Roslyn: Dev nuget Package Microsoft.Net.Compilers.Toolset v 3.8.0-1.20325.3 when using `init` feature in c# 9 raise error: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported

[moh-hassan]

Version Used:
Dev nuget package: Microsoft.Net.Compilers.Toolset v 3.8.0-1.20325.3

Steps to Reproduce:
1- Install Net5 Preview 5 SDK.
2- Create a console project and add the next class to use c# 9 init feature

public class Person
    {        
        public string FirstName { get; init; }
        public string LastName { get; init; }         
    }

3- Add a reference to Microsoft.Net.Compilers.Toolset to the project.
.NET Core SDK Net5 Preview 5 is not supporting init of C# 9 feature yet.

<ItemGroup>     
    <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="3.8.0-1.20325.3">
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
      <PrivateAssets>all</PrivateAssets>
    </PackageReference>
  </ItemGroup>
  ```
4- build

dotnet build

**Expected Behavior**:
Build success.

**Actual Behavior**:
The compiler raise error  
> error CS0518: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported  

5- Add this dummy class to the project:
```cs
namespace System.Runtime.CompilerServices
{
    public sealed class IsExternalInit
    {
    }
}

6- build again
Build success and application run sucessfully.

Is IsExternalInit a missing class?
If IsExternalInit class has no role remove any calling to it.

[joshlang]

Ahhh, I just posted a similar question here:

https://stackoverflow.com/questions/62648189/testing-c-sharp-9-0-in-vs2019-cs0518-isexternalinit-is-not-defined-or-imported

(I am using preview6 sdk though)

[pkanavos]

I've encountered the same problem, and even the latest master branch (June 27) in Sharplab.io generates the same error. I've used the same fix.

[jaredpar]

Thanks for reporting the issue. This is not a bug in the compiler. The IsExternalInit type is a new type added to .NET in the net5 time frame which is necessary to properly emit record types and init properties. That type was added but did not make it into the .NET 5 SDK you used. Once you upgrade to a version which has the type, believe Preview 7, this will go away.

[holc-tkomp]

(SDK Preview 8 + VS Pro 16.8 Preview 2.0)

This works fine now for net5.0 target framework, but if trying to target any of the net461, net472, net48, netcoreapp2.1, netcoreapp3.1, netstandard2.0, netstandard2.1 error will still be thrown.

If dummy class is added, the other tfms works fine.

[kzu]

Trivial workaround: just declare the missing type in your project:

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System.ComponentModel;

namespace System.Runtime.CompilerServices
{
    /// <summary>
    /// Reserved to be used by the compiler for tracking metadata.
    /// This class should not be used by developers in source code.
    /// </summary>
    [EditorBrowsable(EditorBrowsableState.Never)]
    public static class IsExternalInit
    {
    }
}

If you're multitargeting net5.0 and other TFMs, just add this bit of MSBuild to remove it for net5.0 since it's built-in:

<ItemGroup>
  <Compile Remove="IsExternalInit.cs" Condition="'$(TargetFramework)' == 'net5.0'" />
</ItemGroup>

[BoasE]

I stil have this within net standard 2.1 projects with the release from today. an anyone confirm?

[hey-red]

@BoasE I can confirm that.
Edit: As I can see it's not a bug
https://developercommunity.visualstudio.com/content/problem/1244809/error-cs0518-predefined-type-systemruntimecompiler.html

More about target framework https://docs.microsoft.com/en-us/dotnet/standard/net-standard#when-to-target-net50-vs-netstandard

[brantburnett]

I recommend adding this file to your project, based on the example above from @kzu. It will handle multi-targeting and marks the attribute as internal to avoid conflicts with other packages.

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

#if NETSTANDARD2_0 || NETSTANDARD2_1 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NETCOREAPP3_0 || NETCOREAPP3_1 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48

using System.ComponentModel;

// ReSharper disable once CheckNamespace
namespace System.Runtime.CompilerServices
{
    /// <summary>
    /// Reserved to be used by the compiler for tracking metadata.
    /// This class should not be used by developers in source code.
    /// </summary>
    [EditorBrowsable(EditorBrowsableState.Never)]
    internal static class IsExternalInit
    {
    }
}

#endif

[igabesz]

Thanks for the workaround. Still feels like a hack though.

I doubt that it should be the responsibility of the developer to provide internal system classes to make the compiler work properly...

[brantburnett]

Thanks for the workaround. Still feels like a hack though.

I doubt that it should be the responsibility of the developer to provide internal system classes to make the compiler work properly...

I generally agree, but this isn't the first feature like this. A similar (and larger) workaround file is necessary for nullable reference types on older frameworks. I know some language features aren't really usable on older versions of the framework, but this one is. Feels like Microsoft could publish a NuGet package that gives us what we need for older frameworks, much like was done with IAsyncEnumerable in Microsoft.Bcl.AsyncInterfaces. It could probably even be a source code package instead of a binary package. I see a couple of packages from 3rd parties that may do this, but I'd prefer something official and maintained.

[jaredpar]

I doubt that it should be the responsibility of the developer to provide internal system classes to make the compiler work properly...

This is a C# 9 feature and hence only officially supported when targeting net5.0 or higher. The IsExternalInit type is defined there and hence requires no extra work by developers. When targeting other TFs the developer is required to provide the type.

This is how C# has functioned for many features over the years. Extension methods are a very analogous example here. Unless you target a target framework with the suporting ExtensionAttribute type you must define it yourself.

[kzu]

What I added to my approach is bit of msbuild:

<ItemGroup>
  <Compile Remove="IsExternalInit.cs" Condition="'$(TargetFramework)' == 'net5.0'" />
</ItemGroup>

[moh-hassan]

@jaredpar

This class should not be used by developers in source code.

But we have to use the class with Microsoft.Net.Compilers.Toolset in netcore 3.1 to support c# 9.
I checked the version 3.8.0-5.final and also it is not available.
I think the above phrase should be removed from the source and it's better if the missed class is added to Microsoft.Net.Compilers.Toolset.