CsvHelper: writing null strings as special string

Question

I'm trying to configure CsvWriter to use special string "#NULL#" for nullable string properties. For reader it works, by setting csvReader.Configuration.TypeConverterOptionsCache.GetOptions<string>().NullValues.Add("#NULL#"); - it reads "#NULL#" fields in csv as null strings.

The code I'm using for writer is below, but it ignores added NullValues and outputs empty strings instead (default behavior). Is there other config parameter for writer? Thanks.

        public class Entity
    {
        public string Name { get; set; }
        public int Id { get; set; }
    }

    [Test]
    public void csv_write_test()
    {
        var entities = new[] {new Entity {Id = 1, Name = null}, new Entity {Id=2, Name = "SampleName"} };

        var fileName = "C:/Temp/tr/recordings/withNulls/sample-test.csv";
        File.Delete(fileName);

        using (var textWriter = new StreamWriter(fileName))
        {
            var csvWriter = new CsvWriter(textWriter);

            csvWriter.Configuration.TypeConverterOptionsCache.GetOptions<string>().NullValues.Add("#NULL#");

            csvWriter.WriteRecords(entities);
        }
    }

Answer 1

You can use a custom ITypeConverter to accomplish this.

void Main()
{
    using (var stream = new MemoryStream())
    using (var writer = new StreamWriter(stream))
    using (var reader = new StreamReader(stream))
    using (var csv = new CsvWriter(writer))
    {
        var records = new List<Test>
        {
            new Test { Id = 1, Name = "one" },
            new Test { Id = 2, Name = null },
        };

        csv.Configuration.RegisterClassMap<TestMap>();
        csv.WriteRecords(records);

        writer.Flush();
        stream.Position = 0;

        reader.ReadToEnd().Dump();
    }
}

public class Test
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public sealed class TestMap : ClassMap<Test>
{
    public TestMap()
    {
        Map(m => m.Id);
        Map(m => m.Name).TypeConverter<CustomNullTypeConverter<string>>();
    }
}

public class CustomNullTypeConverter<T> : DefaultTypeConverter
{
    public override string ConvertToString(object value, IWriterRow row, MemberMapData memberMapData)
    {
        if (value == null)
        {
            return "#NULL#";
        }

        var converter = row.Configuration.TypeConverterCache.GetConverter<T>();
        return converter.ConvertToString(value, row, memberMapData);
    }
}

If you want it to use the first value in the NullValues option, you'll need to submit a feature request.