Without the backslash, "C:Dir" does not take the child folder under the drive root but takes "whatever directory is the current directory for drive C: at the moment" and concats "Dir" to that, which would yield unexpected test results if you're using drive C: and got some currentdir on it different from the root.

Author: hypersw

ICSharpCode.SharpZipLib.Tests/Zip/ZipTests.cs

@@ -1127,7 +1127,7 @@ public void BasicDirectories()
 			TestDirectory(wnt, "d", "d");
 			TestDirectory(wnt, "absolute/file2", @"absolute\file2");
 
-			string BaseDir1 = Path.Combine("C:", "Dir");
+			string BaseDir1 = Path.Combine("C:\\", "Dir");
 			wnt.BaseDirectory = BaseDir1;
 
 			TestDirectory(wnt, "talofa", Path.Combine(BaseDir1, "talofa"));