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Use C# 9 records in non-net5.0 projects

PreviousPersisting output files from source generatorsNextAsyncLocal never leaks and is safe for CallContext-like state

Last updated 3 years ago

The new C# 9 records syntax is quite nice:

[DebuggerDisplay("{Name} = {Value}")]
record ResourceValue(string Name, string Value, bool HasFormat)
{
    public bool IsIndexed { get; init; }
    public List<string> Format { get; } = new List<string>();
}

When using it in a non-net5.0 project (i.e. netstandard2.0), :

Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported

To workaround the issue, simply 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)]
    sealed 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>

(). Note the class doesn't even need to be public.

compilation fails with an error
as seen elsewhere too