Thresher uses a licensing scheme borrowed form the pnet library. It is made available under the terms of the GPL with a "linking exception" which allows you to link application programs with Thresher regardless of what license has been chosen for your application program. In other words, the "linking exception" causes the GPL to only extend to the C# library's API boundaries and then stop there.
As a special exception, the copyright holder of the Thresher class library gives you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.