parameter error in microsoft SQL connection with unity3d

Hi Unity Members,

In my project I am trying to create sqlClass to connect Microsoft sql server.

I have develop my class to some extend. For example, I can query data according to input of textbox. However, I am doning this query without using any paramater because it gives error. Part of my code-->

 public class SqlClass{
 string connectionString = "Data Source=localhost\\SQLEXPRESS;Initial Catalog=EPTT;User ID=eptt; Password=123; Trusted_Connection=False; ";
 SqlConnection cnn = new SqlConnection();
 public SqlClass()
 {
     cnn.ConnectionString = connectionString;
 }

 public DataTable Fill(string sql, params SqlParameter[] prms)
 {//with parameter
     SqlCommand cmd = new SqlCommand(sql, cnn);
     if (prms != null)
     {
         cmd.Parameters.AddRange(prms);
         
     }
     SqlDataAdapter da = new SqlDataAdapter(cmd);
     DataTable dt = new DataTable();
     da.Fill(dt);
     return dt;
 }

 public DataTable Fill(string sql)
 {//without parameter
     SqlCommand cmd= new SqlCommand(sql,cnn);
     SqlDataAdapter da= new SqlDataAdapter(cmd);
     DataTable dt= new DataTable();
     int dene=da.Fill(dt);
     return dt;
 }
 }

as you can see two types of my query functions. The firs one contain cmd.Parameters.AddRange(prms); line. This line AddRange is unknown in unity (In .Net web apllication this class run without any problem). Therefore, I use Add instead of AddRange but this time it gives error "InvalidCastException:The parameter was not an SqlParameter"

could you help me, I will be glad to read your ideas. Tnx,