How do I add a class item to a database

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C#

ASP.NET

How do I add a class item to a database?

I am getting an error in "@col1" in "classlist[n].col1".

public class Class1
{
    public Int32 id { get; set; }
    public string col1 { get; set; }
    public string col2 { get; set; }
    public string col3 { get; set; }
}

List<Class1> classlist = new List<Class1>();
classlist.Add(new Class1() { id = 1, col1 = "11", col2 = "22", col3="33" });
classlist.Add(new Class1() { id = 2,  col1 = "44", col2 = "55", col3="66" });
classlist.Add(new Class1() { id = 3,  col1 = "77", col2 = "88", col3="99" });
string connStr = ConfigurationManager.ConnectionStrings["MDFdb"].ConnectionString;
string cmdStr = "INSERT INTO [Table1] (col1,col2,col3) VALUES (@col1,@col2,@col3);";
for (int n = 1; n <= classlist.Count; n++)
{
    try
    {
        using (SqlConnection conn = new SqlConnection(connStr))
        {
            using (SqlCommand cmd = new SqlCommand(cmdStr, conn))
            {
                conn.Open();
                cmd.ExecuteNonQuery();
                cmd.Parameters.Add("@col1", classlist[n].col1);
                cmd.Parameters.Add("@col2", classlist[n].col2);
                cmd.Parameters.Add("@col3", classlist[n].col3);
                conn.Close();
                conn.Dispose();
            }
        }
    }
    catch (Exception ex)
    {
        Label1.Text=ex.ToString();
    }
}

Error Code:

System.Data.SqlClient.SqlException (0x80131904): Must declare the scalar variable "@col1". at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction) at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady) at System.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean async, Int32 timeout, Boolean asyncWrite) at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(TaskCompletionSource`1 completion, String methodName, Boolean sendToPipe, Int32 timeout, Boolean asyncWrite) at System.Data.SqlClient.SqlCommand.ExecuteNonQuery() at ClassToMDF._Default.Button1_Click(Object sender, EventArgs e) in C:\Users\User\path\Default.aspx.cs:line 36 ClientConnectionId:e373b91f-0471-4487-9667-bed0597b6391