SqlDataReader.GetSqlDateTime does not support DATETIME2

[bjornbouetsmith]

Describe the bug

It seems like DATETIME2 sql server datatype is not fully or correctly implemented in SqlDataReader.

using method GetDateTime - it handles DATETIME2 correctly

using method GetSqlDateTime - it throws an System.InvalidCastException:

Exception message: Specified cast is not valid 
Stack trace:  at Microsoft.Data.SqlClient.SqlBuffer.get_SqlDateTime()

To reproduce

 [TestMethod]
        public void DateTime2Mapping()
        {
            string Statement = $@"SELECT CAST('2020-12-11 09:17:12.123456' AS DATETIME2) AS [MyDate]";

            var builder = new SqlConnectionStringBuilder()
            {
                InitialCatalog = "master",
                DataSource = @"any sql connection",
                IntegratedSecurity = true,
            };

            var sqlConnection = new SqlConnection(builder.ConnectionString);
            sqlConnection.Open();
            using (SqlCommand command = new SqlCommand(Statement))
            {
                command.CommandType = CommandType.Text;
                command.Connection = sqlConnection;
                command.CommandText = Statement;
                using (var reader = command.ExecuteReader())
                {
                    while (reader.Read())
                    {
                        var dtColDate = reader.GetSqlDateTime(0);
                        Assert.IsFalse(dtColDate.IsNull);
                        Console.WriteLine(dtColDate.Value);
                    }
                }
            }
        }

Expected behavior

I expect that GetSqlDateTime handles DATETIME2 and returns a SqlDateTime struct

Further technical details

Microsoft.Data.SqlClient version: 2.1.0 .NET target: Framework 4.7.2 SQL Server version: 15.0.4079.3 (2019) Operating system: Windows 10

Additional context Why I found this issue, is because I was profiling our code and saw that SqlDataReader.IsDbNull took up a lot of time since we need to check every column for null, since GetXX methods does not handle DBNull - and then I found SqlDataReader.GetSqlXX methods which handles DBNull.

Basically trying to get rid of the overhead in SqlDataReader

 override public bool IsDBNull(int i)
        {
            {
                CheckHeaderIsReady(columnIndex: i);

                SetTimeout(_defaultTimeoutMilliseconds);

                ReadColumnHeader(i);    // header data only
            }

            return _data[i].IsNull;
        }

by swithing to The IsNull property in the SqlTypes

i.e.

  public bool IsNull => !this.m_fNotNull;