Get path of referenced project during unit testing

I suggest you do something like this:

public class MyXslFileLoader
{
    public void Load()
    {
        Load(AppDomain.CurrentDomain.BaseDirectory + "\XML Transformationen\Transformation_01.xslt");
    }

    public void Load(string path)
    {
        Xsl = GetXSLFromFile(path);
    }
}

You would call Load() in your web application, but use the overloaded version of this method in your unittest application. You could consider adding the xslt file as a resource to your project.

You would be able to load the path like this:

var webApplicationDllPath = Path.GetDirectoryName(typeof(ClassInTheWebApplicationDll).Assembly.GetName().CodeBase);

string path;
path = System.IO.Path.GetDirectoryName( 
  System.Reflection.Assembly.GetExecutingAssembly().GetName().CodeBase );

HOW TO: Determine the Executing Application's Path

Getting the path of a executable file in C#

Hope this is helpful.. :)