use null alias

Author: nkolev92

src/NuGet.Core/NuGet.Build.Tasks/GetReferenceNearestTargetFrameworkTask.cs

@@ -169,7 +169,7 @@ private ITaskItem AssignNearestFrameworkForSingleReference(
             // try project fallback frameworks
             foreach (var currentProjectTargetFramework in fallbackNuGetFrameworks)
             {
-                nearestNuGetFramework = packageSpec.GetNearestTargetFramework(currentProjectTargetFramework, null);
+                nearestNuGetFramework = packageSpec.GetNearestTargetFramework(currentProjectTargetFramework, targetAlias: null);
 
                 if (nearestNuGetFramework.FrameworkName != null)
                 {

src/NuGet.Core/NuGet.DependencyResolver.Core/LibraryRangeCacheKey.cs

@@ -17,7 +17,7 @@ namespace NuGet.DependencyResolver
         public LibraryRangeCacheKey(LibraryRange range, NuGetFramework framework, string? alias)
         {
             Framework = framework;
-            Alias = alias ?? string.Empty;
+            Alias = alias ?? string.Empty; // alias may be passed as both null or empty and we need to treat them equivalently.
             LibraryRange = range;
         }
 

src/NuGet.Core/NuGet.ProjectModel/PackageSpecExtensions.cs

@@ -95,7 +95,7 @@ public static ProjectRestoreMetadataFrameworkInfo GetRestoreMetadataFramework(th
         /// <param name="project">The project spec in question.</param>
         /// <param name="targetFramework">The pivot target framework.</param>
         /// <param name="targetAlias">The alias for disambiguation</param>
-        public static TargetFrameworkInformation GetNearestTargetFramework(this PackageSpec project, NuGetFramework targetFramework, string targetAlias)
+        public static TargetFrameworkInformation GetNearestTargetFramework(this PackageSpec project, NuGetFramework targetFramework, string? targetAlias)
         {
             TargetFrameworkInformation? result = null;
             List<TargetFrameworkInformation>? frameworks = null;

src/NuGet.Core/NuGet.ProjectModel/PackageSpecReferenceDependencyProvider.cs

@@ -246,7 +246,7 @@ internal List<LibraryDependency> GetDependenciesFromSpecRestoreMetadata(PackageS
             {
                 if (targetFrameworkInfo.FrameworkName == null && targetFramework is AssetTargetFallbackFramework atfFramework)
                 {
-                    targetFrameworkInfo = packageSpec.GetNearestTargetFramework(atfFramework.AsFallbackFramework(), string.Empty);
+                    targetFrameworkInfo = packageSpec.GetNearestTargetFramework(atfFramework.AsFallbackFramework(), targetAlias: null);
                 }
             }
 

src/NuGet.Core/NuGet.ProjectModel/PublicAPI.Unshipped.txt

@@ -1,5 +1,5 @@
 #nullable enable
-static NuGet.ProjectModel.PackageSpecExtensions.GetNearestTargetFramework(this NuGet.ProjectModel.PackageSpec! project, NuGet.Frameworks.NuGetFramework! targetFramework, string! targetAlias) -> NuGet.ProjectModel.TargetFrameworkInformation!
+static NuGet.ProjectModel.PackageSpecExtensions.GetNearestTargetFramework(this NuGet.ProjectModel.PackageSpec! project, NuGet.Frameworks.NuGetFramework! targetFramework, string? targetAlias) -> NuGet.ProjectModel.TargetFrameworkInformation!
 static readonly NuGet.ProjectModel.LockFileFormat.AliasedVersion -> int
 static readonly NuGet.ProjectModel.LockFileFormat.LegacyVersion -> int
 ~NuGet.ProjectModel.PackageSpecReferenceDependencyProvider.GetLibrary(NuGet.LibraryModel.LibraryRange libraryRange, NuGet.Frameworks.NuGetFramework targetFramework, string alias) -> NuGet.LibraryModel.Library