Question

for (I = 0; I < ds.Tables[0].Rows.Count; I++)
{
 if (Convert.ToBoolean(ds.Tables[0].Rows[I]["IsDefault"].ToString()) == true && Convert.ToBoolean(ds.Tables[0].Rows[I]["IsBill"].ToString()) == true)
.
.
.
.

Error: System.FormatException: String was not recognized as a valid Boolean.

Is default and IsBill are boolean values only

Answer 1

I think some values are null in your table.

if (ds.Tables[0].Rows[I]["IsDefault"] != null && ds.Tables[0].Rows[I]["IsBill"]) != null)
    {
        if (Convert.ToBoolean(ds.Tables[0].Rows[I]["IsDefault"].ToString()) == true && Convert.ToBoolean(ds.Tables[0].Rows[I]["IsBill"]..ToString()) == true)
        {


        }

    }

You might have to use Convert.IsDBNull function to check whether the value is NULL.