ExtraCheckBoxText is used to label the extra checkbox. 
Namespace: Autodesk.Revit.UIAssembly: RevitAPIUI (in RevitAPIUI.dll) Version: 21.0.0.0 (21.1.1.109)
Since: 2017
Syntax
| C# | 
|---|
public string ExtraCheckBoxText { get; set; }  | 
| Visual Basic | 
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Public Property ExtraCheckBoxText As String Get Set  | 
| Visual C++ | 
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public: property String^ ExtraCheckBoxText { String^ get (); void set (String^ value); }  | 
Remarks
If ExtraCheckBoxText is set, a checkbox with the text will be shown. You can get the response of checkbox by checking the return value of the WasExtraCheckBoxChecked() method.
Exceptions
| Exception | Condition | 
|---|---|
| Autodesk.Revit.Exceptions..::..InvalidOperationException | Thrown if the TaskDialog also has EnableMarqueeProgressBar set as the two cannot coincide in the same TaskDialog. |