Is it possible to set SessionState to readonly mode? Readonly mode means, we can only read data from session but we can not write any data in session variable?

No, in ASP.NET, there is no built-in "read-only" mode for session state. By default, session state allows both reading and writing of data. However, you can implement custom logic in your application to achieve a similar effect where you restrict writing to the session state while still allowing read access.

One approach to achieving a read-only session state is by creating a custom session state provider. By implementing a custom session state provider, you can override the methods responsible for writing or modifying session data and restrict those operations. For example, you can throw an exception or simply ignore any attempts to modify session data.

Another approach is to create a wrapper class around the session state object and expose only read-access methods while hiding or disabling write-access methods. This wrapper class can provide read-only access to the session state by exposing properties or methods that retrieve data but prevent modification.

Here's an example of a simple wrapper class that provides read-only access to session state:

public class ReadOnlySessionWrapper
{
    private HttpSessionStateBase session;

    public ReadOnlySessionWrapper(HttpSessionStateBase session)
    {
        this.session = session;
    }

    public object this[string key]
    {
        get { return session[key]; }
    }

    // Other read-access methods or properties as needed
}

By using this wrapper class, you can pass the session state object to the constructor and use the wrapper object throughout your application, allowing only read access to the session data.

It's important to note that implementing a read-only session state requires careful consideration of your application's requirements. Be mindful that certain ASP.NET features or third-party components may rely on read-write access to session state, so thoroughly test your application to ensure it functions correctly in a read-only session state scenario.