diff --git a/library/thread.m b/library/thread.m
index 7a329fc6a..77505b5a5 100644
--- a/library/thread.m
+++ b/library/thread.m
@@ -3,7 +3,7 @@
 %---------------------------------------------------------------------------%
 % Copyright (C) 2000-2001, 2003-2004, 2006-2008, 2010-2011 The University
 % of Melbourne.
-% Copyright (C) 2014-2017 The Mercury Team.
+% Copyright (C) 2014-2018 The Mercury Team.
 % This file may only be copied under the terms of the GNU Library General
 % Public License - see the file COPYING.LIB in the Mercury distribution.
 %---------------------------------------------------------------------------%
@@ -405,17 +405,15 @@ spawn_native(Goal, Res, !IO) :-
     [promise_pure, will_not_call_mercury, thread_safe, tabled_for_io,
         may_not_duplicate],
 "
-    // Only available in .NET 4.0.
-    // System.Threading.Yield();
+    System.Threading.Thread.Yield();
 ").
 
 :- pragma foreign_proc("Java",
-    yield(IO0::di, IO::uo),
+    yield(_IO0::di, _IO::uo),
     [promise_pure, will_not_call_mercury, thread_safe, tabled_for_io,
         may_not_duplicate],
 "
     java.lang.Thread.yield();
-    IO = IO0;
 ").
 
 yield(!IO).
diff --git a/m4/mercury.m4 b/m4/mercury.m4
index 7e6cbc2f1..315f9ae74 100644
--- a/m4/mercury.m4
+++ b/m4/mercury.m4
@@ -1,5 +1,6 @@
 #-----------------------------------------------------------------------------#
 # Copyright (C) 1999,2001-2004, 2006-2012 The University of Melbourne.
+# Copyright (C) 2013-2018 The Mercury team.
 # This file may only be copied under the terms of the GNU General
 # Public Licence - see the file COPYING in the Mercury distribution.
 #-----------------------------------------------------------------------------#
@@ -256,10 +257,10 @@ MS_AL=`basename "$MS_AL"`
 AC_PATH_PROGS([CLI_INTERPRETER], [mono])
 
 # Check for the C# (C sharp) compiler.
-# mcs is the Mono C# compiler targetting all runtimes
-# dmcs is the Mono C# compiler targeting the 4.0 runtime
-# gmcs is the Mono C# compiler targeting the 2.0 runtime (with generics).
-# cscc is the DotGNU C# compiler.
+# csc is the Microsoft C# compiler.
+# mcs is the Mono C# compiler targetting all runtime versions.
+# (dmcs and gmcs are older aliases for the Mono C# compiler which we do
+#  not use.)
 
 AC_CACHE_SAVE
 case "$mercury_cv_with_csharp_compiler" in
@@ -272,7 +273,7 @@ case "$mercury_cv_with_csharp_compiler" in
         exit 1
         ;;
     "")
-        CSC_COMPILERS="csc mcs dmcs gmcs cscc"
+        CSC_COMPILERS="csc mcs"
         ;;
     *)
         CSC_COMPILERS="$mercury_cv_with_csharp_compiler"
@@ -385,14 +386,6 @@ case "$CSC" in
         CSHARP_COMPILER_TYPE=mono
     ;;
 
-    dmcs*)
-        CSHARP_COMPILER_TYPE=mono
-    ;;
-
-    gmcs*)
-        CSHARP_COMPILER_TYPE=mono
-    ;;
-
     *)
         CSHARP_COMPILER_TYPE=unknown
     ;;