an example of an HttpModule that html encodes form data

PrevalidationSanitizer.cs

using System;
using System.Collections.Specialized;
using System.Reflection;
using System.Web;

public class PrevalidationSanitizer : System.Web.IHttpModule
{
    private HttpApplication httpApp;

    public void Init(HttpApplication httpApp)
    {
        this.httpApp = httpApp;
        httpApp.PreRequestHandlerExecute += new System.EventHandler(PreRequestHandlerExecute_Event);
    }

    public void Dispose() { }

    public void PreRequestHandlerExecute_Event(object sender, System.EventArgs args)
    {
        NameValueCollection form = httpApp.Request.Form;

        Type type = form.GetType();

        PropertyInfo prop = type.GetProperty("IsReadOnly", BindingFlags.Instance
            | BindingFlags.IgnoreCase | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy);

        prop.SetValue(form, false, null);

        if (httpApp.Request.RequestType == "POST" != null && httpApp.Request.Form["testinput"])
            httpApp.Request.Form.Set("testinput", httpApp.Server.HtmlEncode(httpApp.Request.Form["testinput"]));
    }
}