[c#] Check if Cookie Exists

From a quick search on Stack Overflow I saw people suggesting the following way of checking if a cookie exists:

HttpContext.Current.Response.Cookies["cookie_name"] != null

or (inside a Page class):

this.Response.Cookies["cookie_name"] != null

However, when I try to use the indexer (or the Cookies.Get method) to retrieve a cookie that does not exist it seems to actually create a 'default' cookie with that name and return that, thus no matter what cookie name I use it never returns null. (and even worse - creates an unwanted cookie)

Am I doing something wrong here, or is there a different way of simply checking for the existance of a specific cookie by name?

There are a lot of right answers here depending on what you are trying to accomplish; here's my attempt at providing a comprehensive answer:

Both the Request and Response objects contain Cookies properties, which are HttpCookieCollection objects.

Request.Cookies:

• This collection contains cookies received from the client
• This collection is read-only
• If you attempt to access a non-existent cookie from this collection, you will receive a null value.

Response.Cookies:

• This collection contains only cookies that have been added by the server during the current request.
• This collection is writeable
• If you attempt to access a non-existent cookie from this collection, you will receive a new cookie object; If the cookie that you attempted to access DOES NOT exist in the Request.Cookies collection, it will be added (but if the Request.Cookies object already contains a cookie with the same key, and even if it's value is stale, it will not be updated to reflect the changes from the newly-created cookie in the Response.Cookies collection.

Solutions

If you want to check for the existence of a cookie from the client, do one of the following

• Request.Cookies["COOKIE_KEY"] != null
• Request.Cookies.Get("COOKIE_KEY") != null
• Request.Cookies.AllKeys.Contains("COOKIE_KEY")

If you want to check for the existence of a cookie that has been added by the server during the current request, do the following:

• Response.Cookies.AllKeys.Contains("COOKIE_KEY") (see here)

Attempting to check for a cookie that has been added by the server during the current request by one of these methods...

• Response.Cookies["COOKIE_KEY"] != null
• Response.Cookies.Get("COOKIE_KEY") != null (see here)

...will result in the creation of a cookie in the Response.Cookies collection and the state will evaluate to true.

You can do something like this to find out the cookies's value:

Request.Cookies[SESSION_COOKIE_NAME].Value

public static class CookieHelper
{
    /// <summary>
    /// Checks whether a cookie exists.
    /// </summary>
    /// <param name="cookieCollection">A CookieCollection, such as Response.Cookies.</param>
    /// <param name="name">The cookie name to delete.</param>
    /// <returns>A bool indicating whether a cookie exists.</returns>
    public static bool Exists(this HttpCookieCollection cookieCollection, string name)
    {
        if (cookieCollection == null)
        {
            throw new ArgumentNullException("cookieCollection");
        }

        return cookieCollection[name] != null;
    }
}

Usage:

Request.Cookies.Exists("MyCookie")

You need to use HttpContext.Current.Request.Cookies, not Response.Cookies.

Side note: cookies are copied to Request on Response.Cookies.Add, which makes check on either of them to behave the same for newly added cookies. But incoming cookies are never reflected in Response.

This behavior is documented in HttpResponse.Cookies property:

After you add a cookie by using the HttpResponse.Cookies collection, the cookie is immediately available in the HttpRequest.Cookies collection, even if the response has not been sent to the client.

Sometimes you still need to know if Cookie exists in Response. Then you can check if cookie key exists:

HttpContext.Current.Response.Cookies.AllKeys.Contains("myCookie")

More info can be found here.

In my case I had to modify Response Cookie in Application_EndRequest method in Global.asax. If Cookie doesn't exist I don't touch it:

string name = "myCookie";
HttpContext context = ((HttpApplication)sender).Context;
HttpCookie cookie = null;

if (context.Response.Cookies.AllKeys.Contains(name))
{
    cookie = context.Response.Cookies[name];
}

if (cookie != null)
{
    // update response cookie
}

Sorry, not enough rep to add a comment, but from zmbq's answer:

Anyway, to see if a cookie exists, you can check Cookies.Get(string), this will not modify the cookie collection.

is maybe not fully correct, as Cookies.Get(string) will actually create a cookie with that name, if it does not already exist. However, as he said, you need to be looking at Request.Cookies, not Response.Cookies So, something like:

bool cookieExists = HttpContext.Current.Request.Cookies["cookie_name"] != null;