diff --git a/configure.ac b/configure.ac
index f88bf49..040ae06 100644
--- a/configure.ac
+++ b/configure.ac
@@ -41,9 +41,11 @@
 
 AX_THRIFT_LIB(csharp, [C#], yes)
 if test "$with_csharp" = "yes";  then
-  PKG_CHECK_MODULES(MONO, mono >= 1.2.6, have_mono=yes, have_mono=no)
+  PKG_CHECK_MODULES(MONO, mono >= 2.0.0, net_3_5=yes, net_3_5=no)
+  PKG_CHECK_MODULES(MONO, mono >= 1.2.4, have_mono=yes, have_mono=no)
 fi
 AM_CONDITIONAL(WITH_MONO, [test "$have_mono" = "yes"])
+AM_CONDITIONAL(NET_2_0, [test "$net_3_5" = "no"])
 
 AX_THRIFT_LIB(java, [Java], yes)
 if test "$with_java" = "yes";  then