SqlConnection db = new SqlConnection(...);
SqlCommand s = new SqlCommand("select * from foo where a=@a", db);
s.Parameters.Add("@a", SqlDbType.VarChar, 8);
s.Prepare();
...
s.Parameters["@a"] = "bozo";
s.Execute();

using (var sqlConnection = new SqlConnection("connstring"))
 {
    sqlConnection.Open();
    using (var sqlCommand = new SqlCommand
       {
          Connection = sqlConnection,
          CommandText = "dbo.MyProc",
          CommandType = CommandType.StoredProcedure,
       })
    {
       // Once-off setup per connection
       // This parameter doesn't vary so is set just once
       sqlCommand.Parameters.Add("ConstantParam0", SqlDbType.Int).Value = 1234;
       // These parameters are defined once but set multiple times
       sqlCommand.Parameters.Add(new SqlParameter("VarParam1", SqlDbType.VarChar));
       sqlCommand.Parameters.Add(new SqlParameter("VarParam2", SqlDbType.DateTime));

       // Tight loop - performance critical
       foreach(var item in itemsToExec)
       {
         // No need to set ConstantParam0
         // Reuses variable parameters, by just mutating values
         sqlParameters["VarParam1"].Value = item.Param1Value; // Or sqlParameters[1].Value
         sqlParameters["VarParam2"].Value = item.Param2Date; // Or sqlParameters[2].Value
         sqlCommand.ExecuteNonQuery();
       }
    }
}