public void AddOrReplaceExtensionWorks()
{
var pb = new VirtualPathBuilder().CombineWith("~/t.cs");
Assert.IsTrue(pb.GetExtension() == "cs");
pb.AddOrReplaceExtension("vb");
Assert.IsTrue(pb.GetExtension() == "vb");
pb.Clear().CombineWith("foo");
Assert.IsFalse(pb.HasExtension());
pb.AddOrReplaceExtension("vb");
Assert.IsTrue(pb.GetExtension() == "vb");
}