Winforms data binding: Can a TypeConverter be used instead of the Format/Parse events?

Question

In a Winforms form, I want to provide visual cues to the user when an input field contains an invalid value. To that end, I want to bind the ForeColor property

Answer 1

My previous answer (now deleted) was incorrect: This can be done, using a custom TypeConverter.

First, one needs a suitable converter. (Unlike with XAML, one does not implement a IValueConverter, but derive from TypeConverter.) For example:

// using System;
// using System.ComponentModel;
// using System.Drawing;
// using System.Globalization;

sealed class BooleanToColorConverter : TypeConverter
{
    public override bool CanConvertTo(ITypeDescriptorContext context,
                                      Type destinationType)
    {
        return destinationType == typeof(Color);
    }

    public override object ConvertTo(ITypeDescriptorContext context,
                                     CultureInfo culture,
                                     object value, 
                                     Type destinationType)
    {
        return (bool)value ? Color.Green : Color.Red;
    }
}

Next, (and also unlike with XAML data binding,) this converter is not applied to the binding itself; it must be attached to the data source's property using the [TypeConverter] attribute:

// using System.ComponentModel;

partial class DataSource : INotifyPropertyChanged
{
    [TypeConverter(typeof(BooleanToColorConverter))] // <-- add this! 
    public bool IsValid { get { … } set { … } }
}

Finally, formatting must be enabled on the data binding:

// Control control = …;
// DataSource dataSource = …;

control.DataBindings.Add("ForeColor", dataSource, "IsValid", formattingEnabled: true);
//                                                           ^^^^^^^^^^^^^^^^^^^^^^^

Note that this example only deals with one-way (data source to control) data binding. For two-way data binding, you would additionally have to override the TypeConverter.ConvertFrom and TypeConverter.CanConvertFrom methods.