Build failed while writing custom controls

[hez2010]

• .NET Core Version: 3.0 preview 6
• Windows version: windows 10 1809 17763.615
• Does the bug reproduce also in WPF for .NET Framework 4.8?: No

Problem description:
Write a custom control, and build it. There's a possibility to get build error: Unknown build error, 'An item with the same key has already been added. Key: themes/generic.baml'.
And if you build it multiple times, it might succeed.

Actual behavior:
C:\Program Files\dotnet\sdk\3.0.100-preview6-012264\Sdks\Microsoft.NET.Sdk.WindowsDesktop\targets\Microsoft.WinFX.targets(687,5): error RG1000: Unknown build error, 'An item with the same key has already been added. Key: themes/generic.baml'

Expected behavior:
No errors.

Minimal repro:
XSecControls.zip

[ryalanms]

Thanks @hez2010. The SDK automatically pulls in all .xaml files, including themes/generic.xaml, which appears to be causing the duplicate ResourceItem at compile time.

https://github.com/dotnet/wpf/blob/master/packaging/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.props#L37

    <Page Include="**/*.xaml"
          Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder);@(ApplicationDefinition)"
          Condition="'$(EnableDefaultPageItems)' != 'false'" >
      <Generator>MSBuild:Compile</Generator>
    </Page>

Two options to fix the error when building the repro project are removing the explicit Page Item include for generic.xaml or setting the 'EnableDefaultPageItems' property to false and explicitly including all .xaml files as Pages.

Removing the Include for Generic.xaml from the .csproj is probably the easiest fix. This file will automatically be pulled in by the SDK:

 <ItemGroup>
    <Page Include="Themes\Generic.xaml" >
      <Generator>MSBuild:Compile</Generator>
    </Page>
  </ItemGroup>

<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <OutputType>library</OutputType>
    <UseWPF>true</UseWPF>
    <Nullable>enable</Nullable>
    <Version>3.0.0.0</Version>
    <Authors>X-Sec</Authors>
    <Product>X-Sec Antivirus</Product>
    <Description>X-Sec Antivirus Custom Controls</Description>
    <AssemblyVersion>1.0.0.0</AssemblyVersion>
    <FileVersion>1.0.0.0</FileVersion>
    <Copyright>(c) X-Sec.  All rights reserved.</Copyright>
  </PropertyGroup>
  
  <ItemGroup>
    <Compile Remove="Themes\**" />
    <EmbeddedResource Remove="Themes\**" />
    <None Remove="Themes\**" />
  </ItemGroup>

 <ItemGroup>
    <Page Include="Themes\Generic.xaml" >
      <Generator>MSBuild:Compile</Generator>
    </Page>
  </ItemGroup>
  
</Project>

[ryalanms]

Please re-open this issue if the above suggestions don't work. Thanks.

[vatsan-madhavan]

Turns out that there is a bug in Microsoft.NET.Sdk.WindowsDesktop.props - it fails to correctly initialize EnableDefaultPageItems, which in turn prevents the duplicate detection logic for Page items from running.

Reactivating this so that we can fix this issue.

[ryalanms]

Adding a default 'true' for unset EnableDefaultPageItems and EnableDefaultApplicationDefinition in Microsoft.NET.Sdk.WindowsDesktop.props causes MSBuild's CheckForDuplicateItems to output the correct error:

C:\Program Files\dotnet\sdk\3.0.100-preview8-013270\Sdks\Microsoft.NET.Sdk.WindowsDesktop\targets\Microsoft.NET.Sdk.WindowsDesktop.targets(21,5): error NETSDK1022: Duplicate 'Page' items were included. The .NET SDK includes 'Page' items from your project directory by default. You can either remove these items from your project file, or set the 'EnableDefaultPageItems' property to 'false' if you want to explicitly include them in your project file. For more information, see https://aka.ms/sdkimplicititems. The duplicate items were: 'Themes\Generic.xaml' [I:\test\XSecControls\XSecControls.csproj]

I'll send a PR shortly.

[ryalanms]

https://docs.microsoft.com/en-us/dotnet/core/tools/csproj#default-compilation-includes-in-net-core-projects needs to be updated.