MaskedTextBox: fixed FormatException with numeric masks

ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/MaskedTextBox/Implementation/MaskedTextBox.cs

@@ -337,14 +337,19 @@ namespace Microsoft.Windows.Controls
             };
         }
 
+        private bool _convertExceptionOccurred = false;
+
         private object ConvertTextToValue(string text)
         {
+
             object convertedValue = null;
 
             Type dataType = ValueType;
 
             string valueToConvert = MaskProvider.ToString().Trim();
 
+            try
+            {
                 if (valueToConvert.GetType() == dataType || dataType.IsInstanceOfType(valueToConvert))
                 {
                     convertedValue = valueToConvert;
@@ -364,6 +369,13 @@ namespace Microsoft.Windows.Controls
                 {
                     convertedValue = Convert.ChangeType(valueToConvert, dataType);
                 }
+            }
+            catch
+            {
+                //if an excpetion occurs revert back to original value
+                _convertExceptionOccurred = true;
+                return Value;
+            }
 
             return convertedValue;
         }
@@ -373,6 +385,12 @@ namespace Microsoft.Windows.Controls
             if (value == null)
                 value = string.Empty;
 
+            if (_convertExceptionOccurred)
+            {
+                value = Value;
+                _convertExceptionOccurred = false;
+            }
+
             //I have only seen this occur while in Blend, but we need it here so the Blend designer doesn't crash.
             if (MaskProvider == null)
                 return value.ToString();