Using Eval() - To take server side value in client side

Question

Using Eval()  how to take server side value in client side

In server side:

in page load, string a="aaaaa";

In client side:

<asp:TextBox ID="txt" runat="server" Text='<%# Eval("a") %>'>
</asp:TextBox>

the textbox is not displaying the values.

How to Proceed?