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

Question

Utpal Vishwas · Answer

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

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

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1 Answers

Key Requirements

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

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

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

Important Notes

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