public class Dev_Sim: IHttpModule

{

    public void Init(HttpApplication app)

    {

        app.BeginRequest += delegate { app.Context.Request.Headers.Add("UserName", "XYZZY"); };

    }

    public void Dispose(){}

}

void changeHeader(HttpApplication app)

        {

            var headers = app.Context.Request.Headers;

            Type hdr = headers.GetType();

            PropertyInfo ro = hdr.GetProperty("IsReadOnly",

                BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.IgnoreCase | BindingFlags.FlattenHierarchy);

            // Remove the ReadOnly property

            ro.SetValue(headers, false, null);

            // Invoke the protected InvalidateCachedArrays method

            hdr.InvokeMember("InvalidateCachedArrays",

                BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Instance,

                null, headers, null);

            // Now invoke the protected "BaseAdd" method of the base class to add the

            // headers you need. The header content needs to be an ArrayList or the

            // the web application will choke on it.

            hdr.InvokeMember("BaseRemove",

                BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Instance,

                null, headers,

                new object[] { "User-Agent" });

            hdr.InvokeMember("BaseAdd",

                BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Instance,

                null, headers,

                new object[] { "User-Agent", new ArrayList { "CustomHeaderContent" } });

            // repeat BaseAdd invocation for any other headers to be added

            // Then set the collection back to ReadOnly

            ro.SetValue(headers, true, null);

        }