CS0234 'Http' does not exist in the namespace 'System.Net' when targeting .NET Fx 4.x with implicit usings

[chucker]

Describe the bug

C# 10 introduces global usings. The .NET 6 SDK ships with a default template for implicit usings. For the regular SDK (e.g., non-Web), this generated C# looks like:

// <auto-generated/>
global using global::System;
global using global::System.Collections.Generic;
global using global::System.IO;
global using global::System.Linq;
global using global::System.Net.Http;
global using global::System.Threading;
global using global::System.Threading.Tasks;

However, System.Net.Http is a namespace that does not exist in .NET Framework 4.x. Therefore, this namespace should not be imported implicitly.

To Reproduce

A csproj like this will suffice:

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

  <PropertyGroup>
    <TargetFramework>net48</TargetFramework>
    <Nullable>enable</Nullable>
    <LangVersion>10.0</LangVersion>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

</Project>

This will fail to build with:

Error | CS0234 | The type or namespace name 'Http' does not exist in the namespace 'System.Net' (are you missing an assembly reference?)

[chucker]

(As a workaround, you can put namespace System.Net.Http { } somewhere in a C# file.)

[pranavkm]

Therefore, this namespace should not be imported implicitly.

The default list of usings are primarily tailored for .NET 6 and newer apps, and aren't enabled by default. You can use the Using itemgroup to specify a custom list of global usings to be used for your app, or use MSBuild to remove ones added by ImplicitUsings=Enable e.g. <Using Remove="System.Net.Http" />.

[Paul-Williams]

<Using Remove="System.Net.Http" /> does the trick.

[xiaoxstz]

Which node to put <Using Remove="System.Net.Http" /> ?

[xiaoxstz]

I solved it by installing the nuget package "System.Net.Http"

[hrumhurum]

An automatic workaround for the whole solution is to put near .sln file a Directory.Build.targets file with the following content:

<Project>

  <!-- Workaround for https://github.com/dotnet/sdk/issues/24146 -->
  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' ">
    <Using Remove="System.Net.Http" />
  </ItemGroup>
  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' AND $([MSBuild]::VersionLessThan($(TargetFrameworkVersion), '4.0')) ">
    <Using Remove="System.Threading.Tasks" />
  </ItemGroup>
  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' AND $([MSBuild]::VersionLessThan($(TargetFrameworkVersion), '3.5')) ">
    <Using Remove="System.Linq" />
  </ItemGroup>

</Project>

[Tyler-H]

An automatic workaround for the whole solution is to put near .sln file a Directory.Build.targets file with the following content:

<Project>

  <!-- Workaround for https://github.com/dotnet/sdk/issues/24146 -->
  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' ">
    <Using Remove="System.Net.Http" />
  </ItemGroup>
  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' AND $([MSBuild]::VersionLessThan($(TargetFrameworkVersion), '4.0')) ">
    <Using Remove="System.Threading.Tasks" />
  </ItemGroup>
  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' AND $([MSBuild]::VersionLessThan($(TargetFrameworkVersion), '3.5')) ">
    <Using Remove="System.Linq" />
  </ItemGroup>

</Project>

Put "near" it? What does that mean? Do you mean put that content in the .sln file?

[KalleOlaviNiemitalo]

A Directory.Build.targets file in the same directory as the solution file would be "near" it in this sense.

It isn't really related to the solution file, though. The important thing is that Directory.Build.targets should be in a directory that contains all the per-project subdirectories, so that it affects all of those projects.

[Varorbc]

This issue has been fixed by #32630