How can we get the exact data in combobox2 on selecting combobo1 value

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C#

hi,
I have two comboboxes were in combobo1 i have data of hospitals and linked hospital users to combobox but while i running the outpit i amnot getting exact list of users who all under certain hopspital, how can we get the same

What I have tried:

void CmbHospitalsSelectedIndexChanged(object sender, EventArgs e)
		{
			            try
            {
                
                //string tag = ((KeyValuePair< string, string >)cmbHospitals.SelectedItem).Key; 
               //MessageBox.Show(tag);            
               // var s=cmbHospitals.SelectedItem;
                //MessageBox.Show(s.key);
                //For Hospital users Dropdown Clear Start
                   dynamic refobj = cmbHospitals.SelectedItem;
                    int key=(int) refobj.Key;
                    Text= Text+cmbhusers.DisplayMember;
                    
					//MessageBox.Show(refobj.Key);
                    cmbhusers.SelectedIndex = -1;
                    cmbhusers.Items.Clear();

                // End Clear
                if(true)
                {
                    WebClient wc = new WebClient();
                    wc.Headers["Content-type"] = "application/x-www-form-urlencoded";
                    NameValueCollection collection = new NameValueCollection();
                    collection.Add("hid",cmbHospitals.SelectedIndex.ToString());
                    collection.Add("sno", "4");
                    byte[] bret = wc.UploadValues(URLAuth, collection);
                    string sret = "";
                    sret = System.Text.Encoding.ASCII.GetString(bret);
                    int count=0;
                    string[,] husers ;                
                   
                     if (sret != "")
                    {                       
                        XmlDocument readDoc = new XmlDocument();
                        readDoc.LoadXml(sret);
                        count = readDoc.SelectNodes("hospitalusers/huids").Count;
                        husers = new string[count, 2];      
                         // alternately, _doc.Load( _strFilename); to read from a file.
                        XmlNodeList xhuid = readDoc.GetElementsByTagName("huid");
                        XmlNodeList xuser_name = readDoc.GetElementsByTagName("user_name");
                        for (int i= 0; i < count; i++)
                        {
                            husers[i,0] = xhuid[i].InnerText;
                            husers[i, 1] = xuser_name[i].InnerText;
                        }
                           cmbhusers.DisplayMember="text";
                             cmbhusers.ValueMember="key";    
                             
                        for (int i = 0; i < husers.GetLength(0); i++)
                        {
                            cmbhusers.Items.Add(new { text = husers[i, 1], key=Convert.ToInt16(husers[i,0])});
                        }
                     }
                }
            }