如何配置NuGet Client SDK指定特定目标框架？

我有一段代码可实现将NuGet包及其依赖项下载至指定文件夹，但这段代码会下载所有受支持的.NET版本（如.NET Standard 2.1、UWP、net45等）的二进制文件。请问如何配置它，使其仅下载适用于特定目标框架（例如.NET 6.0）的包版本？

补充说明：额外请教如何在运行时动态加载目标包所需的程序集，若需修改NuGet以包含json.deps也可接受。

现有代码示例 #

public async Task<HashSet<PackageIdentity>> InstallPackageAndDependenciesAsync(PackageIdentity identity)
{
    bool allowUnlisted = false;
    bool includePrelease = true;

    try
    {
        ResolutionContext resolutionContext = new ResolutionContext(
            DependencyBehavior.Lowest,
            includePrelease,
            allowUnlisted,
            VersionConstraints.ExactMajor | VersionConstraints.ExactMinor);

        INuGetProjectContext projectContext = new BlankProjectContext(Settings.Settings, new NuGetCustomLogger());

        var installActions = await LocalNuGetPackageManager.PreviewInstallPackageAsync(
            LocalNuGetPackageManager.PackagesFolderNuGetProject, identity,
            resolutionContext, projectContext,
            new[] { RemoteNuGetFeed }, Array.Empty<SourceRepository>(),
            CancellationToken.None);

        var sourceCache = new SourceCacheContext();
        await LocalNuGetPackageManager.ExecuteNuGetProjectActionsAsync(LocalNuGetPackageManager.PackagesFolderNuGetProject, installActions, projectContext, sourceCache, CancellationToken.None);

        return new HashSet<PackageIdentity>(installActions.Select(action => action.PackageIdentity));
    }
    catch (Exception ex)
    {
        throw;
    }
} 

一、仅下载指定目标框架的NuGet包 #

要实现只下载对应.NET 6.0的包，核心是让NuGet在解析依赖时只筛选兼容指定框架的包版本，具体修改如下：

1. 定义目标框架
   先明确要适配的目标框架：

   var targetFramework = FrameworkName.Parse("net6.0");
   var targetFrameworks = new[] { targetFramework };
   

2. 自定义NuGet项目类
   默认的PackagesFolderNuGetProject未指定目标框架，需要创建自定义类来暴露目标框架信息：

   public class TargetedPackagesFolderNuGetProject : PackagesFolderNuGetProject
   {
       private readonly IEnumerable<FrameworkName> _targetFrameworks;
   
       public TargetedPackagesFolderNuGetProject(string packagesPath, IEnumerable<FrameworkName> targetFrameworks)
           : base(packagesPath)
       {
           _targetFrameworks = targetFrameworks;
       }
   
       public override IEnumerable<FrameworkName> GetTargetFrameworks()
       {
           return _targetFrameworks;
       }
   }
   

3. 修改原代码逻辑
   替换原有项目实例，并给ResolutionContext绑定目标框架，确保依赖解析时只匹配指定框架：

   // 替换默认的项目实例
   var targetedProject = new TargetedPackagesFolderNuGetProject(LocalNuGetPackageManager.PackagesFolder, targetFrameworks);
   
   // 初始化带目标框架的解析上下文
   var resolutionContext = new ResolutionContext(
       DependencyBehavior.Lowest,
       includePrelease,
       allowUnlisted,
       VersionConstraints.ExactMajor | VersionConstraints.ExactMinor)
   {
       TargetFrameworks = targetFrameworks
   };
   

   后续在PreviewInstallPackageAsync和ExecuteNuGetProjectActionsAsync方法中，用targetedProject替代原来的PackagesFolderNuGetProject即可。

   调整后，NuGet会只下载与.NET 6.0兼容的包二进制文件，不再包含其他无关框架的内容。

二、运行时动态加载程序集 #

结合deps.json和AssemblyLoadContext可以实现依赖的正确加载，步骤如下：

1. 确保保留deps.json文件
   NuGet默认会下载包的完整内容，其中包含deps.json，只需确保解压包时不排除该文件即可。

2. 自定义AssemblyLoadContext
   基于deps.json的依赖信息来加载程序集：

   public class NuGetAssemblyLoadContext : AssemblyLoadContext
   {
       private readonly DependencyContext _dependencyContext;
       private readonly string _packagesFolder;
   
       public NuGetAssemblyLoadContext(string depsJsonPath, string packagesFolder)
           : base(isCollectible: true)
       {
           _packagesFolder = packagesFolder;
           using var stream = File.OpenRead(depsJsonPath);
           _dependencyContext = DependencyContext.Load(stream);
       }
   
       protected override Assembly? Load(AssemblyName assemblyName)
       {
           // 从依赖上下文查找程序集路径
           var library = _dependencyContext.RuntimeLibraries
               .FirstOrDefault(l => l.Name.Equals(assemblyName.Name, StringComparison.OrdinalIgnoreCase));
   
           if (library != null)
           {
               var assemblyPath = Path.Combine(_packagesFolder, $"{library.Name}/{library.Version}", 
                   library.RuntimeAssemblyGroups.First().Assemblies.First().Path);
   
               if (File.Exists(assemblyPath))
               {
                   return LoadFromAssemblyPath(assemblyPath);
               }
           }
   
           // 回退到默认加载逻辑
           return Default.LoadFromAssemblyName(assemblyName);
       }
   }
   

3. 加载示例
   找到目标包的deps.json路径，初始化加载上下文后即可加载程序集：

   // 示例：加载Newtonsoft.Json的程序集
   var depsPath = Path.Combine("./packages/Newtonsoft.Json/13.0.3", "Newtonsoft.Json.deps.json");
   var loadContext = new NuGetAssemblyLoadContext(depsPath, "./packages");
   var assembly = loadContext.LoadFromAssemblyName(new AssemblyName("Newtonsoft.Json"));
   
   // 动态调用示例
   var jsonConvertType = assembly.GetType("Newtonsoft.Json.JsonConvert");
   var serializeMethod = jsonConvertType.GetMethod("SerializeObject", new[] { typeof(object) });
   var result = serializeMethod.Invoke(null, new object[] { new { Name = "Test" } });
   

内容的提问来源于stack exchange，提问作者TomSelleck