<link href="../../../Scripts/jquery-easyui-1.4/themes/default/easyui.css" rel="stylesheet" />
<link href="../../../Scripts/jquery-easyui-1.4/themes/icon.css" rel="stylesheet" />
<script src="../../../Scripts/jquery-easyui-1.4/jquery.min.js"></script>
<script src="../../../Scripts/jquery-easyui-1.4/jquery.easyui.min.js"></script>
<script>
$(function () {
var objval = "param";
var jsonData = [];
$.ajax({
type: "POST",
async:false,
contentType: "application/json; charset=utf-8",
url: "WebService1.asmx/AutoArrayList",
data: "{'Query':'" + objval + "'}",
dataType: "json",
success: function (data) {
jsonData = data.d;
},
error: function (result) {
alert("Error");
}

});

$('#drpSelectStudents').combogrid({
panelWidth: 290,
value: '006',
idField: 'StudentID',
textField: 'StudentName',
//url: '/combogrid/GetStudentsInfo',
//source:jsonData,
columns: [[
{ field: 'StudentName', title: 'StudentName', width: 60 } ]]
});
// get the datagrid object
var g = $('#drpSelectStudents').combogrid('grid');
//assign the data to datagrid
g.datagrid('loadData', jsonData);
});

</script>

<select id="drpSelectStudents" name="Students" style="width:290px;"></select>
And the web service like below.

[System.Web.Script.Services.ScriptService]
public class WebService1 : System.Web.Services.WebService
{

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public List<Student> AutoArrayList(string Query)
{
//according to the Query parameter to do your logic
List<Student> lstStudents = new List<Student>();
lstStudents.Add(new Student { StudentID = 1, StudentName = "ABC"});
lstStudents.Add(new Student { StudentID = 2, StudentName = "DEF"});
lstStudents.Add(new Student { StudentID = 3, StudentName = "GHI"});
lstStudents.Add(new Student { StudentID = 4, StudentName = "JKL"});
return lstStudents;
}

public class Student
{
public int StudentID { get; set; }
public string StudentName { get; set; }
}
}