Can I change the FormsAuthentication cookie name in asp.net dynamically? - asp.net-mvc

Question

I want to set FormsAuthentication cookie name dynammically, for example a guid. how can i do that. I can already change it to whatever I want in web.config. I just can't do it in code and dynamically.Please help.
<authentication mode="Forms">
<forms name="myName" loginUrl="~/Account/Login" defaultUrl="~/Admin/admin" cookieless="UseCookies" slidingExpiration="true" timeout="40320"/>
</authentication>
The reason that I want to do this is, i have several instances of my application on the same host and i do not want them to overwrite each other's cookies.

Answer 1

I have been struggling with Cookies with quite a few days. It has been an awesome learning experience.
So wanted to share the possible ways I found & discovered: There are several HACKs to modify Forms Authentication Cookie name:
You can automate the modification of cookie name under Authenticaiton secion of Web.Config file in Application_Start event in Global.asax. Thanks to Ron for sharing this. But I could not guarantee that the user whose identity would be used to run application domain have enough privileges to modify the file on disk or not. Hence I needed an improvised solution, so I devised following.
Thanks to ILSpy for letting me see inside the FormsAuthentication class, and many thanks to Reflection to let me modify the private field of a class. I used following code to modify the cookie name on run-time with following small piece of code and this worked like a charm !!!
protected void Application_Start(Object sender, EventArgs e)
{
// This will enforce that FormsAuthentication class is loaded from configuration settings for the application.
FormsAuthentication.Initialize();
// The new cookie name whatever you need can go here, I needed some value from my application setting to be prefixed so I used it.
string newCookieName = string.Format("{0}.ASPXAUTH", ConfigurationManager.AppSettings["SomeSettingThatIsUniquetoSite"]);
// Modifying underlying baking field that points to FormsAuthentication.FormsCookieName
Type type = typeof(FormsAuthentication);
System.Reflection.FieldInfo field = type.GetField("_FormsName", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);
field.SetValue(null, newCookieName);
}
Suggestions, loopholes are requested as this is my first answer on this forum.

Answer 2

You can't do this in code; the property FormsAuthentication.FormsCookieName is readonly. I would use the following configuration in web.config:
<authentication mode="Forms">
<forms configSource="forms.config" />
</authentication>
Then give each instance of the application its own forms.config:
<forms name="CookieNameForThisInstance" />