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Home Forums How to create a Cookie in C#, which is both side modifiable. – MindStick
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How to create a Cookie in C#, which is both side modifiable.

ICSM Computer 652 05 May 2025

How to create a Cookie in C#, which is both side modifiable.

c# c#
ICSM Computer
Ask by ICSM Computer
Updated 21 May 2025

1 Answers

Utpal Vishwas
Utpal Vishwas 21 May 2025
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In C#, you can create a cookie that is modifiable from both the client-side (JavaScript) and server-side (C#) by ensuring the cookie meets these two criteria:

Key Requirements

  • HttpOnly = false — so JavaScript can read/modify it.
  • No Secure flag (unless using HTTPS).
  • Accessible Path — set Path = "/" if you want it globally accessible.

1. Set a Modifiable Cookie in C# (Server-Side)

In ASP.NET MVC or WebForms, you can use:

HttpCookie cookie = new HttpCookie("user-preference", "dark-mode");
cookie.Expires = DateTime.Now.AddDays(7);     // Optional: persist cookie
cookie.HttpOnly = false;                      // Required to allow JavaScript access
cookie.Path = "/";                            // Available site-wide

Response.Cookies.Add(cookie);

In ASP.NET Core:

Response.Cookies.Append("user-preference", "dark-mode", new CookieOptions
{
    Expires = DateTimeOffset.Now.AddDays(7),
    HttpOnly = false,
    Path = "/"
});

2. Access and Modify the Cookie via JavaScript (Client-Side)

// Read cookie
function getCookie(name) {
  const match = document.cookie.match(new RegExp('(^| )' + name + '=([^;]+)'));
  return match ? decodeURIComponent(match[2]) : null;
}

// Set or update cookie
document.cookie = "user-preference=light-mode; path=/; max-age=604800"; // 7 days

document.cookie can only access cookies that are not HttpOnly and within the current path/domain.

3. Modify the Cookie in C# Again (Optional)

if (Request.Cookies["user-preference"] != null)
{
    var cookie = new HttpCookie("user-preference", "updated-mode");
    cookie.Expires = DateTime.Now.AddDays(7);
    cookie.HttpOnly = false;
    Response.Cookies.Set(cookie);
}

Important Notes

  1. Avoid storing sensitive data (like tokens, user IDs) in client-accessible cookies.
  2. Cookies sent by the browser will always be the latest set value, whether it was updated client- or server-side.
  3. Cookies are limited in size (~4 KB per cookie).
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