Looping C# list in javascript

Quite simply, you cannot write C# inside javascript - javascript is a scripting language executed on the client, and C# is compiled code that runs on the server.

If you are using ASP.NET you can output javascript to your page though, here is a very simple example:

void WebForm1_PreRender(object sender, EventArgs e)
{
    if (!ClientScript.IsClientScriptBlockRegistered("MyScript"))
    {
        StringBuilder sb = new StringBuilder();
        sb.AppendLine("var myArray = new Array();");
        sb.AppendLine("myArray[0] = 'some value';");
        sb.AppendLine("myArray[1] = 'another value';");
        sb.AppendLine("myArray[2] = 'yet another value';");

        ClientScript.RegisterClientScriptBlock(this.GetType(), "MyScript", sb.ToString(), true);
    }
}

you can then access and iterate this javascript array on the client:

<script language="javascript">
    //first do basic check that the array is available:
    if (typeof(myArray) != 'undefined' && myArray != null) {
        alert(myArray[0]);
    }
</script>

From here it is a simple process to take your prepopulated list and create the javascript list:

void WebForm1_PreRender(object sender, EventArgs e)
{
    List<string> list = new List<string>(new[] { "Foo", "Bar", "Tord", "Bob" });

    if (!ClientScript.IsClientScriptBlockRegistered("MyScript"))
    {
        StringBuilder sb = new StringBuilder();
        sb.AppendLine("var myArray = new Array();");
        for (int i = 0; i < list.Count; i++)
            sb.AppendLine(string.Format("myArray[{0}] = '{1}';", i, list[i]));

        ClientScript.RegisterClientScriptBlock(this.GetType(), "MyScript", sb.ToString(), true);
    }
}

you can use a method that creates the json data:

public static string CreateJsonArray(List<string> list)
{
    if (list.Count > 0)
    {
        StringBuilder sb = new StringBuilder();
        foreach (string item in list)
        {
            sb.AppendFormat("'{0}',", item);
        }
        sb.Remove(sb.Length - 1, 1);
        return String.Format("[{0}]", sb.ToString());
    }
    return "[]";            
}

and than assign it to a javascript script tag

// C#
List<string> list = new List<string>(10);

list.Add("Foo");
list.Add("Bar");
list.Add("Tord");
list.Add("Bob");

ltrResult.Text = CreateJsonArray(list);

// HTML
<script type="text/javascript">
    var arr = <asp:Literal id="ltrResult" runat="server" />;
</script>

@tord: at least for now, you cant use C# code in your javascript file. you would most probably need to convert you List<> to some json equivalent that your javascript can understand. You can use Response.Write from C# to send over the json to the client side

You could go with either injecting the list as an javascript list from your codebehind or if you also whant to show the list you coudl do something simple as:

Front

<asp:Label runat="server" ID="showList"></asp:Label>

Code behind

            List<String> list = new List<string>();
            list.Add("Hello");
            list.Add("How are you doing");
            list.Add("Fine and you?");

            showList.Text += "<ul id='jList'>";
            foreach(String val in list){
                showList.Text += "<li>" + val + "</li>";
            }
            showList.Text += "</ul>";

Then you can access the "jList" from within javascript by GetElementByNode..