Need better error message when generated assembly info collides with one in source.

I usually generate assembly information (such as version, informational version, etc) with custom tasks. However, when trying to use these with the SDK, it fails with errors such as Duplicate 'AssemblyVersion' attribute. If I add <GenerateAssemblyInfo>false</GenerateAssemblyInfo> to the csproj it is built with no issue and my custom generated versions are available.

A simple way to reproduce this is to add the GitVersionTask to the project and attempt to build.

The SDKs I’m using are:

<PackageReference Include="Microsoft.NET.Sdk">
  <Version>1.0.0-alpha-20161104-2</Version>
  <PrivateAssets>All</PrivateAssets>
</PackageReference>
<PackageReference Include="Microsoft.NET.Sdk.Web">
  <Version>1.0.0-alpha-20161104-2-112</Version>
  <PrivateAssets>All</PrivateAssets>
</PackageReference>

[twsouthwick]

This is a regression from previous behavior - an even simpler repro is add the following to a file:

[assembly: System.Reflection.AssemblyInformationalVersion("test")]

This is how assembly information is added in all other .NET projects and is now broken with the new SDK without adding this property. The assembly level attributes should only be created if they are not already available.

[rkeithhill]

Perhaps we just need some guidance on the landing page for this error:

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/cs0579

I tried to add a community comment:

If you get this error on one of the Assembly-level attributes such as AssemblyCompanyAttribute in a .NET Core/Standard project, the error might be occurring because you are compiling a “ported” AssemblyInfo.cs file with these attributes.

The new CSPROJ / MSBuild / SDK project system automatically generate these attributes which is why you get “duplicate attribute” errors when porting a project from the old build system to the new build system.  You can tell the new project system to skip auto-generating these attributes with <GenerateAssemblyInfo>false</GenerateAssemblyInfo>.

But the site wouldn’t accept my Hotmail account. Go figure. Anyway, I think this would be enough to address the issue.