BulkCopy writes binary data to String column incorrectly in 0.9

[MaceWindu]

Describe the bug

        using var cn = new ClickHouseConnection(cs);
        cn.Open();

        using var cmd = cn.CreateCommand();

        cmd.CommandText = @"DROP TABLE IF EXISTS TestTable";
        cmd.ExecuteNonQuery();

        cmd.CommandText = @"
CREATE TABLE TestTable
(
	ID                Int32,
	StringValue       Nullable(String),
	FixedStringValue  Nullable(FixedString(20)),
) ENGINE = MergeTree() ORDER BY ID PRIMARY KEY ID
";
        cmd.ExecuteNonQuery();

        using var bc = new ClickHouseBulkCopy(cn)
        {
            DestinationTableName = "TestTable",
            ColumnNames = ["ID", "StringValue", "FixedStringValue"]
        };

        await bc.InitAsync();

        await bc.WriteToServerAsync([[1, new byte[] { 2 }, Enumerable.Range(0, 20).Select(r => (byte)r).ToArray()], [2, "123", "12345678901234567890"]]);

Configuration

Environment

• Client version: 0.9

ClickHouse server

• ClickHouse Server version: 25.11

[alex-clickhouse]

Hi, thanks for reporting this. Just to clarify, the expectation is that one can insert straight bytes as with FixedString, yes?

[MaceWindu]

Yes, both String and FixedString are binary types, so it makes sense to be able read and write them as binary without data loss on binary<->utf8 conversions.

[MaceWindu]

BTW, I was rising it before already DarkWanderer/ClickHouse.Client#138

[MaceWindu]

Currently in linq2db we test both string and binary mappings for both types for all 3 supported providers: CH.Driver, Octonica and MySqlConnector and only Octonica pass this test. This provider and mysql apply utf8 conversion in some cases leading to data loss for non-uft8 compatible binary data.

[alex-clickhouse]

I think maybe the best approach is to make the behavior adjustable by the user: 1) whether they want byte[] or string, and 2) the default encoding to use.