add const modifier
diff --git a/src/csharp/ext/grpc_csharp_ext.c b/src/csharp/ext/grpc_csharp_ext.c
index 0d2a74a..0eb2365 100644
--- a/src/csharp/ext/grpc_csharp_ext.c
+++ b/src/csharp/ext/grpc_csharp_ext.c
@@ -850,7 +850,7 @@
 typedef void(GPR_CALLTYPE *test_callback_funcptr)(gpr_int32 success);
 
 /* Version info */
-GPR_EXPORT char *GPR_CALLTYPE grpcsharp_version_string() {
+GPR_EXPORT const char *GPR_CALLTYPE grpcsharp_version_string() {
   return grpc_version_string();
 }