diff --git a/src/Mod/Sketcher/App/ExternalGeometryExtension.cpp b/src/Mod/Sketcher/App/ExternalGeometryExtension.cpp
index 578d5ff4683f..f229e690e323 100644
--- a/src/Mod/Sketcher/App/ExternalGeometryExtension.cpp
+++ b/src/Mod/Sketcher/App/ExternalGeometryExtension.cpp
@@ -33,7 +33,7 @@ using namespace Sketcher;
 
 //---------- Geometry Extension
 
-constexpr std::array<const char *,ExternalGeometryExtension::NumFlags> ExternalGeometryExtension::flag2str;
+constexpr std::array<const char *, ISketchExternalGeometryExtension::NumFlags> ISketchExternalGeometryExtension::flag2str;
 
 TYPESYSTEM_SOURCE(Sketcher::ExternalGeometryExtension,Part::GeometryExtension)
 
diff --git a/src/Mod/Sketcher/App/ExternalGeometryExtension.h b/src/Mod/Sketcher/App/ExternalGeometryExtension.h
index e7ec183ed4c8..58557fdc1fde 100644
--- a/src/Mod/Sketcher/App/ExternalGeometryExtension.h
+++ b/src/Mod/Sketcher/App/ExternalGeometryExtension.h
@@ -25,13 +25,13 @@
 
 #include <Mod/Part/App/Geometry.h>
 #include <array>
+#include <bitset>
 
 namespace Sketcher
 {
 
-class SketcherExport ExternalGeometryExtension : public Part::GeometryExtension
+class ISketchExternalGeometryExtension
 {
-    TYPESYSTEM_HEADER_WITH_OVERRIDE();
 public:
     // START_CREDIT_BLOCK: Credit under LGPL for this block to Zheng, Lei (realthunder) <realthunder.dev@gmail.com>
     enum Flag {
@@ -46,6 +46,25 @@ class SketcherExport ExternalGeometryExtension : public Part::GeometryExtension
 
     constexpr static std::array<const char *,NumFlags> flag2str {{ "Defining", "Frozen", "Detached","Missing", "Sync" }};
 
+public:
+    // Identification information
+    // START_CREDIT_BLOCK: Credit under LGPL for this block to Zheng, Lei (realthunder) <realthunder.dev@gmail.com>
+    virtual bool testFlag(int flag) const = 0;
+    virtual void setFlag(int flag, bool v=true) = 0;
+    // END_CREDIT_BLOCK: Credit under LGPL for this block to Zheng, Lei (realthunder) <realthunder.dev@gmail.com>
+
+    virtual bool isClear() const = 0;
+    virtual size_t flagSize() const = 0;
+
+    virtual const std::string& getRef() const = 0;
+    virtual void setRef(const std::string & ref) = 0;
+};
+
+class SketcherExport ExternalGeometryExtension : public Part::GeometryExtension, public ISketchExternalGeometryExtension
+{
+    TYPESYSTEM_HEADER_WITH_OVERRIDE();
+public:
+
     ExternalGeometryExtension() = default;
     virtual ~ExternalGeometryExtension() override = default;
 
@@ -59,15 +78,15 @@ class SketcherExport ExternalGeometryExtension : public Part::GeometryExtension
     virtual PyObject *getPyObject(void) override;
 
     // START_CREDIT_BLOCK: Credit under LGPL for this block to Zheng, Lei (realthunder) <realthunder.dev@gmail.com>
-    bool testFlag(int flag) const { return Flags.test((size_t)(flag)); }
-    void setFlag(int flag, bool v=true) { Flags.set((size_t)(flag),v); }
+    virtual bool testFlag(int flag) const override { return Flags.test((size_t)(flag)); }
+    virtual void setFlag(int flag, bool v=true) override { Flags.set((size_t)(flag),v); }
     // END_CREDIT_BLOCK: Credit under LGPL for this block to Zheng, Lei (realthunder) <realthunder.dev@gmail.com>
 
-    bool isClear() const {return Flags.none();}
-    size_t flagSize() const {return Flags.size();}
+    virtual bool isClear() const override {return Flags.none();}
+    virtual size_t flagSize() const override {return Flags.size();}
 
-    const std::string& getRef() const {return Ref;}
-    void setRef(const std::string & ref) {Ref = ref;}
+    virtual const std::string& getRef() const override {return Ref;}
+    virtual void setRef(const std::string & ref) override {Ref = ref;}
 
 private:
     ExternalGeometryExtension(const ExternalGeometryExtension&) = default;
diff --git a/src/Mod/Sketcher/App/ExternalGeometryExtensionPyImp.cpp b/src/Mod/Sketcher/App/ExternalGeometryExtensionPyImp.cpp
index bdf010574ccc..f1addee1407f 100644
--- a/src/Mod/Sketcher/App/ExternalGeometryExtensionPyImp.cpp
+++ b/src/Mod/Sketcher/App/ExternalGeometryExtensionPyImp.cpp
@@ -55,7 +55,7 @@ std::string ExternalGeometryExtensionPy::representation(void) const
 
         bool first=true;
 
-        for(size_t i=0;i<ExternalGeometryExtension::NumFlags;i++) {
+        for(size_t i=0;i<ISketchExternalGeometryExtension::NumFlags;i++) {
             if(getExternalGeometryExtensionPtr()->testFlag(i)) {
                 if(first) {
                     first=false;