added method to detect if a yaml fragment is a sequence (#720)

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

@@ -198,4 +198,21 @@ public void RoundTripFromYaml(string path)
     }
 
     #endregion
+
+    #region Is Sequence checks
+
+    [TestMethod]
+    [DataRow(@"_TestData/SchemaV3_0/Examples/Src/App.pa.yaml", false)]
+    [DataRow(@"_TestData/SchemaV3_0/Examples/Src/Screens/Screen1.pa.yaml", false)]
+    [DataRow(@"_TestData/SchemaV3_0/Examples/Src/Components/MyHeaderComponent.pa.yaml", false)]
+    [DataRow(@"_TestData/ValidYaml-CI/With-list-of-controls.pa.yaml", true)]
+    [DataRow(@"_TestData/InvalidYaml/Dupliacte-Keys.pa.yaml", false)]
+    public void IsSequenceCheckShouldWorkAsExpected(string path, bool expected)
+    {
+        var yaml = File.ReadAllText(path);
+        var isSequence = PaYamlSerializer.CheckIsSequence(yaml);
+        isSequence.Should().Be(expected);
+    }
+
+    #endregion
 }

src/Persistence/PaYaml/Serialization/PaYamlSerializer.cs

@@ -128,5 +128,45 @@ private static void WriteTextWriter<TValue>(TextWriter writer, in TValue? value,
         // See: fluentvalidation.net
         // See: https://www.youtube.com/watch?v=jblRYDMTtvg
     }
+
+    /// <summary>
+    /// checks the input source is a sequence of YAML items.
+    /// </summary>
+    /// <param name="yaml">The YAML text to check.</param>
+    /// <returns></returns>
+    /// <exception cref="ArgumentNullException"></exception>
+    public static bool CheckIsSequence(string yaml)
+    {
+        _ = yaml ?? throw new ArgumentNullException(nameof(yaml));
+
+        using var reader = new StringReader(yaml);
+        return CheckIsSequence(reader);
+    }
+
+    /// <summary>
+    /// checks the input source is a sequence of YAML items.
+    /// </summary>
+    /// <param name="reader"></param>
+    /// <returns></returns>
+    /// <exception cref="ArgumentNullException"></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;
+        }
+    }
+
     #endregion
 }

src/Persistence/PaYaml/Serialization/YamlSequenceTesterConverter.cs

@@ -0,0 +1,42 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Collections;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+
+namespace Microsoft.PowerPlatform.PowerApps.Persistence.PaYaml.Serialization;
+
+/// <summary>
+/// custom converter used to test if the input is a sequence of YAML items.
+/// </summary>
+internal sealed class YamlSequenceTesterConverter : IYamlTypeConverter
+{
+    public bool Accepts(Type type)
+    {
+        return type == typeof(IEnumerable) || type.IsSubclassOf(typeof(IEnumerable)) ||
+            type == typeof(IEnumerable<object>) || type.IsSubclassOf(typeof(IEnumerable<object>)) ||
+            type == typeof(object[]);
+    }
+
+    public object? ReadYaml(IParser parser, Type type)
+    {
+        if (parser.Current is not SequenceStart)
+            throw new YamlException(parser.Current!.Start, parser.Current.End, $"Expected sequence start but got {parser.Current.GetType().Name}");
+
+        while (!parser.Accept<SequenceEnd>(out _))
+        {
+            parser.MoveNext();
+        }
+
+        parser.MoveNext();
+
+        return Array.Empty<object>();
+    }
+
+    public void WriteYaml(IEmitter emitter, object? value, Type type)
+    {
+        throw new NotImplementedException();
+    }
+}