[C#] 実行中の .NET バージョンを取得する方法

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net8.0;net7.0;net6.0;net5.0;netcoreapp3.1;net48</TargetFrameworks>
  </PropertyGroup>

</Project>

// .NET 5 以降の Environment.Version のコード抜粋
public static Version Version
{
    get
    {
        string? versionString = typeof(object).Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion;

        ReadOnlySpan<char> versionSpan = versionString.AsSpan();

        // Strip optional suffixes
        int separatorIndex = versionSpan.IndexOfAny('-', '+', ' ');
        if (separatorIndex != -1)
            versionSpan = versionSpan.Slice(0, separatorIndex);

        // Return zeros rather then failing if the version string fails to parse
        return Version.TryParse(versionSpan, out Version? version) ? version : new Version();
    }
}

// .NET 5 以降の RuntimeInformation.FrameworkDescription のコード抜粋
private const string FrameworkName = ".NET";
private static string? s_frameworkDescription;

public static string FrameworkDescription
{
    get
    {
        if (s_frameworkDescription == null)
        {
            ReadOnlySpan<char> versionString = typeof(object).Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion;

            // Strip the git hash if there is one
            int plusIndex = versionString.IndexOf('+');
            if (plusIndex >= 0)
            {
                versionString = versionString.Slice(0, plusIndex);
            }

            s_frameworkDescription = !versionString.Trim().IsEmpty ? $"{FrameworkName} {versionString}" : FrameworkName;
        }

        return s_frameworkDescription;
    }
}

// .NET Core 3.1 の Environment.Version のコード抜粋
public static Version Version
{
    get
    {
        // FX_PRODUCT_VERSION is expected to be set by the host
        string? versionString = (string?)AppContext.GetData("FX_PRODUCT_VERSION");

        if (versionString == null)
        {
            // Use AssemblyInformationalVersionAttribute as fallback if the exact product version is not specified by the host
            versionString = typeof(object).Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion;
        }

        ReadOnlySpan<char> versionSpan = versionString.AsSpan();

        // Strip optional suffixes
        int separatorIndex = versionSpan.IndexOfAny("-+ ");
        if (separatorIndex != -1)
            versionSpan = versionSpan.Slice(0, separatorIndex);

        // Return zeros rather then failing if the version string fails to parse
        return Version.TryParse(versionSpan, out Version? version) ? version : new Version();
    }
}

// .NET Core 3.1 の RuntimeInformation.FrameworkDescription のコード抜粋
private const string FrameworkName = ".NET Core";
private static string s_frameworkDescription;

public static string FrameworkDescription
{
    get
    {
        if (s_frameworkDescription == null)
        {
            string versionString = (string)AppContext.GetData("FX_PRODUCT_VERSION");

            if (versionString == null)
            {
                // Use AssemblyInformationalVersionAttribute as fallback if the exact product version is not specified by the host
                versionString = typeof(object).Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion;

                // Strip the git hash if there is one
                int plusIndex = versionString.IndexOf('+');
                if (plusIndex != -1)
                    versionString = versionString.Substring(0, plusIndex);
            }

            s_frameworkDescription = $"{FrameworkName} {versionString}";
        }

        return s_frameworkDescription;
    }
}

// .NET Framework 4.8.1 の Environment.Version のコード抜粋
public static Version Version => new Version(4, 0, 30319, 42000);

// .NET Framework 4.8.1 の RuntimeInformation.FrameworkDescription のコード抜粋
private static string s_frameworkDescription;

public static string FrameworkDescription
{
  get
  {
    if (RuntimeInformation.s_frameworkDescription == null)
      RuntimeInformation.s_frameworkDescription = ".NET Framework " + ((AssemblyFileVersionAttribute)typeof(object).GetTypeInfo().Assembly.GetCustomAttribute(typeof(AssemblyFileVersionAttribute))).Version;
    return RuntimeInformation.s_frameworkDescription;
  }
}

// .NET 5 以降および .NET Core 3.1 の AppContext.TargetFrameworkName のコード抜粋
public static string? TargetFrameworkName =>
    // The Target framework is not the framework that the process is actually running on.
    // It is the value read from the TargetFrameworkAttribute on the .exe that started the process.
    Assembly.GetEntryAssembly()?.GetCustomAttribute<TargetFrameworkAttribute>()?.FrameworkName;

// .NET Framework 4.8.1 の AppContext.TargetFrameworkName のコード抜粋
public static class AppContext
{
    public static string TargetFrameworkName
    {
        // 1. AppDomainSetup.TargetFrameworkName プロパティを参照している
        get => AppDomain.CurrentDomain.SetupInformation.TargetFrameworkName;
    }
}

public sealed class AppDomainSetup : IAppDomainSetup
{
    [OptionalField(VersionAdded = 5)]
    private string _TargetFrameworkName;

    // 2. ここにセットされる値は AppDomain.GetTargetFrameworkName() で取得している
    public string TargetFrameworkName
    {
        get => this._TargetFrameworkName;
        set => this._TargetFrameworkName = value;
    }
}

public sealed class AppDomain
{
    private AppDomainSetup _FusionStore;

    // 3. ここで実際に取得する
    internal String GetTargetFrameworkName()
    {
        String targetFrameworkName = _FusionStore.TargetFrameworkName;

        if (targetFrameworkName == null && IsDefaultAppDomain() && !_FusionStore.CheckedForTargetFrameworkName)
        {
            // This should only be run in the default appdomain.  All other appdomains should have
            // values copied from the default appdomain and/or specified by the host.
            Assembly assembly = Assembly.GetEntryAssembly();
            if (assembly != null)
            {
                TargetFrameworkAttribute[] attrs = (TargetFrameworkAttribute[])assembly.GetCustomAttributes(typeof(TargetFrameworkAttribute));
                if (attrs != null && attrs.Length > 0)
                {
                    Contract.Assert(attrs.Length == 1);
                    targetFrameworkName = attrs[0].FrameworkName;
                    _FusionStore.TargetFrameworkName = targetFrameworkName;
                }
            }
            _FusionStore.CheckedForTargetFrameworkName = true;
        }

        return targetFrameworkName;
    }
}