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copied!<p>WPF's TextBox and RichTextBox classes do not directly support multiselection, but as with most parts of WPF it's extremely easy to customize its existing RichTextBox to get this ability.</p> <p>The steps are:</p> <ul> <li>Create a class deriving from RichTextBox</li> <li>Add an "AdditionalRanges" property of type <code>ObservableCollection<TextRange></code> which will contain all selected ranges except the current TextSelection</li> <li>Override <code>OnPreviewMouseLeftButtonDown</code>: If Ctrl is pressed, combine the current TextSelection into your "AdditionalRanges" property and clear Selection, otherwise clear "AdditionalRanges".</li> <li>In the constructor, add a <code>CollectionChanged</code> handler to "AdditionalRanges" that uses <code>TextRange.ApplyPropertyValue()</code> to make added ranges in the collection appear hilighted and removed ranges appear normally.</li> </ul> <p>In your implementation I also recommend you implement a few more properties for convenience:</p> <ul> <li>An "AllRanges" property that combines the TextSelection with AdditionalRanges</li> <li>A bindable "Text" property</li> <li>A bindable "SelectedText" property</li> </ul> <p>These are all quite trivial to implement.</p> <p>Final notes:</p> <ul> <li>When updating AdditionalRanges or computing AllRanges, if the TextSelection overlaps an existing AdditionalRange, replace it with a combined range otherwise add the TextSelection to the list.</li> <li>You can add a <code>TextChanged</code> handler to know when to update the "Text" property, and a PropertyChangedCallback to know when to update the FlowDocument</li> </ul>

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