Integration and unit tests no longer work on ASP.NET Core 2.1 failing to find assemblies at runtime

Update: This has been made easier with 2.2 Tooling. Make sure that your dotnet --version SDK version is at least 2.2.100, even when buidling 2.1 applications

Just add a versionless package reference to your project while keeping the Microsoft.NET.Sdk:

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

      <PropertyGroup>
        <TargetFramework>netcoreapp2.1</TargetFramework>
      </PropertyGroup>

      <ItemGroup>
        <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="2.1.1" />
        <PackageReference Include="Microsoft.AspNetCore.App" />
        <!-- other references to xunit, test SDK etc. -->
      </ItemGroup>

      <ItemGroup>
        <ProjectReference Include="..\AspNetCoreAppToTest\AspNetCoreAppToTest.csproj" />
      </ItemGroup>

    </Project>

Original:

ASP.NET Core 2.1 uses a new “shared framework” to run ASP.NET Core applications on. Test projects need to be modified/updated to also use this shared framework using one of the following approaches:

1. Change the test project’s <Project> tag in the first line to use the web SDK (Microsoft.NET.Sdk.Web instead of Microsoft.NET.Sdk) and add a package reference to Microsoft.AspNetCore.App (or .All if you are using that inside the web project) without specifying a version

   The project file (.csproj) of the test project should now look like this:

   <Project Sdk="Microsoft.NET.Sdk.Web">
   
     <PropertyGroup>
       <TargetFramework>netcoreapp2.1</TargetFramework>
     </PropertyGroup>
   
     <ItemGroup>
       <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="2.1.1" />
       <PackageReference Include="Microsoft.AspNetCore.App" />
       <!-- other references to xunit, test SDK etc. -->
     </ItemGroup>
   
     <ItemGroup>
       <ProjectReference Include="..\AspNetCoreAppToTest\AspNetCoreAppToTest.csproj" />
     </ItemGroup>
   
   </Project>
   

2. Alternative: Leave the Sdk as-is and add a PackageReference to the shared framework package but specify a version.

   This can be done by simply adding a NuGet reference to Microsoft.AspNetCore.App. However, this may cause issues since the SDK may choose to update the reference when a new patch release of the ASP.NET Core is released and the tooling is updated to reflect this. You will need update the NuGet reference for every patch release.