How do I get a JSON string out of a class object

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C#

ASP.NET

I have changed the static from string to object with no success.

The inputs mo, dy and yr exist.

I have written a working class statement using a string including: idd, datetime, col1, col2 and col3.

How do I get a JSON string out of a class object?

Error Code:

[object Object]

Default.aspx.cs

public static string GetDate(string mo, string dy, string yr)
{
    List<dataclass> returndata = new List<dataclass>();
    dataclass dc = new dataclass();
    var json = "";
    string date1 = mo.ToString() + "/" + dy.ToString() + "/" + yr.ToString() + " 8:00 AM";
    if (date1.Length > 6)
    {
        DataSet ds = new DataSet();
        DataTable dt = new DataTable();
        string connStr = ConfigurationManager.ConnectionStrings["jsonobject"].ConnectionString;
        string cmdStr = "SELECT * FROM [jsondata] WHERE [datetime1]=@datetime1;";
        using (SqlConnection conn = new SqlConnection(connStr))
        {
            using (SqlCommand cmd = new SqlCommand(cmdStr, conn))
            {
                conn.Open();
                cmd.Parameters.AddWithValue("@datetime1", date1);
                using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                {
                    da.Fill(ds);
                    dt = ds.Tables[0];
                    foreach (DataRow dr in dt.Rows)
                    {
                        dc.idd = dr["idd"].ToString();
                        dc.datetime = dr["datetime1"].ToString();
                        dc.col1 = dr["col1"].ToString();
                        dc.col2 = dr["col2"].ToString();
                        dc.col3 = dr["col3"].ToString();
                        returndata.Add(dc);
                    }
                }
            }
        }
        System.Web.Script.Serialization.JavaScriptSerializer serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
        json = serializer.Serialize(returndata);
    }
    else
    {
        json = "mo,dy,yr were null";
    }
    return json;
}