I am implementing a control inherited from System.Web.UI.WebControls.BaseValidator,I want that the control perfoms client side validation against a web service in the evaluationfunction of the control.  I already do the script and web service registration and the call to the webservice inside the call to the evaluationfunction, the problem is that this call is asyncronous. There is a way to do this call syncronous or to wait to the callback function of the webservice?.

 Hi,

To call a web service synchronously, you can use XmlHttpRequest object, and specify it to send request synchronously. For example:

 UserName:<input id="Text2" type="text" />
    <input id="Button1" type="button" value="Client Check" onclick="CheckUserExist();" /><br />
    Result:<input id="Text1" type="text" />
<script type="text/javascript">
var client;

function CheckUserExist()
{
    if (window.XMLHttpRequest)
    {
        client = new XMLHttpRequest();       
    }
    else
    {
        if (window.ActiveXObject)
        {
            client = new ActiveXObject("Microsoft.XMLHTTP");
        }
    }
   
   // client.onreadystatechange = handler;
    client.open("POST", "http://localhost/Samples/MyService.asmx/IsUserExist", false); // This parameter sepcifies to request synchronously
    client.setrequestheader("Content-type", "application/x-www-form-urlencoded");
    client.send("userName=" + document.getElementById("Text2").value);
    alert(client.responseText);
}

[WebMethod]
    public string IsUserExist()
    {
        System.Threading.Thread.Sleep(3000);
        return "Hello World";
    }

Hope this helps. 

Thanks a lot.

Follow the Asp.net Ajax version http://geekswithblogs.net/rashid/archive/2007/07/04/SJAX-Call.aspx