Object reference not set toan instance of anobject. C#combob

Question

hi

 

i have written below code to get values in combobox nut i am getting error as object reference not set .please suggest

void CmbhospitalsSelectedIndexChanged(object sender, EventArgs e)
{
try
{
WebClient wc = new WebClient();
wc.Headers["Content-type"] = "application/x-www-form-urlencoded";
NameValueCollection collection = new NameValueCollection();
collection.Add("hid","0");
collection.Add("sno", "2");
byte[] bret = wc.UploadValues(URLAuth, collection);
string sret = "";
sret = System.Text.Encoding.ASCII.GetString(bret);
int count=0;
string[,] hospitalslist ;

if (sret != "")
{
XmlDocument readDoc = new XmlDocument();
readDoc.LoadXml(sret);
count = readDoc.SelectNodes("hoskeys/hkids").Count;
hospitalslist = new string[count, 2];
// alternately, _doc.Load( _strFilename); to read from a file.
XmlNodeList xhkid = readDoc.GetElementsByTagName("hkid");
XmlNodeList xhos_key = readDoc.GetElementsByTagName("hos_key");
for (int i= 0; i <count; i++)
{
hospitalslist[i, 0] = xhkid[i].InnerText;
hospitalslist[i, 1] = xhos_key[i].InnerText;
}
for (int i = 0; i < hospitalslist.GetLength(0); i++)
{
cmbhusers.Items.Add(new { Text = hospitalslist[i, 1], Key=Convert.ToInt16(hospitalslist[i, 0]) });
}
cmbhusers.DisplayMember = "Text";
cmbhusers.ValueMember = "Key";
}
}
catch (Exception e1)
{
MessageBox.Show(e1.Message);
}
}