skip non-sdk stype projects + some more null dereference handling

src/NuGet.Core/NuGet.CommandLine.XPlat/Commands/WhyCommand/DependencyGraphFinder.cs

@@ -38,13 +38,13 @@ internal static class DependencyGraphFinder
             {
                 foreach (var (targetFrameworkAlias, topLevelReferences) in topLevelReferencesByFramework)
                 {
-                    LockFileTarget target = assetsFile.GetTarget(targetFrameworkAlias, runtimeIdentifier: null);
+                    LockFileTarget? target = assetsFile.GetTarget(targetFrameworkAlias, runtimeIdentifier: null);
 
                     // get all package libraries for the framework
-                    IList<LockFileTargetLibrary> packageLibraries = target.Libraries;
+                    IList<LockFileTargetLibrary>? packageLibraries = target?.Libraries;
 
                     // if the project has a dependency on the target package, get the dependency graph
-                    if (packageLibraries.Any(l => l?.Name?.Equals(targetPackage, StringComparison.OrdinalIgnoreCase) == true))
+                    if (packageLibraries?.Any(l => l?.Name?.Equals(targetPackage, StringComparison.OrdinalIgnoreCase) == true) == true)
                     {
                         doesProjectHaveDependencyOnPackage = true;
                         dependencyGraphPerFramework.Add(targetFrameworkAlias,
@@ -272,7 +272,7 @@ private static Dictionary<string, List<string>> GetTopLevelPackageAndProjectRefe
         /// <param name="packageLibraries">All package libraries for a given framework.</param>
         private static Dictionary<string, string> GetAllResolvedVersions(IList<LockFileTargetLibrary> packageLibraries)
         {
-            var versions = new Dictionary<string, string>();
+            var versions = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
             foreach (var package in packageLibraries)
             {

src/NuGet.Core/NuGet.CommandLine.XPlat/Commands/WhyCommand/WhyCommandRunner.cs

@@ -39,35 +39,48 @@ public static int ExecuteCommand(WhyCommandArgs whyCommandArgs)
             foreach (var projectPath in projectPaths)
             {
                 Project project = MSBuildAPIUtility.GetProject(projectPath);
-                LockFile? assetsFile = GetProjectAssetsFile(project, whyCommandArgs.Logger);
 
-                if (assetsFile != null)
-                {
-                    ValidateFrameworksOptions(assetsFile, whyCommandArgs.Frameworks, whyCommandArgs.Logger);
+                string usingNetSdk = project.GetPropertyValue("UsingMicrosoftNETSdk");
 
-                    Dictionary<string, List<DependencyNode>?>? dependencyGraphPerFramework = DependencyGraphFinder.GetAllDependencyGraphsForTarget(
-                        assetsFile,
-                        whyCommandArgs.Package,
-                        whyCommandArgs.Frameworks);
+                if (!string.IsNullOrEmpty(usingNetSdk))
+                {
+                    LockFile? assetsFile = GetProjectAssetsFile(project, whyCommandArgs.Logger);
 
-                    if (dependencyGraphPerFramework != null)
+                    if (assetsFile != null)
                     {
-                        whyCommandArgs.Logger.LogMinimal(
-                            string.Format(
-                                Strings.WhyCommand_Message_DependencyGraphsFoundInProject,
-                                assetsFile.PackageSpec.Name,
-                                targetPackage));
-
-                        DependencyGraphPrinter.PrintAllDependencyGraphs(dependencyGraphPerFramework, targetPackage, whyCommandArgs.Logger);
+                        ValidateFrameworksOptions(assetsFile, whyCommandArgs.Frameworks, whyCommandArgs.Logger);
+
+                        Dictionary<string, List<DependencyNode>?>? dependencyGraphPerFramework = DependencyGraphFinder.GetAllDependencyGraphsForTarget(
+                            assetsFile,
+                            whyCommandArgs.Package,
+                            whyCommandArgs.Frameworks);
+
+                        if (dependencyGraphPerFramework != null)
+                        {
+                            whyCommandArgs.Logger.LogMinimal(
+                                string.Format(
+                                    Strings.WhyCommand_Message_DependencyGraphsFoundInProject,
+                                    assetsFile.PackageSpec.Name,
+                                    targetPackage));
+
+                            DependencyGraphPrinter.PrintAllDependencyGraphs(dependencyGraphPerFramework, targetPackage, whyCommandArgs.Logger);
+                        }
+                        else
+                        {
+                            whyCommandArgs.Logger.LogMinimal(
+                                string.Format(
+                                    Strings.WhyCommand_Message_NoDependencyGraphsFoundInProject,
+                                    assetsFile.PackageSpec.Name,
+                                    targetPackage));
+                        }
                     }
-                    else
-                    {
-                        whyCommandArgs.Logger.LogMinimal(
+                }
+                else
+                {
+                    whyCommandArgs.Logger.LogMinimal(
                             string.Format(
-                                Strings.WhyCommand_Message_NoDependencyGraphsFoundInProject,
-                                assetsFile.PackageSpec.Name,
-                                targetPackage));
-                    }
+                                Strings.WhyCommand_Message_NonSDKStyleProjectsAreNotSupported,
+                                project.GetPropertyValue("MSBuildProjectName")));
                 }
 
                 ProjectCollection.GlobalProjectCollection.UnloadProject(project);

src/NuGet.Core/NuGet.CommandLine.XPlat/Strings.resx

@@ -932,6 +932,9 @@ Non-HTTPS access will be removed in a future version. Consider migrating to 'HTT
     <value>Unable to run 'dotnet nuget why'. Missing or invalid project/solution file '{0}'.</value>
     <comment>{0} - Project/solution file path</comment>
   </data>
+  <data name="WhyCommand_Message_NonSDKStyleProjectsAreNotSupported" xml:space="preserve">
+    <value>Unable to run 'dotnet nuget why' for project '{0}'. This command only works on SDK-style projects.</value>
+  </data>
   <data name="WhyCommand_Error_InvalidAssetsFile" xml:space="preserve">
     <value>The assets file {0} is invalid. Please run restore for project '{1}' before running this command.</value>
     <comment>{0} - Assets file path, {1} - Project name</comment>