Cannot add element with the same key to resource dictionary - it already exists

Question

while parsing one xaml file I get an exception:

Result Message:
Test method ThemeResourceDictionaryTest.ParseXaml threw exception: System.Windows.Markup.XamlParseException: 'Add value to dictionary of type 'System.Windows.ResourceDictionary' threw an exception.' Line number '1397' and line position '4'. ---> System.ArgumentException: Item has already been added. Key in dictionary: 'System.Windows.Controls.TextBox' Key being added: 'System.Windows.Controls.TextBox'

and my element:

<Style x:Key="MyVeryUniqueKey" BasedOn="{StaticResource TextBoxStyle}" TargetType="TextBox">
    <Setter Property="MinWidth" Value="90"/>
</Style>

Why is the key of the element set to TextBox, when I explicitly set the key to be MyVeryUniqueKey, which is unique.

I have other styles whose target type is TextBox, but they all have different keys.

Answer 1

I suspect that, in a particular resource dictionary scope, you have more than one TextBox style where the TargetType is set but the x:Key is not.

If you don't explicitly set the dictionary key for a TextBox style it will default to {x:Type TextBox}. You obviously can't have two of these duplicated as keys in a dictionary.

For example, if you put this kind of thing in resources:

<Window.Resources>
    <Style TargetType="{x:Type TextBox}" />
    <Style TargetType="{x:Type TextBox}" />
</Window.Resources>

Then you will get a runtime XamlParseException:

Item has already been added. Key in dictionary: 'System.Windows.Controls.TextBox' Key being added: 'System.Windows.Controls.TextBox'"