Use reflection to instantiate VSGitExt implementations

We can't reference all of the GitHub.TeamFoundation.* projects at once, so instead
we use reflection to retrieve and instantiate the appropriate VSGitExt implementation.

Author: jcansdale

src/GitHub.TeamFoundation.14/Services/VSGitExt.cs

@@ -15,24 +15,6 @@
 
 namespace GitHub.VisualStudio.Base
 {
-    // This is a workaround to avoid using reference aliases, which don't currently work with <PackageReference>.
-#if TEAMEXPLORER14
-    public class VSGitExt14 : VSGitExt
-    {
-        public VSGitExt14(IServiceProvider serviceProvider, IGitService gitService) : base(serviceProvider, gitService) { }
-    }
-#elif TEAMEXPLORER15
-    public class VSGitExt15 : VSGitExt
-    {
-        public VSGitExt15(IServiceProvider serviceProvider, IGitService gitService) : base(serviceProvider, gitService) { }
-    }
-#elif TEAMEXPLORER16
-    public class VSGitExt16 : VSGitExt
-    {
-        public VSGitExt16(IServiceProvider serviceProvider, IGitService gitService) : base(serviceProvider, gitService) { }
-    }
-#endif
-
     /// <summary>
     /// This service acts as an always available version of <see cref="IGitExt"/>.
     /// </summary>

src/GitHub.VisualStudio/Services/VSGitExtFactory.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Reflection;
 using GitHub.Logging;
 using GitHub.VisualStudio.Base;
 using Serilog;
@@ -20,24 +21,18 @@ public VSGitExtFactory(int vsVersion, IServiceProvider serviceProvider, IGitServ
             this.gitService = gitService;
         }
 
+        // The GitHub.TeamFoundation.* assemblies target different .NET and Visual Studio versions.
+        // We can't reference all of their projects directly, so instead we use reflection to retrieve
+        // and instantiate the correct implementation.
         public IVSGitExt Create()
         {
-            switch (vsVersion)
+            if(Type.GetType($"GitHub.VisualStudio.Base.VSGitExt, GitHub.TeamFoundation.{vsVersion}", false) is Type type)
             {
-                case 14:
-                    return Create(() => new VSGitExt14(serviceProvider, gitService));
-                case 15:
-                    return Create(() => new VSGitExt15(serviceProvider, gitService));
-                case 16:
-                    return Create(() => new VSGitExt16(serviceProvider, gitService));
-                default:
-                    log.Error("There is no IVSGitExt implementation for DTE version {Version}", vsVersion);
-                    return null;
+                return (IVSGitExt)Activator.CreateInstance(type, serviceProvider, gitService);
             }
-        }
 
-        // NOTE: We're being careful to only reference VSGitExt14 and VSGitExt15 from inside a lambda expression.
-        // This ensures that only the type that's compatible with the running DTE version is loaded.
-        static IVSGitExt Create(Func<IVSGitExt> factory) => factory.Invoke();
+            log.Error("There is no IVSGitExt implementation for DTE version {Version}", vsVersion);
+            return null;
+        }
     }
 }