Make HasRows report whether rows exist

src/FirebirdSql.Data.FirebirdClient.Tests/FbDataReaderTests.cs

@@ -54,6 +54,39 @@ public async Task ReadTest()
 		}
 	}
 
+	[Test]
+	public async Task HasRowsIsFalseForAnEmptyResultTest()
+	{
+		// DbDataReader.HasRows: "true if the DbDataReader contains one or more rows; otherwise false."
+		await using (var command = new FbCommand("select INT_FIELD from TEST where 1 = 0", Connection))
+		{
+			await using (var reader = await command.ExecuteReaderAsync())
+			{
+				Assert.IsFalse(reader.HasRows);
+				Assert.IsFalse(await reader.ReadAsync());
+			}
+		}
+	}
+
+	[Test]
+	public async Task HasRowsDoesNotSkipTheFirstRowTest()
+	{
+		await using (var command = new FbCommand("select INT_FIELD from TEST where INT_FIELD in (1, 2) order by INT_FIELD", Connection))
+		{
+			await using (var reader = await command.ExecuteReaderAsync())
+			{
+				Assert.IsTrue(reader.HasRows);
+				Assert.IsTrue(reader.HasRows);
+				Assert.IsTrue(await reader.ReadAsync());
+				Assert.AreEqual(1, reader.GetInt32(0));
+				Assert.IsTrue(reader.Read());
+				Assert.AreEqual(2, reader.GetInt32(0));
+				Assert.IsFalse(reader.Read());
+				Assert.IsTrue(reader.HasRows, "HasRows describes the result, not the current position");
+			}
+		}
+	}
+
 	[Test]
 	public async Task ReadClobTest()
 	{

src/FirebirdSql.Data.FirebirdClient/FirebirdClient/FbDataReader.cs

@@ -51,6 +51,9 @@ public sealed class FbDataReader : DbDataReader
 	private bool _eof;
 	private bool _isClosed;
 	private int _position;
+	private DbValue[] _peekedRow;
+	private bool _hasPeekedRow;
+	private bool _hasRows;
 	private int _recordsAffected;
 	private Dictionary<string, int> _columnsIndexesOrdinal;
 	private Dictionary<string, int> _columnsIndexesOrdinalCI;
@@ -102,9 +105,25 @@ public override int Depth
 		}
 	}
 
+	/// <summary>
+	/// DbDataReader.HasRows: "true if the DbDataReader contains one or more rows; otherwise false." Before the first
+	/// Read this fetches one row ahead; Read then returns that row.
+	/// </summary>
 	public override bool HasRows
 	{
-		get { return _command.HasFields; }
+		get
+		{
+			if (!_hasRows && !_isClosed && !_hasPeekedRow && _position == StartPosition && _command != null && _command.HasFields && !IsCommandBehavior(CommandBehavior.SchemaOnly))
+			{
+				using (var explicitCancellation = ExplicitCancellation.Enter(CancellationToken.None, _command.Cancel))
+				{
+					_peekedRow = _command.Fetch();
+				}
+				_hasPeekedRow = true;
+				_hasRows = _peekedRow != null;
+			}
+			return _hasRows;
+		}
 	}
 
 	public override bool IsClosed
@@ -166,6 +185,8 @@ public override void Close()
 			_command = null;
 			_connection = null;
 			_row = null;
+			_peekedRow = null;
+			_hasPeekedRow = false;
 			_schemaTable = null;
 			_fields = null;
 		}
@@ -195,6 +216,8 @@ public override async Task CloseAsync()
 			_command = null;
 			_connection = null;
 			_row = null;
+			_peekedRow = null;
+			_hasPeekedRow = false;
 			_schemaTable = null;
 			_fields = null;
 		}
@@ -229,10 +252,11 @@ public override bool Read()
 		{
 			using (var explicitCancellation = ExplicitCancellation.Enter(CancellationToken.None, _command.Cancel))
 			{
-				_row = _command.Fetch();
+				_row = TakePeekedRow(out var peeked) ? peeked : _command.Fetch();
 				if (_row != null)
 				{
 					_position++;
+					_hasRows = true;
 					return true;
 				}
 				else
@@ -259,10 +283,11 @@ public override async Task<bool> ReadAsync(CancellationToken cancellationToken)
 		{
 			using (var explicitCancellation = ExplicitCancellation.Enter(cancellationToken, _command.Cancel))
 			{
-				_row = await _command.FetchAsync(explicitCancellation.CancellationToken).ConfigureAwait(false);
+				_row = TakePeekedRow(out var peeked) ? peeked : await _command.FetchAsync(explicitCancellation.CancellationToken).ConfigureAwait(false);
 				if (_row != null)
 				{
 					_position++;
+					_hasRows = true;
 					return true;
 				}
 				else
@@ -1012,6 +1037,15 @@ private FbDbType GetProviderType(int i)
 		return (FbDbType)_fields[i].DbDataType;
 	}
 
+	private bool TakePeekedRow(out DbValue[] row)
+	{
+		row = _peekedRow;
+		var hadPeekedRow = _hasPeekedRow;
+		_peekedRow = null;
+		_hasPeekedRow = false;
+		return hadPeekedRow;
+	}
+
 	private void UpdateRecordsAffected()
 	{
 		if (_command != null && !_command.IsDisposed)