Telerik Rad Combo fill by using web service creating problem

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C#

ASP.NET

C#4.0

private const int ItemsPerRequest = 10;
[WebMethod]
public RadComboBoxItemData[] GetAccount(object context)
{

RadComboBoxContext obj = (RadComboBoxContext)context;
DataTable data = GetDataAccount(obj.Text);

RadComboBoxData comboData = new RadComboBoxData();
int itemOffset = obj.NumberOfItems;
int endOffset = Math.Min(itemOffset + ItemsPerRequest, data.Rows.Count);
comboData.EndOfItems = endOffset == data.Rows.Count;

List<radcomboboxitemdata> result = new List<radcomboboxitemdata>(endOffset - itemOffset);

for (int i = itemOffset; i < endOffset; i++)
{
RadComboBoxItemData itemData = new RadComboBoxItemData();
itemData.Value = data.Rows[i]["AccountLevelNo"].ToString();
itemData.Text = data.Rows[i]["AccountDesc3"].ToString();
itemData.Attributes.Add("Level6", data.Rows[i]["AccountDesc2"].ToString());
itemData.Attributes.Add("Level1", data.Rows[i]["AccountDesc1"].ToString());


result.Add(itemData);
}

comboData.Items = result.ToArray();
// comboData.Message = GetStatusMessage(endOffset, data.Rows.Count);

return comboData.Items.ToArray();
}
private static DataTable GetDataAccount(string text)
{
int accCode = 0;
string query = "select COA.LevelAccountNo,COA.AccountDesc as AccountDesc3,Level1.AccountDesc as AccountDesc1, Level2.AccountDesc as AccountDesc2 from COA COA,(select LevelAccountNo,AccountDesc " +
"from COA where len(LevelAccountNo)=2)as Level1,(select LevelAccountNo,AccountDesc from COA where len(LevelAccountNo)=5)as Level2 " +
"where Level1.LevelAccountNo=left(COA.LevelAccountNo,2)and Level2.LevelAccountNo=left(COA.LevelAccountNo,5) and len(COA.LevelAccountNo)>6";


try
{

accCode = Convert.ToInt32(text);
query = query + " COA.LevelAccountNo like '" + text + "%'";

}
catch (Exception ex)
{

query = query + " COA.AccountDesc3 like '%" + text + "%'";

}



SqlConnection con = new SqlConnection(ConfigurationSettings.AppSettings["ConnectionString"].ToString());
// string constr=ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;



SqlDataAdapter adapter = new SqlDataAdapter(query, con);

// adapter.SelectCommand.Parameters.AddWithValue("@text", text);

DataTable data = new DataTable();
adapter.Fill(data);
con.Close();
return data;

}
this is my web service code

<telerik:RadComboBox ID="cboAccount"  runat="server" Height="200" Width="200" EmptyMessage="Select an Account"
                                        EnableLoadOnDemand="true" ShowMoreResultsBox="true"  EnableVirtualScrolling="true">
                                        <HeaderTemplate>
                                        <h3>Accounts</h3>
                                        </HeaderTemplate>
                                        <ClientItemTemplate>
                                        <div>
                                        <ul>
                                            <li><span><b>Name:#= Text # </b></span></li>
                                            <li><span>Level6 #= Attributes.Level6 # </span></li>
                                            <li><span>Level1: #= Attributes.Level4 # </span></li>
                                            <li><span>Level4 #= Attributes.Level1 # </span></li>
                                        </ul>
                                       </div>
                                        <br></br>
                                        </ClientItemTemplate>
                                        <WebServiceSettings Method="GetAccount" Path="InvestmentDropDownWebService.asmx" />
                                        </telerik:RadComboBox>

this is Rad combo box html code and call web service
After running the project i will recieved this error please help me
The type Telerik.Web.UI.RadComboBoxContext is not supported because it implements IDictionary.