preventing cross-site request forgery (csrf) attacks in asp.net web forms

You could try the following. In the Web-Form add:

<%= System.Web.Helpers.AntiForgery.GetHtml() %>

This will add a hidden field and a cookie. So if you fill out some form data and post it back to the server you need a simple check:

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
        AntiForgery.Validate(); // throws an exception if anti XSFR check fails.
}

AntiForgery.Validate(); throws an exception if anti XSFR check fails.