catch SigarException only

bindings/dotnet/examples/Df.cs

@@ -21,7 +21,7 @@ public class Df {
                 avail = usage.Avail;
                 total = usage.Total;
                 pct = (ulong)(usage.UsePercent * 100);
-            } catch (ApplicationException) {
+            } catch (SigarException) {
                 used = avail = total = pct = 0;
                 continue;
             }

bindings/dotnet/examples/Ifconfig.cs

@@ -74,7 +74,7 @@ public class Ifconfig {
                     "  " +
                     "TX bytes:" + txBytes + 
                     " (" + Sigar.FormatSize(txBytes) + ")");
-        } catch { }
+        } catch (SigarException) { }
 
         println("");
     }