MahApps.Metro - ValidationHelper

Applies to any UIElement. Four properties, all of them about the popup MahApps shows next to a control whose binding failed validation.

Property	Type	Default
`CloseOnMouseLeftButtonDown`	`bool`	`false`	clicking anywhere dismisses the popup
`ShowValidationErrorOnMouseOver`	`bool`	`false`	show the error while the pointer is over the red triangle rather than only while the control has focus
`ShowValidationErrorOnKeyboardFocus`	`bool`	`true`	the focus route itself, which this is what switches off
`AlwaysShowValidationError`	`bool`	`false`	keep the message up whatever the focus and the pointer do

The last two are new in develop and are not in 2.4.11, which has the first two.

<TextBox Text="{Binding Age, ValidatesOnDataErrors=True}"
         mah:ValidationHelper.ShowValidationErrorOnMouseOver="True"
         mah:ValidationHelper.CloseOnMouseLeftButtonDown="True" />

By default the error popup appears while the control has keyboard focus. That works while the user is filling a form in, but it means an error on a field they have moved away from is invisible — ShowValidationErrorOnMouseOver gives them a way to read it again without tabbing back.

CloseOnMouseLeftButtonDown suits a long message that would otherwise cover the controls beneath it.

Both can be set once for the whole window rather than per control:

<Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}">
    <Setter Property="mah:ValidationHelper.ShowValidationErrorOnMouseOver" Value="True" />
</Style>

The template the popup uses is MahApps.Templates.ValidationError, which the input control styles set as Validation.ErrorTemplate. Replace that resource to change what an error looks like rather than how it behaves.

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