参数传不进去！急

Question

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        ObjectDataSource1.SelectParameters.Clear();
        Parameter paradt = new Parameter();
        Parameter paraisauth = new Parameter();
        switch (DropDownList1.SelectedValue)
        {
            case "0":                paradt.Type = TypeCode.DateTime;
                paradt.Direction = ParameterDirection.Input;
                paradt.DefaultValue = Convert.ToString(DateTime.Now - TimeSpan.FromMinutes(50000));                paraisauth.Type = TypeCode.Boolean;
                paraisauth.DefaultValue = "true";
                paradt.Direction = ParameterDirection.Input;
                break;
            case "5":
                
                
                paradt.Type = TypeCode.DateTime;
                paradt.DefaultValue = Convert.ToString(DateTime.Now - TimeSpan.FromMinutes(5)) ;
                paradt.Direction = ParameterDirection.Input;
                
                paraisauth.Type = TypeCode.Boolean;
                paraisauth.DefaultValue = "true";
                paradt.Direction = ParameterDirection.Input;
                break;
            case "15":
                paradt.Type = TypeCode.DateTime;
                paradt.DefaultValue = Convert.ToString(DateTime.Now - TimeSpan.FromMinutes(15));
                paradt.Direction = ParameterDirection.Input;                paraisauth.Type = TypeCode.Boolean;
                paraisauth.DefaultValue = "true";
                paradt.Direction = ParameterDirection.Input;
                break;
            case "30":
                paradt.Type = TypeCode.DateTime;
                paradt.DefaultValue = Convert.ToString(DateTime.Now - TimeSpan.FromMinutes(30));
                paradt.Direction = ParameterDirection.Input;                paraisauth.Type = TypeCode.Boolean;
                paraisauth.DefaultValue = "true";
                paradt.Direction = ParameterDirection.Input;
                break;
                
        }
        ObjectDataSource1.SelectParameters.Add(paradt);
        ObjectDataSource1.SelectParameters.Add(paraisauth);        GridView1.DataBind();
   }
代码如上，想用dropdownlist的变化来向objectdatasource添加参数，但不知为啥datetime类型的那个参数传不进去
高手帮忙！