"The Name 'XXX' has already been defined" build error occurs if ControlTemplate has visual element and VisualState with the same names

[pavelovcharov]

• .NET Core Version: 3.0.100-preview-010184
• Windows version: 1809 (17763.253)
• Does the bug reproduce also in WPF for .NET Framework 4.8?: No

Problem description:
Build error occurs if ControlTemplate contains a visual element and VisualState with the same names

Actual behavior:
Build error "MC4101: The Name 'Normal' has already been defined. Names must be unique." occurs

Expected behavior:
No build errors

Minimal repro:
Create new .Net Core 3 wpf project, add the following ControlTemplate

    <Window.Resources>
        <ControlTemplate TargetType="Button" x:Key="buttonStyle">
            <Border x:Name="Normal">
                <VisualStateManager.VisualStateGroups>
                    <VisualStateGroup x:Name="TestGroup">
                        <VisualState x:Name="Normal" />
                    </VisualStateGroup>
                </VisualStateManager.VisualStateGroups>
            </Border>
        </ControlTemplate>
    </Window.Resources>

and try to build the project.

[stevenbrix]

Thanks @pavelovcharov for filing this issue! This is interesting, in a .NET Framework WPF app, you see an intellisense error, and an error shows up in the ErrorsList, but doesn't cause your build to error. The UWP Xaml Compiler also treats this as an error condition as well, and your build will fail. The build error does seem like correct behavior, as the names would collide.

@ryalanms i'm assigning this to you. i'd imagine we should make sure this doesn't cause errors so that porting from .NET Framework -> .NET Core is as easy as possible. however, we should continue having discussions so that these behaviors are consistent and both UWP and WPF behave the same

[ryalanms]

This difference was due to a bug in BamlMapTable.