MaxLength property of a TextBox control does not work when the TextMode property is set to Multiline. Here is small work around using regualr expression validator..

<asp:TextBox ID="txtComment" runat="server" Width="466px" TextMode ="MultiLine" ></asp:TextBox> 
<asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server" ControlToValidate="txtComment" 
ErrorMessage="Not more than 50 charcters" Display="Static" ValidationExpression='^[\s\S]{0,50}$'> 
</asp:RegularExpressionValidator> 

Here is sample code for adding tooltip to DropDownList. The key here is adding "title" attribute to the list items in DataBound event.

<asp:DropDownList ID="DropDownList1" runat="server" ondatabound="DropDownList1_DataBound">
</asp:DropDownList>

protected void DropDownList1_DataBound(object sender, EventArgs e)
{
    DropDownList ddl = sender as DropDownList;
    if (ddl != null)
    {
        foreach (ListItem li in ddl.Items)
        {
           li.Attributes["title"] = li.Text; // setting text value of item as tooltip

        }
    }

}

 
<script type="text/javascript" language="javascript">
function CustomValidator1_ClientValidate(source,args)
{ 
var listbox = document.getElementById("<%=ListBox1.ClientID %>");
var total = 0;
var i=0;
for( i = 0; i < listbox.options.length; i++ )
{
 if (listbox.options[i].selected) 
  {
   total ++; 
    if (total >10) {
      args.IsValid = false;
      return;
  }
  } 
 } 
 args.IsValid = true; return;
}
</script>
 
<asp:ListBox ID="ListBox1" runat="server" SelectionMode="Multiple" Height="100%"
Width="100%"></asp:ListBox>
<asp:Button ID="Button3" runat="server" OnClick="Button1_Click" Text="Button" />
<asp:CustomValidator ID="CustomValidator1" runat="server" Display="Dynamic" ErrorMessage="Please select 10 items only."
ClientValidationFunction="CustomValidator1_ClientValidate" OnServerValidate="CustomValidator1_ServerValidate">
</asp:CustomValidator>

Code Behind:

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args) 
{ 
  int count = 0; 
  for (int i = 0; i < ListBox1.Items.Count; i++) 
  { 
        if (ListBox1.Items[i].Selected) 
            count++; 
  } 
 args.IsValid = (count > 10) ? false : true; 
} 

Some times we need to call server side function from Javascript function.For example,callig a PoupWindow(with value from code behind) on HyperLink click. 

This must be implemented by creating a additional request to the web server and that’s the only way to invoke methods on the server. Page methods are used for this.

What is PageMethod?

A PageMethod is a public static method that is exposed in the code-behind of a page and can be called from the client script. PageMethods are annotated with [System.Web.Services.WebMethod] attribute.

The code is simple..

Server side function

[code:c#]

[System.Web.Services.WebMethod] public static void Score(string ID, string name, string Role) {

         try {

                //Some data manipulations 

                  return finalScore ; 

              }

         catch (Exception ex) { 

               //Response.Write(ex.ToString());

                                  } 

            } 

[/code]

//Client Side function 

[code:c#]

function score()

              {               

                  PageMethods.Score(txtID.value,txtName.value,txtRole.value);            

               } 

[/code]

<asp:HyperLink ID="btn_Test" runat="server" Text="Calculate" OnClientClick="javascript:return score();"/>