diff --git a/compiler/cpp/src/generate/t_csharp_generator.cc b/compiler/cpp/src/generate/t_csharp_generator.cc
index 2d72598..f215e30 100644
--- a/compiler/cpp/src/generate/t_csharp_generator.cc
+++ b/compiler/cpp/src/generate/t_csharp_generator.cc
@@ -573,6 +573,8 @@
         print_const_value(out, "this." + prop_name(*m_iter), t, (*m_iter)->get_value(), true, true);
       } else {
         print_const_value(out, "this._" + (*m_iter)->get_name(), t, (*m_iter)->get_value(), true, true);
+        // Optionals with defaults are marked set
+        indent(out) << "this.__isset." << (*m_iter)->get_name() << " = true;" << endl;
       }
     }
   }