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Data binding and formatting multiple values in Silverlight

by ondrejsv 16. September 2009 09:15

This article introduces a way to produce formatted strings with standard .NET formatting patterns (as used by the String.Format method) in data-binding on the Silverlight platform.

Oftentimes we require some kind of formatting when binding a value from a data source on the form. In ASP.NET many of the data-bound controls have the DataFormatString property in addition to the DataField property. But if you want to bind against more than one data field and concatenate their values into one formatted string, you usually need to create a template field. An example could be a Contact object from the AdventureWorks model with its FirstName and LastName properties which we want to concatenate and show in a single GridView column.

Windows Presentation Foundation features a more advanced framework for data-binding. However, there is no built-in support for formatting data-bound values. Fortunately, with extensibility in mind, WPF supports so-called converters which help to convert raw values coming from data sources into the final object consumable by the target property. They realize the IValueConverter interface:

public interface IValueConverter { object Convert(object value, Type targetType, object parameter, CultureInfo culture); object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture); }

or IMultiValueConverter interface for converters taking more than one input:

public interface IMultiValueConverter { object Convert(object[] values, Type targetType, object parameter, CultureInfo culture); object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture); }

To achieve our goal we can use one of the already built multi-value converters from the WPF Convertes project, FormatConverter:

<my:DataGridTextColumn Header="Name">
<MultiBinding Converter="{cx:FormatConverter {}{0} {1}}">
<Binding Path="Individual.Contact.FirstName" />
<Binding Path="Individual.Contact.LastName" />
</MultiBinding >

However, multi-value converters are not supported on the Silverlight platform, so it is not the WPF Convertes package.

To perform a similar string formatting we must write our own value converter. We make use of the parameter parameter of the IValueConverter.Convert method which is filled with any string supplied by the user in markup (through the ConverterParameter attribute of the Binding markup extension). We will support any number of input data fields and also property paths (to enable binding against object graphs). Our converter will be able to process format specified as in this example:

<my:DataGridTextColumn Header="Number" Binding="{Binding Converter={StaticResource MultiStringConverter}, ConverterParameter='[Individual.Contact.FirstName] [Individual.Contact.LastName]'}" />

Notice that data fields values of which we want to substitute, are enclosed in brackets (braces would reflect more nicely the syntax used by the String.Format method but we would need to escape them with a backslash – not so practical).

The code works by picking up all the property paths in the format string and assigning them unique index values. These indices are inserted to the rest of the format string. Then property paths are evaluated against the binding target (we use the code similar to the one in my article Binding object graphs to the ASP.NET GridView control) and passed to the String.Format method together with new format string. More detailed code description follows.

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { Dictionary<string , paramvalueentity> paramValue = new Dictionary<string, ParamValueEntity>(); string formatString = (string)parameter; StringBuilder realFormatString = new StringBuilder(); int position = 0; string currentParam = ""; int lastIndex = 0;

We will store all found property paths in the format string together with their assigned index in a ParamValueEntity object (justs a wrapper for this pair). realFormatString variable stores the new format string.

We’ll proceed in a while loop by looking at the characters of the original format string (TagStart constant refers to '['):

while (position < formatString.Length) { if (formatString[position] == TagStart) { // We've just entered a parameter

… escaping is handled by repeating the left bracket…:

// Handle escaping (double TagStart) if (position+1<formatString.Length && formatString[position+1] == TagStart) { realFormatString.Append(TagStart); position += 2; continue; }

… otherwise we found beginning of a property path and we pick it up…:

position++; currentParam = ""; int startPosition = position; while ((char.IsLetterOrDigit(formatString[position]) || formatString[position] == '.') && position < formatString.Length) { position++; } currentParam = formatString.Substring(startPosition, position - startPosition);

… we check if such property path has already been found. If so, we put its index in the new format string (we evaluate a single property path only once – both for performance reasons but more importantly the property getter could have side effects and its repeated evaluations could return different values). If this is the first occurrence of the property path, we assign a new index value… :

// Have we met it already? ParamValueEntity pve; if (!paramValue.TryGetValue(currentParam, out pve)) { pve = new ParamValueEntity(); pve.Index = lastIndex++; paramValue[currentParam] = pve; }

… the we pick anything up to the finalizing right bracket (for example any formatting patterns – think of [Person.BirthDate:D])… :

// Parse up to the finalizing ']' startPosition = position; while (formatString[position] != TagEnd && position < formatString.Length) position++; string rest = ""; if (startPosition < position) rest = formatString.Substring(startPosition, position - startPosition);
... and put the index value to the new format string...:
// Add to the string builder realFormatString.Append("{" + pve.Index.ToString() + rest + "}"); // Pass the } if (position < formatString.Length) position++;

… all other characters are copied verbatim…:

} else { realFormatString.Append(formatString[position++]); } }

… then we evaluate all property paths…:

Type type = value.GetType(); // Let's find values foreach (var pvde in paramValue) { pvde.Value.Value = GetValueFromPropertyPath(value, pvde.Key); }

… and call the String.Format method on the new format string and just evaluated values…:

var q = from pve in paramValue.Values orderby pve.Index select pve.Value; return string.Format(realFormatString.ToString(), q.ToArray());

The GetValueFromPropertyPath method evaluates a property path against an object graph. It’s code is almost identical to the one described in my previous article (Binding object graphs to the ASP.NET GridView control)) but it uses pure Reflection instead of the TypeDescriptor and PropertyDescriptor wrappers as these are unavailable in Silverlight.

You can download the complete code together with a sample featuring a DataGrid showing some data from the AdventureWorks. Happy coding!

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