which are yet to be loaded. If the control cannot load a certain file (for example, if the file is not a valid image format file), an "error" thumbnail will be used in the item. You can access and change this thumbnail image through the ErrorThumbnail property.

During the loading operation, you can check the IsLoadingThumbnails property to determine if the control is still loading thumbnails in the background. The FinishedLoadingThumbnails event will fire when the loading operation is done. You can subscribe to the LoadThumbnail event to get feedback about the item currently being loaded. You use this event also to update a progress bar on your application and to cancel the load process if needed. The CancelLoadingThumbnails method lets you cancel the load operation at any time.

The ThumbnailSizeFlags property lets you control the quality of the thumbnail images created by the RasterThumbnailBrowser control.


This example creates an instance of a RasterThumbnailBrowser control and adds it to a form Next it displays thumbnail images of all images in a directory

using Leadtools.WinForms; 
using Leadtools; 
using Leadtools.Codecs; 
using Leadtools.Drawing; 
class MyForm : Form 
   public RasterThumbnailBrowser theBrowser; 
   public bool cancelOperation;        // Informs us if the user canceled the operation 
   public Button buttonCancel;         // Cancel browse loading 
   public Button buttonBrowse;         // Diplays the browse control 
   public ProgressBar progressBar;     // shows progress of loading browse images 
   void buttonBrowse_Click(object sender, EventArgs e) 
      // If we are already loading thumbails, cancel 
      // this operation 
      if (theBrowser.IsLoadingThumbnails) 
      // Clean all the items 
      // Update the application state 
      buttonCancel.Enabled = true; 
      progressBar.Value = 0; 
      // And load the new thumbnails 
      cancelOperation = false; 
      string folderPath = LEAD_VARS.ImagesDir; 
      theBrowser.LoadStamp = true; 
      theBrowser.LoadThumbnails(folderPath, "*.*", RasterThumbnailBrowserLoadFlags.Block); 
      theBrowser.EnableBrowserWatcher = true; 
   private void thumbnailBrowser_LoadThumbnail(object sender, RasterThumbnailBrowserLoadThumbnailEventArgs e) 
      // If this is the first iteration, update the progress bar minimum and maximum values 
      if (e.Index == 0) 
         progressBar.Minimum = 0; 
         progressBar.Maximum = e.Total - 1; 
      // Update where we are in the loading operation 
      progressBar.Value = e.Index; 
      // Check if we need to cancel (due to the user clicking the Cancel button) 
      if (cancelOperation) 
         e.Cancel = true; 
   private void thumbnailBrowser_AddFile(object sender, RasterThumbnailBrowserAddFileEventArgs e) 
      Console.WriteLine("AddFile: {0}    Add: {1}", e.FileName, e.Add); 
   private void thumbnailBrowser_FinishedLoadingThumbnails(object sender, EventArgs e) 
      buttonCancel.Enabled = false; 
   private void buttonCancel_Click(object sender, EventArgs e) 
      // The user has clicked the cancel button 
      this.cancelOperation = true; 
   // Create  custom images to use as the "error" and "loading" thumbnails. 
   void CreateErrorThumbnail() 
      // Get the image size 
      Size imageSize = theBrowser.ItemImageSize; 
      // No "loading" thumbnail 
      theBrowser.LoadingThumbnail = null; 
      // For the "error" thumbnail, create a red X image 
      RasterColor[] palette = new RasterColor[0]; 
      RasterImage image = new RasterImage( 
      IntPtr hdc = RasterImagePainter.CreateLeadDC(image); 
      Graphics g = Graphics.FromHdc(hdc); 
      g.FillRectangle(Brushes.Magenta, 0, 0, imageSize.Width, imageSize.Height); 
      g.DrawLine(Pens.Red, 0, 0, imageSize.Width, imageSize.Height); 
      g.DrawLine(Pens.Red, imageSize.Width, 0, 0, imageSize.Height); 
      // Make this image transparent 
      image.Transparent = true; 
      image.TransparentColor = RasterColor.FromKnownColor(RasterKnownColor.Magenta); 
      theBrowser.ErrorThumbnail = image; 
   public void Cleanup() 
      buttonBrowse.Click -= new EventHandler(buttonBrowse_Click); 
      buttonCancel.Click -= new EventHandler(buttonCancel_Click); 
      theBrowser.LoadThumbnail -= new EventHandler<RasterThumbnailBrowserLoadThumbnailEventArgs>(thumbnailBrowser_LoadThumbnail); 
      theBrowser.FinishedLoadingThumbnails -= new EventHandler(thumbnailBrowser_FinishedLoadingThumbnails); 
   public MyForm() 
      Size = new Size(300, 200); 
      // create the browser 
      theBrowser = new RasterThumbnailBrowser(); 
      theBrowser.Codecs = new RasterCodecs(); 
      theBrowser.Dock = DockStyle.Fill; 
      theBrowser.ItemSpacingSize = new Size(10, 10); 
      theBrowser.ThumbnailSizeFlags = RasterSizeFlags.Bicubic; 
      theBrowser.LoadThumbnail += new EventHandler<RasterThumbnailBrowserLoadThumbnailEventArgs>(thumbnailBrowser_LoadThumbnail); 
      theBrowser.FinishedLoadingThumbnails += new EventHandler(thumbnailBrowser_FinishedLoadingThumbnails); 
      theBrowser.AddFile += new EventHandler<RasterThumbnailBrowserAddFileEventArgs>(thumbnailBrowser_AddFile); 
      // Create a thumbnail image to be displayed on error 
      // add a panel 
      Panel panel = new Panel(); 
      panel.Dock = DockStyle.Left; 
      panel.Width = 100; 
      // add a "browse" button 
      buttonBrowse = new Button(); 
      buttonBrowse.Text = "Browse"; 
      buttonBrowse.Dock = DockStyle.Top; 
      buttonBrowse.Click += new EventHandler(buttonBrowse_Click); 
      // add a "cancel" button 
      buttonCancel = new Button(); 
      buttonCancel.Text = "Cancel"; 
      buttonBrowse.Dock = DockStyle.Bottom; 
      buttonCancel.Click += new EventHandler(buttonCancel_Click); 
      // add a progress bar 
      progressBar = new ProgressBar(); 
      progressBar.Dock = DockStyle.Bottom; 
   private void InitializeComponent() 
      // MyForm 
      this.ClientSize = new System.Drawing.Size(315, 273); 
      this.Name = "MyForm"; 
public void RasterThumbnailBrowser_RasterThumbnailBrowser() 
   MyForm form = new MyForm(); 
static class LEAD_VARS 
   public const string ImagesDir = @"C:\LEADTOOLS22\Resources\Images"; 


Target Platforms

See Also


RasterThumbnailBrowser Members

Leadtools.WinForms Namespace

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© 1991-2023 LEAD Technologies, Inc. All Rights Reserved.