How to set ASP.net dropdownlist index to 0 from the front-end

Question

I have the following code in ASP.net page:

Answer 1

There are many ways to do this but here is the solution with JavaScript:

function setSelectedIndex(dropdownlist, selVal)
    var ddl1 = document.getElementById(dropdownlist);
    //alert(ddl1.selectedIndex); //displays the proper index...
    if(ddl1.length >= selVal)
    {
       ddl1.selectedIndex = selVal;
    }
}

Call the above function as per you required as below:

setSelectedIndex('<%=ddl1.ClientID %>', 2);

UPDATE As you said you have already set ClientIDMode, try the following updated function:

function setSelectedIndex(selVal){
    var ddl1 = document.getElementById('ddl1');
    if(ddl1.length >= selVal)
    {
       ddl1.selectedIndex = selVal;
    }
}

and call it as:

setSelectedIndex(0);