gistfile1.cs

public static class Mocks
{
    public static Mock<HttpRequestBase> HttpRequest()
    {
        var httpRequest = new Mock<HttpRequestBase>();
        httpRequest.Setup(x => x.Cookies).Returns(new HttpCookieCollection());
        return httpRequest;
    }

    public static Mock<HttpResponseBase> HttpResponse()
    {
        var httpResponse = new Mock<HttpResponseBase>();
        httpResponse.Setup(x => x.Cookies).Returns(new HttpCookieCollection());
        return httpResponse;
    }
}

// This method is from my CookieContainerTests class

[TestMethod]
public void SetValue_UpdatesExistingCookie()
{
    // Arrange
    const string cookieName = "myCookie";
    const string cookieValue = "myValue";
    DateTime cookieExpires = new DateTime(2009, 1, 1, 0, 0, 0);

    var httpRequest = Mocks.HttpRequest();
    var httpResponse = Mocks.HttpResponse();
    var cookieContainer = new CookieContainer(httpRequest.Object, httpResponse.Object);
    
    httpResponse.Object.Cookies.Add(new HttpCookie(cookieName, "oldValue"));

        // Act
    _cookieContainer.SetValue(cookieName, cookieValue, cookieExpires);

    // Assert
    HttpCookie cookie = httpResponse.Object.Cookies["myCookie"];
    Assert.IsNotNull(cookie);
    Assert.AreEqual(cookie.Name, cookieName);
    Assert.AreEqual(cookie.Value, cookieValue);
    Assert.AreEqual(cookie.Expires, cookieExpires);
}