Client_RowsCells.aspx try to add rows to multiselect combo in a javsascript?

In order to add multiselect item you will need to add the checkboc element yourself after updateui is invoked. Here is the snippet:

case "Add New Row":
    var rows = wc4.GetRows(); // get the WebCombo4's rows for addition
    var newRow = wc4.NewRow("NEWCUST"); // instantiate new row with NEWCUST as the KeyValue, containing cells with empty value
    var cells = newRow.GetCells(); // get the cells of the new row for value assignment

    cells.GetNamedItem("ContactName").Text = "New Customer"; // write codes in the same way as in server-side object model, with similar object hierarchy.
    cells.GetNamedItem("Address").Text = "New Address"; // assign value to Address Cell
    cells.GetNamedItem("CustomerID").Text = "NEWCUST";

    rows.Add(newRow); //add the newRow to the rows collection
    wc4.UpdateUI(); // instruct WebCombo to update the user interface accordingly to the changes in Rows object.

    var rowElem = wc4.GetRows();
    var latestRow = rowElem[rowElem.length - 1];

    var newInput = latestRow.RowElement.childNodes[0].document.createElement('input');

    newInput.setAttribute('type', "checkbox");
    newInput.setAttribute('hideFocus', "hideFocus");

    latestRow.RowElement.childNodes[0].className = "WC4-C";
    latestRow.RowElement.childNodes[0].setAttribute("noWrap", "noWrap");
    latestRow.RowElement.childNodes[0].appendChild(newInput);

    alert("NEWCUST row has been added");
    break;

WebComboLinked1.aspx try to use WebCombo2_InitializeDataSource eventhandler to populate WebCombo on a server.

You will need to set the MultipleSelectionSettings.AllowBrowseAll to true in order for the DropDownList to show the data. Here is the snippet:

void FormatWebCombo2()
{
    WebCombo2.DataMember = "Orders";
    WebCombo2.DataTextField = "CustomerID";
    WebCombo2.DataValueField = "OrderID";

    WebCombo2.LayoutSettings.ComboMode = Mode.MultipleColumns;
    WebCombo2.LayoutSettings.StatusBoxVisible = false;

    WebCombo2.Columns.Add("CustomerID");
    WebCombo2.Columns.Add("ShipAddress");
    WebCombo2.Columns.Add("OrderID");

    WebCombo2.MultipleSelectionSettings.Enabled = true;
    WebCombo2.MultipleSelectionSettings.AllowBrowseAll = true;
    WebCombo2.LayoutSettings.TextBoxMode = TextboxMode.ReadOnly;
    WebCombo2.AllowAutoQueryHandler = false;
    WebCombo2.AllowAutoDataCaching = false;
}

WebComboLinked2.aspx try to execute [System.Web.Services.WebMethod] from javascript to populate WebCombo2 in a JavaScript

You will need to set the WebCombo2 BoundMode property to ForceUnbound under the layout settings. Here is the snippet:

<LayoutSettings ComboMode="MultipleColumns" TextBoxMode="ReadOnly" BoundMode="ForceUnbound">

Glen,

Thank you for prompt response. Unfortunatelly neither solution you proposed is working. Can you concentrate on the first page (Client_RowsCells.aspx). I think it may satisfy me. When I introduced your changes I still got error "property not supported.."  (vbae71.checked =false) 

I am very sorry I forgot to mention that you will need to update your WebCombo and WebUI Framework to the latest version in order for the snippet to work. In the attached project, you are still using WebCombo 4 build 12 and WebUI Framework 3 build 507. Please update the WebCombo and the WebUI Framework to build 218 and build 750 respectively.

Yes it is works for latest version, but I can't use upgraded version. Is it possible to create combo with checkboxes which I can use for multiselection using WebCombo 4 build 12 and WebUI Framework 3 build 507. Thank you

Since the control have been updated for numerous fix since the previous build, the build number must be incremented. We could not release a fix for a specific build number.

Since you are already using the latest version of WebCombo and WebUI Framework, version 4 and 3 respectively. You will only need to use the  Update Manager in order to retrieve the latest update automatically.

I am having difficulty replicating the issue with the provided snippet in our environment. Do you mind sending us the complete code? How do you invoke the fnPopulateCombo function in order to fill in the wcBooks web combo?

I am using latest version of WebCombo and WebUI Framework, version 4 and 3 respectively. Are latest updates for these versions fully backward compatible?

The control with the same major version is designed to be backward compatible. For example updating from WebCombo 4 previous build to the latest build will not result in any issue. However, some issue might occurs if you are updating from previous version, for example WebCombo 3 to WebCombo 4.

Did you have any issue when updating from the previous build of WebCombo 4 and WebUI Framework 3?