C#: Detect SDK elements in project files

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

Author: michaelnebel

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

@@ -275,7 +275,11 @@ private void Parse(string file)
                         isSdkProjectStructureUsed = 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" />`
+                    case "sdk":
+                        var sdkName = reader.GetAttribute("Name");
+                        isSdkProjectStructureUsed |= sdkName != null;
+                        useAspNetCoreDlls |= string.Equals(sdkName, "Microsoft.NET.Sdk.Web", StringComparison.OrdinalIgnoreCase);
+                        break;
                 }
             });
         }

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

@@ -247,6 +247,44 @@ public void TestFileContent_SdkProjectImport()
             Assert.False(isLegacy);
         }
 
+        /// <summary>
+        /// Verifies that an SDK element indicates SDK project structure.
+        /// </summary>
+        [Fact]
+        public void TestFileContent_SdkElement()
+        {
+            // Setup
+            var fileContent = new TestFileContent("<Project><Sdk Name='Microsoft.NET.Sdk' /></Project>");
+
+            // Execute
+            var isSdk = fileContent.IsSdkProjectStructureUsed;
+            var isLegacy = fileContent.IsLegacyProjectStructureUsed;
+
+            // Verify
+            Assert.True(isSdk);
+            Assert.False(isLegacy);
+        }
+
+        /// <summary>
+        /// Verifies that an ASP.NET SDK element indicates ASP.NET Core usage.
+        /// </summary>
+        [Fact]
+        public void TestFileContent_AspNetCoreSdkElement()
+        {
+            // Setup
+            var fileContent = new TestFileContent("<Project><Sdk Name='Microsoft.NET.Sdk.Web' /></Project>");
+
+            // Execute
+            var isSdk = fileContent.IsSdkProjectStructureUsed;
+            var useAspNetCoreDlls = fileContent.UseAspNetCoreDlls;
+            var isAspNetCoreDetected = fileContent.IsAspNetCoreDetected;
+
+            // Verify
+            Assert.True(isSdk);
+            Assert.True(useAspNetCoreDlls);
+            Assert.True(isAspNetCoreDetected);
+        }
+
         /// <summary>
         /// Verifies that `FileContent` correctly handles XML syntax with namespaces and various elements.
         /// </summary>