OpenFileDialog dlg = new OpenFileDialog();

// Show all files
dlg.Filter = string.Empty;

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Show all files
dlg.Filter = null;

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Filter by Word Documents
dlg.Filter = "Word Documents|*.doc";

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Filter by Excel Worksheets
dlg.Filter = "Excel Worksheets|*.xls";

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Filter by PowerPoint Presentations
dlg.Filter = "PowerPoint Presentations|*.ppt";

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Filter by Office Files
dlg.Filter = "Office Files|*.doc;*.xls;*.ppt";

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Filter by All Files
dlg.Filter = "All Files|*.*";

dlg.ShowDialog();

OpenFileDialog dlg = new OpenFileDialog();

// Filter by Word Documents OR Excel Worksheets OR PowerPoint Presentations 
//           OR Office Files 
//           OR All Files
dlg.Filter = "Word Documents|*.doc|Excel Worksheets|*.xls|PowerPoint Presentations|*.ppt" +
             "|Office Files|*.doc;*.xls;*.ppt" +
             "|All Files|*.*";

dlg.ShowDialog();