Question

I have defined a dependency property that uses an enum to update the background colour of a button. When it runs I get a “Default value type does not match type of property ‘CurrentWarningLevel'” exception. The dependency property is as follows:

public enum WarningLevels
{
    wlError=0,
    wlWarning,
    wlInfo
};

public class WarningLevelButton : Button
{
    static WarningLevelButton()
    {
    }

    public WarningLevels CurrentWarningLevel
    {
        get { return (WarningLevels)GetValue(WarningLevelProperty); }
        set { base.SetValue(WarningLevelProperty, value); }
    }

    public static readonly DependencyProperty WarningLevelProperty =
      DependencyProperty.Register("CurrentWarningLevel", typeof(WarningLevels), typeof(WarningLevelButton), new PropertyMetadata(false));
}

I am trying to use the property as follows:

<Trigger Property="local:WarningLevelButton.CurrentWarningLevel" Value="wlError">
    <Setter TargetName="ButtonBody" Property="Background" Value="{StaticResource CtlRedBrush}" />
    <Setter TargetName="ButtonBody" Property="BorderBrush" Value="{StaticResource CtlRedBrush}" />
</Trigger>
<Trigger Property="local:WarningLevelButton.CurrentWarningLevel" Value="wlWarning">
    <Setter TargetName="ButtonBody" Property="Background" Value="{StaticResource GreyGradientBrush}" />
    <Setter TargetName="ButtonBody" Property="BorderBrush" Value="{StaticResource BorderBrush}" />
</Trigger>

Answer 1

1)
Because you can’t cast false(bool) into WarningLevels.

Remember the first parameter of PropertyMetadata is your default value.

2) You propably will run into another problem. Your Trigger value

Value="wlError"

is a string, which can’t be converted into an enum aswell, without a type converter. The easiest way to fix that is to extend your value:

<Trigger Property="local:WarningLevelButton.CurrentWarningLevel" >
    <Trigger.Value>
        <my:WarningLevels>wlError</my:WarningLevels>
    </Trigger.Value>
    <Setter TargetName="ButtonBody" Property="Background" Value="{StaticResource CtlRedBrush}" />
    <Setter TargetName="ButtonBody" Property="BorderBrush" Value="{StaticResource CtlRedBrush}" />
</Trigger>