public void DirectoryExists()
 {
     Assert.ThrowsExact <ArgumentNullException>(() => Assert.DirectoryExists(null as string));
     Assert.ThrowsExact <ArgumentNullException>(() => Assert.DirectoryExists(null as DirectoryInfo));
     Assert.ThrowsExact <AssertionException>(() => Assert.DirectoryExists(NonExistingDirectory));
     Assert.ThrowsExact <AssertionException>(() => Assert.DirectoryExists(NonExistingDirectoryInfo));
     Assert.ThrowsExact <AssertionException>(() => Assert.DirectoryExists(new DirectoryInfo(NonExistingDirectory)));
     Assert.DoesNotThrow(() => Assert.DirectoryExists(ExistingDirectory));
     Assert.DoesNotThrow(() => Assert.DirectoryExists(ExistingDirectoryInfo));
 }