improved sequence check (#721)

src/Persistence.Tests/PaYaml/Serialization/PaYamlSerializerTests.cs

@@ -214,5 +214,51 @@ public void IsSequenceCheckShouldWorkAsExpected(string path, bool expected)
         isSequence.Should().Be(expected);
     }
 
+    [TestMethod]
+    [DataRow("")]
+    [DataRow("   ")]
+    [DataRow("not yaml")]
+    [DataRow("{ prop1: str2 }")]
+    public void IsSequenceCheckShouldReturnFalseForInvalidSequences(string yaml)
+    {
+        var isSequence = PaYamlSerializer.CheckIsSequence(yaml);
+        isSequence.Should().BeFalse();
+    }
+
+    [TestMethod]
+    [DataRow("[]")]
+    [DataRow("['str',333]")]
+    public void IsSequenceCheckShouldReturnTrueForInlineSequences(string yaml)
+    {
+        var isSequence = PaYamlSerializer.CheckIsSequence(yaml);
+        isSequence.Should().BeTrue();
+    }
+
+    [TestMethod]
+    public void IsSequenceCheckShouldReturnTrueWhenSequenceInFragmentStartsWithComments()
+    {
+        var yaml = """
+            # comment
+            - name: control1
+            - name: control2
+            """;
+
+        var isSequence = PaYamlSerializer.CheckIsSequence(yaml);
+        isSequence.Should().BeTrue();
+    }
+
+    [TestMethod]
+    public void IsSequenceCheckShouldReturnTrueWhenSequenceInFragmentBeginsWithDocumentStart()
+    {
+        var yaml = """
+            ---
+            - name: control1
+            - name: control2
+            """;
+
+        var isSequence = PaYamlSerializer.CheckIsSequence(yaml);
+        isSequence.Should().BeTrue();
+    }
+
     #endregion
 }

src/Persistence/PaYaml/Serialization/PaYamlSerializer.cs

@@ -4,6 +4,7 @@
 using System.Globalization;
 using System.Text;
 using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
 using YamlDotNet.Serialization;
 
 namespace Microsoft.PowerPlatform.PowerApps.Persistence.PaYaml.Serialization;
@@ -132,12 +133,15 @@ private static void WriteTextWriter<TValue>(TextWriter writer, in TValue? value,
     /// <summary>
     /// checks the input source is a sequence of YAML items.
     /// </summary>
-    /// <param name="yaml">The YAML text to check.</param>
+    /// <remarks>
+    /// This method does not perform validation on the input fragment.
+    /// </remarks>
+    /// <param name="yaml"></param>
     /// <returns></returns>
-    /// <exception cref="ArgumentNullException"></exception>
+    /// <exception cref="ArgumentNullException">thrown when <paramref name="yaml"/> is null.</exception>
     public static bool CheckIsSequence(string yaml)
     {
-        _ = yaml ?? throw new ArgumentNullException(nameof(yaml));
+        ArgumentNullException.ThrowIfNull(yaml);
 
         using var reader = new StringReader(yaml);
         return CheckIsSequence(reader);
@@ -146,26 +150,23 @@ public static bool CheckIsSequence(string yaml)
     /// <summary>
     /// checks the input source is a sequence of YAML items.
     /// </summary>
+    /// <remarks>
+    /// This method does not perform validation on the input fragment.
+    /// </remarks>
     /// <param name="reader"></param>
     /// <returns></returns>
-    /// <exception cref="ArgumentNullException"></exception>
+    /// <exception cref="ArgumentNullException">thrown when <paramref name="reader"/> is null.</exception>
     public static bool CheckIsSequence(StringReader reader)
     {
         ArgumentNullException.ThrowIfNull(reader);
 
-        var builder = new DeserializerBuilder()
-            .IgnoreUnmatchedProperties()
-            .WithTypeConverter(new YamlSequenceTesterConverter());
-        var deserializer = builder.Build();
-        try
-        {
-            var results = deserializer.Deserialize<object[]>(reader);
-            return results is not null;
-        }
-        catch (YamlException)
-        {
-            return false;
-        }
+        var parser = new Parser(reader);
+
+        // Need to consume the StreamStart and DocumentStart events to get to the root sequence
+        parser.TryConsume<StreamStart>(out _);
+        parser.TryConsume<DocumentStart>(out _);
+
+        return parser.TryConsume<SequenceStart>(out _);
     }
 
     #endregion