How set value of DropDown after getting the list

Question

I need to set value of the dropdown after getting the list for dataset. But I'm having an error of:

The ViewData item that has the key 'txtCustomer' is of type 'System.String' but must be of type 'IEnumerable<SelectListItem>'

Here's my Controller:

wmsSR = new WMSSOFT_WCF_Service.WMSSOFT_InterfaceClient();
dsl = new DataSet();
dsO = new DataSet();
a = "GetSRMCustomers";
b = string.Empty;

//b = wmsSR.fCallWMSSOFTService(a, null, ref dsl, ref dsO);
b = wmsSR.fCallWMSSOFTService(a, "", ref dsl, ref dsO);

if (b == "OK ~~")
{
    if (dsO.Tables[0].Rows.Count >= 0)
    {
        List<SelectListItem> item = new List<SelectListItem>();
        for (int i = 0; i < dsO.Tables[0].Rows.Count; i++)
        {
            item.Add(new SelectListItem { Text = dsO.Tables[0].Rows[i]["NAMECUST"].ToString(), Value = dsO.Tables[0].Rows[i]["IDCUST"].ToString().Replace("  ", String.Empty) });
        }

        ViewBag.txtCustomer = item;

        ViewBag.txtCustomer = "ABC ";
    }
}