Update implicit global usings feature to address issues

The new implicit global usings feature added to the SDK in 6.0.100-preview.7 has caused a few issues in some important scenarios such that some changes will be made to mitigate them and generally improve its usability.

These changes are targeting 6.0.100-rc.1

Items:

• Rename the MSBuild property from <DisableImplicitNamespaceImports> to <ImplicitUsings>
  • Note this is for C# only
  • Visual Basic will continue to use the <DisableImplicitNamespaceImports> property for disabling implicit namespace imports
• Add the <ImplicitUsings> property to the schema file so it appears in statement completion
• Change the defined conditions to only enable adding the implicit usings from the SDK when the <ImplictUsings> property is set to “true” or “enable”
  • The existing conditions based on TFM should be removed
  • The only condition for adding the implicit usings as defined in the SDK will be the check that <ImplicitUsings> is equal to “true” or “enable”
• Rename the item type that specifies the global namespaces to emit to the generated .cs file from <Import> to <Using>
  • Note this is for C# only
  • Visual Basic will continue to use the <Import> item type for specifying namespaces to import
• dotnet/msbuild#6745
• Ensure the generated file is still emitted if any <Using> items are declared (even if <ImplicitUsings> is false)
• Ensure that implicit <Import> items for Visual Basic are not changed from what they were in .NET 5
• Update processing of <Using> items to support defining using [alias] and using static [type] as well as using [namespace], e.g.:
  • <Using Include="Microsoft.AspNetCore.Http.Results" Alias="Results" /> emits global using Results = global::Microsoft.AspNetCore.Http.Results;
  • <Using Include="Microsoft.AspNetCore.Http.Results" Static="True" /> emits global using static global::Microsoft.AspNetCore.Http.Results;
  • <Using Include="Microsoft.AspNetCore.Http" /> emits global using global::Microsoft.AspNetCore.Http;
• Update the C# project templates to enable implicit usings for new .NET 6 projects, i.e. include <ImplicitUsings>enable</ImplicitUsings> in the .csproj file
  • This will be tracked by issues in the relevant repos for each template
  • dotnet/aspnetcore#35131
  • dotnet/templating#3619
  • dotnet/winforms#5074

Example project file content after these changes:

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

  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

  <ItemGroup>
    <Using Include="My.Awesome.Namespace" />
  </ItemGroup>
</Project>

[dsplaisted]

@DamianEdwards I do not think we should support enable as a value for the ImplicitUsings property. Other Boolean MSBuild properties just support true. The Nullable property uses enable instead of true because it has 4 different supported values instead of just true and false: https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references#nullable-contexts

[jmaillet]

Why is this not just driven by the template? Have an _Usings.cs in the project by default, like Razor does. Much easier to see what namespaces included by default. No need to google the documentation. Personally, I’d much rather just stay in my c# headspace and edit that file to add or remove namespaces, as opposed to mucking with the csproj file with a completely different syntax.