public static void SignOut()
{
    Initialize();
    HttpContext current = HttpContext.Current;
    bool flag = current.CookielessHelper.DoesCookieValueExistInOriginal('F');
    current.CookielessHelper.SetCookieValue('F', null);
    if (!CookielessHelperClass.UseCookieless(current, false, CookieMode) || current.Request.Browser.Cookies)
    {
        string str = string.Empty;
        if (current.Request.Browser["supportsEmptyStringInCookieValue"] == "false")
        {
            str = "NoCookie";
        }
        HttpCookie cookie = new HttpCookie(FormsCookieName, str);
        cookie.HttpOnly = true;
        cookie.Path = _FormsCookiePath;
        cookie.Expires = new DateTime(0x7cf, 10, 12);
        cookie.Secure = _RequireSSL;
        if (_CookieDomain != null)
        {
            cookie.Domain = _CookieDomain;
        }
        current.Response.Cookies.RemoveCookie(FormsCookieName);
        current.Response.Cookies.Add(cookie);
    }
    if (flag)
    {
        current.Response.Redirect(GetLoginPage(null), false);
    }
}

current.Request.Browser["supportsEmptyStringInCookieValue"]

HttpContext.Current.Request.Browser.SupportsEmptyStringInCookieValue

CookielessHelperClass.UseCookieless(current, false, CookieMode)

HttpContext.Current.Request.Browser = new HttpBrowserCapabilities() { Capabilities = new Dictionary<string, string> { { "supportsEmptyStringInCookieValue", "false" } } };

public interface IFormsAuthenticationWrap
{
    void SignOut();
}

public class FormsAuthenticationWrap : IFormsAuthenticationWrap
{
    public void SignOut()
    {
        FormsAuthentication.SignOut();
    }
}

public class LogOutClass
{
    private readonly IFormsAuthenticationWrap _formsAuthentication;

    public LogOutClass() : this (new FormsAuthenticationWrap())
    {
    }

    public LogOutClass(IFormsAuthenticationWrap formsAuthentication)
    {
        _formsAuthentication = formsAuthentication;
    }

    public void LogOutMethod()
    {
        // Code before SignOut

        _formsAuthentication.SignOut();

        // Code after SignOut
    }
}

public class FormsAuthenticationStub : IFormsAuthenticationWrap
{
    public void SignOut()
    {
    }
}

[TestMethod]
    public void TestLogOutMethod()
    {
        var logOutClass = new LogOutClass(new FormsAuthenticationStub());
        logOutClass.LogOutMethod();
    }