Here is my final code. The doc's are real bad , no samples and every small change gives catastrophic error with no error message.
Have fun
private string geo2json(IFeature feature)
{
IJSONWriter jsonWriter = new JSONWriterClass();
jsonWriter.WriteToString();
IJSONSerializer jsonSerializer = new JSONSerializerGdb();
IPropertySet props = new PropertySetClass();
props.SetProperty("Format", "GeoJSON");
jsonSerializer.InitSerializer(jsonWriter, props);
((IExternalSerializerGdb)jsonSerializer).WriteGeometry(null, feature.Shape);
var geoJson = Encoding.UTF8.GetString(jsonWriter.GetStringBuffer());
return geoJson;
}