Increase Session id length

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ASP.NET

I want to increase session id length from 24 characters to 32 characters. I put a class file which is given below

public class GuidSessionIDManager:SessionIDManager
{
    public GuidSessionIDManager()
    {
        //
        // TODO: Add constructor logic here
        //
    }
    public override string CreateSessionID(HttpContext context)
    {
        return Guid.NewGuid().ToString();

        // Here you can create your own max 80 character unique GUID ID.
    }


    public override bool Validate(string id)
    {
        try
        {

            Guid testGuid = new Guid(id);
            if (id == testGuid.ToString())
                return true;
        }
        catch
        {
        }

        return false;
    }
}

and in webconfig file

<sessionstate sessionidmanagertype="GuidSessionIDManager">

But still now i cannot change session id value. It is very urgent please help me .