Note that the default value of this property is null. You must set the property to an instance of ValueInput.ValueConstraint object.

ValueConstraint is used to limit what the user can input into the editor. More accurately, the editor will consider user input invalid if it doesn't satisfy one or more criteria specified by the ValueConstraint. The ValueInput.IsValueValid property can be used to find out if editor considers current value valid.

When an invalid value is entered into the editor, there are behavioral implications based on various settings. For example, by default the editor doesn't exit edit mode when the current input is invalid. The action taken by the editor can be controlled using the ValueInput.InvalidValueBehavior property.

Reference