Custom CheckBoxList with SelectedValues Property

How many times did you faced the problem of using a CheckBoxList control in your web application and use it as control parameter of an ObjectDataSource control?
I face similar situation and I searched over internet and I didn't found any good work around which is not involve javascript functions on selected event or using hidden fields used to store values of selected items. What If I do not want to use javascript?
So I decided to create a custom control which extends CheckBoxList and add a simple property called SelectedValues. It will retrieve a list of selected values without using any javascript at all. So here this is

public class CustomCheckBoxList : System.Web.UI.WebControls.CheckBoxList
{
    public ListItems SelectedValues
    {
        get
        {
            ListItems selected = new ListItems();
            foreach (ListItem item in this.Items)
            {
                if (item.Selected)
                    selected.Add(item.Value);
            }
            return selected;
        }
    }

ListItems is a custom List.It extends List<string></string> and implements IConvertible interface, because I need to use it in an ObjectDataSource as a control parameter and then it's necessary to convert it into a string. The conversion is done combining all selected values using comma as separator.

[Serializable]
public class ListItems : List<String>, IConvertible
{
    #region IConvertible Members

    public TypeCode GetTypeCode()
    {
        return this.GetTypeCode();
    }
    /// <summary>
    /// Returns logic evaluation of selected values (in &)
    /// </summary>
    /// <param name="provider"></param>
    /// <returns>True if all values are true , false otherwise</returns>
    public bool ToBoolean(IFormatProvider provider)
    {
        foreach (string item in this)
        {
            if (!Convert.ToBoolean(item, provider))
                return false;
        }
        return true;
    }

    public byte ToByte(IFormatProvider provider)
    {
        throw new NotImplementedException();
    }

    public char ToChar(IFormatProvider provider)
    {
        throw new NotImplementedException();
    }

    public DateTime ToDateTime(IFormatProvider provider)
    {
        throw new NotImplementedException();
    }

    public decimal ToDecimal(IFormatProvider provider)
    {
        decimal retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToDecimal(item, provider);
        }
        return retVal;
    }

    public double ToDouble(IFormatProvider provider)
    {
        double retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToDouble(item, provider);
        }
        return retVal;
    }

    public short ToInt16(IFormatProvider provider)
    {
        short retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToInt16(item, provider);
        }
        return retVal;
    }

    /// <summary>
    /// Returns the sum of selected values
    /// </summary>
    /// <param name="provider"></param>
    /// <returns></returns>
    public int ToInt32(IFormatProvider provider)
    {
        int retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToInt32(item, provider);
        }
        return retVal;
    }

    public long ToInt64(IFormatProvider provider)
    {
        long retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToInt64(item, provider);
        }
        return retVal;
    }

    public sbyte ToSByte(IFormatProvider provider)
    {
        sbyte retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToSByte(item, provider);
        }
        return retVal;
    }

    public float ToSingle(IFormatProvider provider)
    {
        float retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToInt64(item, provider);
        }
        return retVal;
    }

    /// <summary>
    /// Returns the String Concatenation of selected values
    /// </summary>
    /// <param name="provider"></param>
    /// <returns></returns>
    public string ToString(IFormatProvider provider)
    {
        if (this.Count == 0)
            return null;
        return string.Join(",", this.ToArray());
    }

    public object ToType(Type conversionType, IFormatProvider provider)
    {
        throw new NotImplementedException();
    }

    public ushort ToUInt16(IFormatProvider provider)
    {
        ushort retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToUInt16(item, provider);
        }
        return retVal;
    }

    public uint ToUInt32(IFormatProvider provider)
    {
        uint retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToUInt32(item, provider);
        }
        return retVal;
    }

    public ulong ToUInt64(IFormatProvider provider)
    {
        ulong retVal = 0;

        foreach (string item in this)
        {
            retVal += Convert.ToUInt64(item, provider);
        }
        return retVal;
    }

    #endregion
}

Finally we can use our custom property with an ObjectDataSource control

<asp:ControlParameter ControlID="cblStates"Name="states" PropertyName="SelectedValues"Type="String" />