The Windows 8 platform provides an API to inject touch events (WM_TOUCH..) via a C++ API. The API is relatively easy to use, but not available in C#, unless you do DllImport/PInvoke.
I found this to be kind of tricky, but finally I succeeded to make a C# library that can be used to use the TouchInjection API from C#. You can find TCD.System.TouchInjection on NuGet, or have a look at the TouchInjection.cs file itself.
You may notice that I renamed all the strucs, enums and classes to look nicer than their C++ counterparts. The IntelliSense descriptions are available for almost everything! (I got them from the documentation on msdn..)
I’ve made a sample application for the MSDN Code Gallery as well.
Here’s a small collection of useful resources regarding the TouchInjection API: