我们如何将SOAP web服务的返回类型指定为JSON格式

这是我的方法，返回xml格式，但我需要返回Json类型的格式。

[WebMethod]
public string GetAllCity(long UID, string IMEINo)
{
    DataSet ds = new DataSet();
    ExeWeb2 cd = new ExeWeb2();
    DataTable dt = new DataTable();
    //booking
    string qyr = "";
    // self booking
    ds = cd.retData("select city_name from city order by city_name");
    dt = ds.Tables[0];
    DataSet ds1 = new DataSet();
    ds1.Tables.Add(dt.Copy());
    ds1.Tables[0].TableName = "City";

    System.IO.MemoryStream s = new System.IO.MemoryStream();
    ds1.WriteXml(s, XmlWriteMode.IgnoreSchema);
    return System.Text.Encoding.UTF8.GetString(s.ToArray());
}

  DataTable dt11 = ds1.Tables[0];
    System.Web.Script.Serialization.JavaScriptSerializer serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
    List<Dictionary<string, object>> rows = new List<Dictionary<string, object>>();

    Dictionary<string, object> row;
    foreach (DataRow dr in dt11.Rows)
    {
        row = new Dictionary<string, object>();
        foreach (DataColumn col in dt11.Columns)
        {
           row.Add(col.ColumnName, dr[col]);
       }
        rows.Add(row);
    }
    return serializer.Serialize(rows);