Spoof the current CultureInfo in a unit test.

SpoofCulture.cs

    public class SpoofCulture : IDisposable
    {
        private readonly CultureInfo _previousCultureInfo;
        private readonly Thread _threadToSpoof;

        public SpoofCulture(CultureInfo cultureToUse, Thread threadToSpoof = null)
        {
            _threadToSpoof = threadToSpoof ?? Thread.CurrentThread;

            _previousCultureInfo = CultureInfo.CurrentCulture;

            _threadToSpoof.CurrentCulture = cultureToUse;
        }

        public void Dispose()
        {
            _threadToSpoof.CurrentCulture = _previousCultureInfo;
        }
    }

SpoofCultureTests.cs

    public class SpoofCultureTests
    {
        [Test]
        public void Can_spoof_the_current_culture_on_the_current_thread()
        {
            var cultureBeforeSpoofing = CultureInfo.CurrentCulture;

            var cultureToSpoof = CultureInfo.GetCultures(CultureTypes.SpecificCultures).First();
            if (cultureToSpoof.Equals(cultureBeforeSpoofing))
            {
                cultureToSpoof = CultureInfo.GetCultures(CultureTypes.SpecificCultures).Skip(1).First();
            }

            Assert.AreEqual(CultureInfo.CurrentCulture, cultureBeforeSpoofing);
            Assert.AreNotEqual(CultureInfo.CurrentCulture, cultureToSpoof);

            using (new SpoofCulture(cultureToSpoof))
            {
                Assert.AreEqual(CultureInfo.CurrentCulture, cultureToSpoof);
                Assert.AreNotEqual(CultureInfo.CurrentCulture, cultureBeforeSpoofing);
            }

            Assert.AreEqual(CultureInfo.CurrentCulture, cultureBeforeSpoofing);
            Assert.AreNotEqual(CultureInfo.CurrentCulture, cultureToSpoof);
        }
    }