added grpc extension library for C#
diff --git a/build.json b/build.json
index 1cd8775..e7ab14b 100644
--- a/build.json
+++ b/build.json
@@ -441,6 +441,18 @@
         "grpc",
         "gpr"
       ]
+    },
+    {
+      "name": "grpc_csharp_ext",
+      "build": "all",
+      "language": "c",
+      "deps": [
+        "gpr",
+        "grpc"
+      ],
+      "src": [
+        "src/csharp/ext/grpc_csharp_ext.c"
+      ]
     }
   ],
   "targets": [