Threading Concept WaitHandle[] executing Error

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XML

multi-threading

webservice

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i am new in threading concept i applied threading concept in my code when i run logic i have two beigninvoke concept when i executed it pass two beigninvoke then it will get output or else it executed only one beigninvoke means it will not get ouput it will get error in WaitHandle[] because of another thread will be pass in null how can i solve this problem

[WebMethod]
public XmlDocument GIHotelInventory(string GIMonth, string GIYear, string username)
{
    XmlDocument Ctxml = new XmlDocument();
    XmlDocument Mmtxml = new XmlDocument();
    XmlDocument xml = new XmlDocument();

    var dt = ProviderDetails(username);

    CtCaller ct = new CtCaller(*****Inventory);
    MmtCaller mmt = new MmtCaller(*****Inventory);
    IAsyncResult ctreslt = null;
    IAsyncResult mmtreslt = null;
    string ProviderUserName = string.Empty;
    string ProviderPassword = string.Empty;
    foreach (DataRow a in dt.Rows)
    {
        CollectionProviderDetails prd = new CollectionProviderDetails();
        prd.GiUserName = Convert.ToString(a["GiUserName"]);
        prd.ProviderName = Convert.ToString(a["ProviderName"]);
        ProviderUserName = Convert.ToString(a["PrUserName"]);
        ProviderPassword = Convert.ToString(a["PrPassword"]);

        if (prd.ProviderName == Portal.*****.ToString())
        {
            ctreslt = ct.BeginInvoke(GIMonth, GIYear, ProviderUserName, ProviderPassword, null, null);

        }

        else if (prd.ProviderName == Portal.*****.ToString())
        {
            mmtreslt = mmt.BeginInvoke(GIMonth, GIYear, null, null);
        }
    }
    WaitHandle[] handleArray = { ctreslt.AsyncWaitHandle, mmtreslt.AsyncWaitHandle };
    WaitHandle.WaitAll(handleArray);
    Ctxml = ct.EndInvoke(ctreslt);
    Mmtxml = mmt.EndInvoke(mmtreslt);
    String Allxml = "<inventorydetails>\n";
    string GIxml = "<inventorydetails>\n" + Ctxml.InnerXml + " " + Mmtxml.InnerXml + "</inventorydetails>";
    Allxml += "</inventorydetails>";
    xml.LoadXml(GIxml);

    return xml;
}