A simple example shows that no such loss of precision occurs with correctly written code:
static void Main(string[] args)
{
decimal d = 34.09090909090909M;
Console.WriteLine(d);
SqlConnectionStringBuilder scsb = new SqlConnectionStringBuilder();
scsb.IntegratedSecurity = true;
scsb.DataSource = @".\sql2012";
using (SqlConnection conn = new SqlConnection(scsb.ConnectionString)) {
conn.Open();
using (SqlCommand cmd = new SqlCommand(
"select cast(@d as DECIMAL(20,15))", conn))
{
cmd.Parameters.AddWithValue("@d", d);
decimal rd = (decimal) cmd.ExecuteScalar();
Console.WriteLine(rd);
}
}
}
Therefore I must conclude the problem is with your code, which is not posted.