Add regression test for ParseAttributeArgumentList with invalid input (fixes #8699)

src/Compilers/CSharp/Test/Syntax/Syntax/SyntaxFactoryTests.cs

@@ -714,5 +714,20 @@ public void TestParseMethodsKeepParseOptionsInTheTree()
             var typeName = SyntaxFactory.ParseTypeName("", options: parseOptions);
             Assert.Same(parseOptions, typeName.SyntaxTree.Options);
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/8699")]
+        public void TestParseAttributeArgumentListWithInvalidString()
+        {
+            // Regression test for issue where ParseAttributeArgumentList would throw NullReferenceException
+            // when given an invalid string without parentheses
+            var result = SyntaxFactory.ParseAttributeArgumentList("somethingWithoutBrackets");
+
+            Assert.NotNull(result);
+            Assert.True(result.GetDiagnostics().Any(), "Expected diagnostics for invalid input");
+
+            // Verify the structure is as expected - should have missing open/close parens and errors
+            Assert.True(result.OpenParenToken.IsMissing);
+            Assert.True(result.CloseParenToken.IsMissing);
+        }
     }
 }