'Declaration Public Property Nullable As Nullable(Of Boolean)
public Nullable<bool> Nullable {get; set;}
Nullable property if set to False constraints the value to be non-null. The value must not be one of null (Nothing in VB) or DBNull.
If the ValueConstraint object is associated with an editor's ValueConstraint property, Nullable indicates if the user is allowed to delete all the contents of the editor. If Nullable is set to False then the user is not allowed to delete all the contents. The user must enter a value in the editor.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2