diff --git a/MyClientCpp/Main.cpp b/MyClientCpp/Main.cpp
index 817bef2..b13a7a7 100644
--- a/MyClientCpp/Main.cpp
+++ b/MyClientCpp/Main.cpp
@@ -82,7 +82,16 @@ int main() {
 
         try {
             auto cruncher = server->GetNumberCruncher();
-            double pi = cruncher->ComputePi();
+            // try to cast to INumberCruncher2 to check if the new interface is implemented
+            MyInterfaces::INumberCruncher2Ptr cruncher2 = cruncher;
+            double pi = 0;
+            if (cruncher2) {
+                // use new interface
+                pi = cruncher2->ComputePi2();
+            } else {
+                // fallback to old interface
+                pi = cruncher->ComputePi();
+            }
             std::wcout << L"pi = " << pi << std::endl;
 
             auto callback = CreateLocalInstance<MyClient>();
diff --git a/MyClientCs/Program.cs b/MyClientCs/Program.cs
index 9ddfc3e..21c4b3e 100644
--- a/MyClientCs/Program.cs
+++ b/MyClientCs/Program.cs
@@ -13,7 +13,16 @@ static void CommunicateWithServer()
 
             {
                 var cruncher = server.GetNumberCruncher();
-                double pi = cruncher.ComputePi();
+                double pi = 0;
+                try {
+                    // try to cast to INumberCruncher2 to check if the new interface is implemented
+                    var cruncher2 = (MyInterfaces.INumberCruncher2)cruncher;
+                    // use new interface
+                    pi = cruncher2.ComputePi2();
+                } catch (System.InvalidCastException) {
+                    // fallback to old interface
+                    pi = cruncher.ComputePi();
+                }
                 Console.WriteLine($"pi = {pi}");
 
                 // release reference to help GC clean up (not strctly needed)
diff --git a/MyExeServerCpp/MyServerImpl.hpp b/MyExeServerCpp/MyServerImpl.hpp
index 012f19e..99d5bab 100644
--- a/MyExeServerCpp/MyServerImpl.hpp
+++ b/MyExeServerCpp/MyServerImpl.hpp
@@ -18,7 +18,8 @@ using namespace MyInterfaces;
 class ATL_NO_VTABLE NumberCruncher :
     public CComObjectRootEx<CComMultiThreadModel>, // also compatible with STA
     public CComCoClass<NumberCruncher>, // no CLSID needed
-    public INumberCruncher
+    public INumberCruncher,
+    public INumberCruncher2
 {
 public:
     NumberCruncher() {
@@ -33,12 +34,21 @@ class ATL_NO_VTABLE NumberCruncher :
         if (!val)
             return E_INVALIDARG;
 
+        *val = M_PI + 0.01; // old inaccurate impl.
+        return S_OK;
+    }
+
+    HRESULT STDMETHODCALLTYPE raw_ComputePi2(/*out*/double* val) override {
+        if (!val)
+            return E_INVALIDARG;
+
         *val = M_PI;
         return S_OK;
     }
 
     BEGIN_COM_MAP(NumberCruncher)
         COM_INTERFACE_ENTRY(INumberCruncher)
+        COM_INTERFACE_ENTRY(INumberCruncher2)
     END_COM_MAP()
 };
 
diff --git a/MyExeServerCs/MyServerImpl.cs b/MyExeServerCs/MyServerImpl.cs
index 5688bf4..195315d 100644
--- a/MyExeServerCs/MyServerImpl.cs
+++ b/MyExeServerCs/MyServerImpl.cs
@@ -126,7 +126,7 @@ public void Unsubscribe(IMyClient client)
     }
 
     /** Non-creatable COM class that doesn't need any CLSID. */
-    public sealed class NumberCruncher : ComSupport.ComClass, MyInterfaces.INumberCruncher
+    public sealed class NumberCruncher : ComSupport.ComClass, MyInterfaces.INumberCruncher, MyInterfaces.INumberCruncher2
     {
         public NumberCruncher()
         {
@@ -139,6 +139,11 @@ public NumberCruncher()
         }
 
         public double ComputePi ()
+        {
+            return Math.PI + 0.01; // old inaccurate impl.
+        }
+
+        public double ComputePi2()
         {
             return Math.PI;
         }
diff --git a/MyInterfaces/MyInterfaces.idl b/MyInterfaces/MyInterfaces.idl
index 029cae2..74f0de3 100644
--- a/MyInterfaces/MyInterfaces.idl
+++ b/MyInterfaces/MyInterfaces.idl
@@ -90,6 +90,15 @@ interface INumberCruncher : IUnknown {
     HRESULT ComputePi([out, retval] double *ret);
 };
 
+[object,
+oleautomation,
+uuid(E847D928-753B-45AC-A795-8716D87FABB0),
+helpstring("New and better number crunching interface")]
+interface INumberCruncher2 : IUnknown {
+    [helpstring("Test method")]
+    HRESULT ComputePi2([out, retval] double* ret);
+};
+
 [object,
  oleautomation,
  uuid(F586D6F4-AF37-441E-80A6-3D33D977882D),
@@ -107,6 +116,8 @@ interface IMyServer : IUnknown {
 
 [uuid(46F3FEB2-121D-4830-AA22-0CDA9EA90DC3)]
 library MyInterfaces {
+    interface INumberCruncher2;
+
     [uuid(AF080472-F173-4D9D-8BE7-435776617347)]
     coclass MyServer {
         [default] interface IMyServer;
diff --git a/UNREGISTER.bat b/UNREGISTER.bat
index 3794efd..26fa481 100644
--- a/UNREGISTER.bat
+++ b/UNREGISTER.bat
@@ -10,6 +10,7 @@ for %%P in (32 64) do (
   reg delete "HKCR\Interface\{BE3FF6C1-94F5-4974-913C-237C9AB29679}" /f /reg:%%P 2> NUL
   reg delete "HKCR\Interface\{F586D6F4-AF37-441E-80A6-3D33D977882D}" /f /reg:%%P 2> NUL
   reg delete "HKCR\Interface\{B5506675-17E0-4709-A31A-305E36D0E2FA}" /f /reg:%%P 2> NUL
+  reg delete "HKCR\Interface\{E847D928-753B-45AC-A795-8716D87FABB0}" /f /reg:%%P 2> NUL
 )
 
 :: Delete MyInterfaces TypeLib