Custom Markup Extensions & Type Converters

There are times when we need to extend the xaml to support some special features for our application. A very simple example is like We have a textbox which takes firstname and lastname and displays it in the format 'LastName, FirstName', We can obviously format the string, but just to demonstrate what Markup extensions can do:-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Markup;

namespace WpfApplication3
{
    public class MyMarkupExtension:MarkupExtension
    {
        [ConstructorArgument("FirstName")]
        public string FirstName { get; set; }

        [ConstructorArgument("LastName")]
        public string LastName { get; set; }


        public MyMarkupExtension()
        {

        }

        public MyMarkupExtension(string FirstName, string LastName)
        {
            this.FirstName = FirstName;
            this.LastName = LastName;
        }



        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            return LastName + ", " + FirstName;
        }
    }
}

and in the xaml we write:-

<TextBlock Text="{local:MyMarkup FirstName=Shivakant, LastName=Upadhyay}"/>   

The result is visible even in the design time environment:-

Now for type converters, suppose you want to bind a special type to your program, for which there is no built in sonverter. For example you have a type called StringList which can hold string in comma seperated format and convert it back and forth to Enumerable. You want to supply this as ItemsSource of a list. For this you need the following TypeConverter

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace WpfApplication3

{

    public class MyTypeconverter:TypeConverter

    {

        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)

        {

            return sourceType == typeof(string);

        }

        public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)

        {

            return destinationType == typeof(string);

        }

        public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)

        {

            return value==null?string.Empty:string.Join(",", string.Join(",", ((StringList)value)));

        }

        public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)

        {

            return new StringList((string)value);

        }

    }

}

Also you will need to decorate your type with TypeConverter attribute:-

using System;

using System.Collections;

using System.Collections.Generic;

using System.ComponentModel;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace WpfApplication3

{

    [TypeConverter(typeof(MyTypeconverter))]

    public class StringList : IEnumerable<string>

    {

        private readonly IEnumerable<string> _enumerable;

        private readonly string _original;

        public StringList(string value)

        {

            _original = value;

            if (!string.IsNullOrEmpty(value))

            {

                _enumerable = value.Split(",;".ToCharArray()).

                  Where(i => !string.IsNullOrWhiteSpace(i)).Select(i => i.Trim());

            }

        }

        protected StringList(IEnumerable<string> value)

        {

            _enumerable = value;

            _original = string.Join(", ", value);

        }

        public StringList() { }

        public IEnumerator<string> GetEnumerator()

        {

            return _enumerable.GetEnumerator();

        }

        IEnumerator IEnumerable.GetEnumerator()

        {

            return _enumerable.GetEnumerator();

        }

        public override string ToString()

        {

            return _original;

        }

    }

}

That's all, now you can use your type in xaml.