How to get values from dynamically added (using javascript) elements?

When you add the elements, assign unique IDs to them, and then retrieve their values using Request.Form["UniqueIdHere"] (C#) or Request.Form("UniqueIdHere") (VB.NET).

Create a loop that loops through each input and select object, that grabs the name/id of the current object and its corresponding value. Then add those items to an array and once the loop is completed, pass those values to your aspx file.

You can view an example with this approach at: http://jsfiddle.net/euHeX/. It currently just alerts the values, but you could easily modify it to pass the values as a parameter via ajax to your handler aspx file. The code will add new inputs or select boxes based off of the input provided. This would of course be modified to reflect your current setup.

HTML:

<div id="dynamic"></div>
<input type="button" id="submit-form" value="Submit>>">

JavaScript (using jQuery):

function createInput(type){
    for(var i=0; i<5; i++){
        if(type==0){
            var obj = '<input type="text" id="'+i+'" class="dynamicContent">';
        }else if(type==1){
            var obj = '<select id="'+i+'" class="dynamicContent"><option>--Select--</option></select>';
        }
        $("#dynamic").append(obj);
    }
}

function getContent(){
    var inputArray = [];
    $(".dynamicContent").each(function(k,v){
        var o = $(this);
        var oType;
        if(o.is("input")){ oType = "input"; }
        if(o.is("select")){ oType = "select"; }        
        var oID = oType+o.attr("id");
        var oValue = o.val();
        inputArray.push(oID+'='+oValue);
    });
    alert(inputArray);
}

$("#submit-form").click(function(){
    getContent();    
});

// Set type to 0 for input or 1 for select
var type = '1';
createInput(type);

If you're using jQuery you can use .live() to achive this like a peace of cake! http://api.jquery.com/live/

I don't know if your controls will survive the postback the way you're creating them, but a good technique for accessing dynamically generated controls (assuming that you've figured out how to persist them) is to do something like the following:

Add a panel to your page. Add your dynamically created controls to this panel.

In the OnClick event handler (or other method), do something like the following:

foreach (DropDownList ddl in Panel1.Controls.OfType<DropDownList>())
{
    //put code here
}

foreach (TextBox txt in Panel1.Controls.OfType<TextBox>())
{
   //put code here
}