Fix: Show source project in graph build errors for invalid project references

src/Build.UnitTests/Graph/ProjectGraph_Tests.cs

@@ -2923,6 +2923,77 @@ public void GetTargetListsWithSolutionInvalidTargets(string entryTarget)
             }
         }
 
+        [Fact]
+        [UseInvariantCulture]
+        public void InvalidProjectReferenceErrorIncludesReferringProject()
+        {
+            using (var env = TestEnvironment.Create())
+            {
+                // Create a project that references a non-existent project
+                TransientTestFile project1 = env.CreateFile("project1.proj", @"
+<Project>
+  <ItemGroup>
+    <ProjectReference Include=""missing.proj"" />
+  </ItemGroup>
+</Project>");
+
+                // Attempt to create a graph should throw an exception
+                var exception = Should.Throw<AggregateException>(() => new ProjectGraph(project1.Path));
+
+                // The exception should be an InvalidProjectFileException
+                exception.InnerExceptions.ShouldHaveSingleItem();
+                var innerException = exception.InnerExceptions[0].ShouldBeOfType<InvalidProjectFileException>();
+
+                // The error message should mention the referring project
+                innerException.Message.ShouldContain("project1.proj");
+                innerException.Message.ShouldContain("Referenced by:");
+            }
+        }
+
+        [Fact]
+        [UseInvariantCulture]
+        public void InvalidProjectReferenceErrorIncludesMultipleReferringProjects()
+        {
+            using (var env = TestEnvironment.Create())
+            {
+                // Create two projects that both reference a non-existent project
+                TransientTestFile project1 = env.CreateFile("project1.proj", @"
+<Project>
+  <ItemGroup>
+    <ProjectReference Include=""missing.proj"" />
+  </ItemGroup>
+</Project>");
+
+                TransientTestFile project2 = env.CreateFile("project2.proj", @"
+<Project>
+  <ItemGroup>
+    <ProjectReference Include=""missing.proj"" />
+  </ItemGroup>
+</Project>");
+
+                TransientTestFile main = env.CreateFile("main.proj", @"
+<Project>
+  <ItemGroup>
+    <ProjectReference Include=""project1.proj"" />
+    <ProjectReference Include=""project2.proj"" />
+  </ItemGroup>
+</Project>");
+
+                // Attempt to create a graph should throw an exception
+                var exception = Should.Throw<AggregateException>(() => new ProjectGraph(main.Path));
+
+                // The exception should be an InvalidProjectFileException
+                exception.InnerExceptions.ShouldHaveSingleItem();
+                var innerException = exception.InnerExceptions[0].ShouldBeOfType<InvalidProjectFileException>();
+
+                // The error message should mention at least one referring project
+                innerException.Message.ShouldContain("Referenced by:");
+                bool hasProject1 = innerException.Message.Contains("project1.proj");
+                bool hasProject2 = innerException.Message.Contains("project2.proj");
+                (hasProject1 || hasProject2).ShouldBeTrue();
+            }
+        }
+
         public void Dispose()
         {
             _env.Dispose();

src/Build/Graph/GraphBuilder.cs

@@ -53,6 +53,14 @@ internal class GraphBuilder
         private IReadOnlyDictionary<string, IReadOnlyCollection<string>> _solutionDependencies;
         private ConcurrentDictionary<ConfigurationMetadata, Lazy<ProjectInstance>> _platformNegotiationInstancesCache = new();
 
+        /// <summary>
+        /// Tracks which projects are referencing each project. Used to provide better error messages
+        /// when a referenced project fails to load.
+        /// Key: The project being referenced
+        /// Value: Set of projects that reference it
+        /// </summary>
+        private readonly ConcurrentDictionary<ConfigurationMetadata, ConcurrentBag<string>> _projectReferrers = new();
+
         public GraphBuilder(
             IEnumerable<ProjectGraphEntryPoint> entryPoints,
             ProjectCollection projectCollection,
@@ -528,10 +536,36 @@ private ParsedProject ParseProject(ConfigurationMetadata configurationMetadata)
             // TODO: ProjectInstance just converts the dictionary back to a PropertyDictionary, so find a way to directly provide it.
             var globalProperties = configurationMetadata.GlobalProperties.ToDictionary();
 
-            var projectInstance = _projectInstanceFactory(
-                configurationMetadata.ProjectFullPath,
-                globalProperties,
-                _projectCollection);
+            ProjectInstance projectInstance;
+            try
+            {
+                projectInstance = _projectInstanceFactory(
+                    configurationMetadata.ProjectFullPath,
+                    globalProperties,
+                    _projectCollection);
+            }
+            catch (InvalidProjectFileException ex) when (_projectReferrers.TryGetValue(configurationMetadata, out var referrers) && !referrers.IsEmpty)
+            {
+                // Enrich the exception with information about which project(s) referenced this project
+                string referrerList = string.Join(", ", referrers.Distinct().OrderBy(r => r));
+
+                string enrichedMessage = ResourceUtilities.FormatResourceStringIgnoreCodeAndKeyword(
+                    "ProjectGraphProjectFileCannotBeLoadedWithReferrers",
+                    referrerList,
+                    ex.Message);
+
+                throw new InvalidProjectFileException(
+                    ex.ProjectFile ?? configurationMetadata.ProjectFullPath,
+                    ex.LineNumber,
+                    ex.ColumnNumber,
+                    ex.EndLineNumber,
+                    ex.EndColumnNumber,
+                    enrichedMessage,
+                    ex.ErrorSubcategory,
+                    ex.ErrorCode,
+                    ex.HelpKeyword,
+                    ex.InnerException);
+            }
 
             if (projectInstance == null)
             {
@@ -585,6 +619,10 @@ private void SubmitProjectForParsing(ConfigurationMetadata projectToEvaluate)
                         referenceInfo.ReferenceConfiguration.ProjectFullPath));
                 }
 
+                // Track that this project is referencing the target project
+                _projectReferrers.GetOrAdd(referenceInfo.ReferenceConfiguration, _ => new ConcurrentBag<string>())
+                    .Add(parsedProject.ProjectInstance.FullPath);
+
                 SubmitProjectForParsing(referenceInfo.ReferenceConfiguration);
 
                 referenceInfos.Add(referenceInfo);

src/Build/Resources/Strings.resx

@@ -1830,6 +1830,14 @@ Utilization:          {0} Average Utilization: {1:###.0}</value>
       request a target to build itself (perhaps via a chain of other targets)
     </comment>
   </data>
+  <data name="ProjectGraphProjectFileCannotBeLoadedWithReferrers" xml:space="preserve">
+    <value>Referenced by: {0}. {1}</value>
+    <comment>
+      This message is prepended to the original error when a referenced project file cannot be loaded during static graph construction.
+      {0} is a comma-separated list of projects that reference the failed project
+      {1} is the complete original error message
+    </comment>
+  </data>
   <data name="CacheMissesNotAllowedInIsolatedGraphBuilds" xml:space="preserve">
     <value>MSB4252: Project "{0}" with global properties
     ({1})