THRIFT-467. csharp: Fix compilation on Debian Sid

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@769114 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/csharp/src/Collections/THashSet.cs b/lib/csharp/src/Collections/THashSet.cs
index cd4e19b..a995769 100644
--- a/lib/csharp/src/Collections/THashSet.cs
+++ b/lib/csharp/src/Collections/THashSet.cs
@@ -37,7 +37,7 @@
 
 		public bool IsReadOnly
 		{
-			get { return set.IsReadOnly; }
+			get { return false; }
 		}
 
 		public void Add(T item)