C#: Detect SDK-style project imports

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/FileContent.cs

@@ -246,12 +246,12 @@ private void Parse(string file)
                         break;
                     case "project":
                         var sdk = reader.GetAttribute("Sdk");
-                        isSdkProjectStructureUsed |= sdk != null;
+                        isSdkProjectStructureUsed |= sdk is not null;
                         useAspNetCoreDlls |= string.Equals(sdk, "Microsoft.NET.Sdk.Web", StringComparison.OrdinalIgnoreCase);
                         break;
                     case "frameworkreference":
                         var framework = reader.GetAttribute("Include");
-                        isSdkProjectStructureUsed |= framework != null;
+                        isSdkProjectStructureUsed |= framework is not null;
                         useAspNetCoreDlls |= string.Equals(framework, "Microsoft.AspNetCore.App", StringComparison.OrdinalIgnoreCase);
                         break;
                     case "implicitusings":
@@ -271,8 +271,11 @@ private void Parse(string file)
                         }
                         break;
                     case "import":
-                        isLegacyProjectStructureUsed = isLegacyProjectStructureUsed || reader.GetAttribute("Project")?.EndsWith("Microsoft.CSharp.targets", StringComparison.OrdinalIgnoreCase) == true;
+                        var project = reader.GetAttribute("Project");
+                        isSdkProjectStructureUsed |= project is not null && reader.GetAttribute("Sdk") is not null;
+                        isLegacyProjectStructureUsed = isLegacyProjectStructureUsed || project?.EndsWith("Microsoft.CSharp.targets", StringComparison.OrdinalIgnoreCase) == true;
                         break;
+                        // TODO: we could also check `<Sdk Name="Microsoft.NET.Sdk" />`
                 }
             });
         }

csharp/extractor/Semmle.Extraction.Tests/FileContent.cs

@@ -229,6 +229,24 @@ public void TestFileContent_SdkProjectStructure()
             Assert.False(isLegacy);
         }
 
+        /// <summary>
+        /// Verifies that an SDK attribute on an import indicates SDK project structure.
+        /// </summary>
+        [Fact]
+        public void TestFileContent_SdkProjectImport()
+        {
+            // Setup
+            var fileContent = new TestFileContent("<Project><Import Project='Sdk.props' Sdk='Microsoft.NET.Sdk' /></Project>");
+
+            // Execute
+            var isLegacy = fileContent.IsLegacyProjectStructureUsed;
+            var isSdk = fileContent.IsSdkProjectStructureUsed;
+
+            // Verify
+            Assert.True(isSdk);
+            Assert.False(isLegacy);
+        }
+
         /// <summary>
         /// Verifies that `FileContent` correctly handles XML syntax with namespaces and various elements.
         /// </summary>