Fix return value of ExecuteScalar.

It must return the first column of the first row of the first result set.

Author: bgrainger

src/MySqlConnector/Core/TextCommandExecutor.cs

@@ -37,13 +37,19 @@ public virtual async Task<int> ExecuteNonQueryAsync(string commandText, MySqlPar
 		public virtual async Task<object> ExecuteScalarAsync(string commandText, MySqlParameterCollection parameterCollection,
 			IOBehavior ioBehavior, CancellationToken cancellationToken)
 		{
+			var hasSetResult = false;
 			object result = null;
 			using (var reader = (MySqlDataReader) await ExecuteReaderAsync(commandText, parameterCollection, CommandBehavior.SingleResult | CommandBehavior.SingleRow, ioBehavior, cancellationToken).ConfigureAwait(false))
 			{
 				do
 				{
-					if (await reader.ReadAsync(ioBehavior, cancellationToken).ConfigureAwait(false))
-						result = reader.GetValue(0);
+					var hasResult = await reader.ReadAsync(ioBehavior, cancellationToken).ConfigureAwait(false);
+					if (!hasSetResult)
+					{
+						if (hasResult)
+							result = reader.GetValue(0);
+						hasSetResult = true;
+					}
 				} while (await reader.NextResultAsync(ioBehavior, cancellationToken).ConfigureAwait(false));
 			}
 			return result;

tests/SideBySide/QueryTests.cs

@@ -498,6 +498,40 @@ id bigint(20) not null primary key
 			}
 		}
 
+		[Fact]
+		public void ExecuteScalarReturnsFirstValue()
+		{
+			var result = m_database.Connection.ExecuteScalar("select 1; select 2;");
+			Assert.Equal(1L, result);
+		}
+
+		[Fact]
+		public async Task ExecuteScalarAsyncReturnsFirstValue()
+		{
+			var result = await m_database.Connection.ExecuteScalarAsync("select 1; select 2;");
+			Assert.Equal(1L, result);
+		}
+
+		[Fact]
+		public void ExecuteScalarReturnsNull()
+		{
+			m_database.Connection.Execute(@"drop table if exists empty_table;
+			create table empty_table(id bigint not null primary key);");
+			var result = m_database.Connection.ExecuteScalar("select * from empty_table; select 2;");
+			Assert.Null(result);
+		}
+
+		[Fact]
+		public void ExecuteScalarReturnsDBNull()
+		{
+			using (var command = m_database.Connection.CreateCommand())
+			{
+				command.CommandText = "select null; select 2;";
+				var result = command.ExecuteScalar();
+				Assert.Equal(DBNull.Value, result);
+			}
+		}
+
 		[Fact]
 		public void SumBytes()
 		{