What is the namespace of NUnit CollectionAssert?

I have the following:

using System;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading;
using NUnit.Framework;
using Selenium;
using System.Collections.Generic;

but still can not get CollectionAssert which I found on CollectionA开发者_JAVA百科ssert (NUnit 2.4)

The printscreen:

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It's definitely in the NUnit.Framework namespace in the nunit.framework assembly - I've just downloaded NUnit 2.5.10 to check.

I suggest you check which version of nunit.framework you're really referencing, and open it up in Reflector to check for the existence of CollectionAssert.