create table TestUni
(
  UniCol nvarchar(20)
)

insert into TestUni values ('😅')
insert into TestUni values ('電')

select len(UniCol), UniCol from TestUni

2    ??
1    ?

var sqlConnection = new SqlConnection("***");

sqlConnection.Open();

var sqlCommand = new SqlCommand("select * from TestUni", sqlConnection);
        var reader = sqlCommand.ExecuteReader();

var str = string.Empty;

while(reader.Read())
{
    str += reader["UniCol"];
}

File.WriteAllText("d:\test.txt", str);