Saturday, December 4, 2010

WPF Sample Series – Using WPF Binding StringFormat Property with Nullable Types

One of the great features introduced in .NET 3.5 Framework Service Pack 1 is the BindingBase StringFormat property.  When used in a data binding expression the StringFormat property replaces most IValuteConverters that were used for formatting of values in  WPF 3.0 & WPF 3.5 applications.
Here are two examples of the StringFormat property in action formatting a currency field:
<TextBox Text="{Binding Path=Salary, StringFormat=c}" />

<TextBox Text="{Binding Path=Salary, StringFormat=\{0:c\}}" />

Nullable Type in the Mix

If you are binding to a non-Nullable data type the above will work just fine.
However, if you are binding to a Nullable data type the above will not work.
This breaks down when a UI Control like a TextBox as a value and the user clears the TextBox and press TAB.  The default implementation of StringFormat will attempt to set the Target Property to an empty string which in the the case of a Nullable Decimal property will cause the following or similar data binding exception.  The below data binding exception can be viewed in the Visual Studio Output window.
System.Windows.Data Error: 7 : ConvertBack cannot convert value ” (type ‘String’). BindingExpression:Path=Age; DataItem=’Customer’ (HashCode=31884011); target element is ‘TextBox’ (Name=”); target property is ‘Text’ (type ‘String’) FormatException:’System.FormatException: Input string was not in a correct format.
at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
at System.String.System.IConvertible.ToInt32(IFormatProvider provider)
at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
at MS.Internal.Data.SystemConvertConverter.ConvertBack(Object o, Type type, Object parameter, CultureInfo culture)
at System.Windows.Data.BindingExpression.ConvertBackHelper(IValueConverter converter, Object value, Type sourceType, Object parameter, CultureInfo culture)’

TargetNullVaue to the Rescue

In the above example, the exception is thrown because of a type mismatch.  In WPF 3.0 and 3.5 I had a set of Nullable ValueConverters for my applications to handle this problem.
Another great feature introduced in .NET 3.5 Framework Service Pack 1 is the BindingBase TargetNullValue property.  This property can be used to handled the type mismatch problem when converting an empty string from a TextBox to Nothing (null) when the binding pipeline sets the target Nullable property.
Let’s have a look at the two below TextBoxes.
    Text="{Binding Path=NumberOfComputers,
            TargetNullValue={x:Static sys:String.Empty},
            StringFormat=\{0:D\}}" />

    Text="{Binding Path=Age, StringFormat=\{0:D\}}" />
These TextBoxes are both bound to Nullable properties.  The first TextBox takes advantage of the TargetNullValue property and works as expected.  The second does not.   The data binding pipeline will throw an exception when the second TextBox is changed to an empty string and it attempts to assign the value of String.Empty to the Target property.
Here is a potential problem.  If the exception gets thrown and swallowed because the code didn’t check for this, the user thinks they cleared a value but the property the TextBox is bound to never gets changed due to the exception being thrown.