Xamarin.Forms で IValueConverter を使う ref: http://qiita.com/ytabuchi/items/623eca74c4377d8afba1

file0.xml

<ContentPage.Resources>
  <ResourceDictionary>
    <cv:StringCaseConverter x:Key="scConverter" />
    <cv:StringToLengthConverter x:Key="s2lConverter" />
  </ResourceDictionary>
</ContentPage.Resources>

<StackLayout>
  <Editor x:Name="editor" Text="test" />
  <!-- ConverterParameter は "True" で UpperCase、"False" で LowerCase になります。
       今は固定してしまっていますが、Switch とかから IsToggled を参照したいですね。 -->
  <Label BindingContext="{x:Reference Name=editor}"
         Text="{Binding Text, 
                Converter={StaticResource scConverter},
                ConverterParameter=True}" />
  <Label BindingContext="{x:Reference Name=editor}"
         Text="{Binding Text, 
                Converter={StaticResource s2lConverter},
                StringFormat='{0} letters'}" />
</StackLayout>

file1.cs

var editor = new Editor { Text = "test" };

var sclabel = new Label { Text = "" };
sclabel.BindingContext = editor;
sclabel.SetBinding(Label.TextProperty, 
    new Binding("Text", 
        converter: new Converters.StringCaseConverter(),
        converterParameter: "True"));

var sllabel = new Label { Text = "" };
sllabel.BindingContext = editor;
sllabel.SetBinding(Label.TextProperty,
    new Binding("Text",
        converter: new Converters.StringToLengthConverter(),
        stringFormat: "{0} letters"));

Content = new StackLayout
{
    Children = {
        editor,
        sclabel,
        sllabel
    }
};

file1.xml

<ContentPage.Resources>
  <ResourceDictionary>
    <local:StringToUpperConverter x:Key="s2uConverter" />
    <local:StringToIntConverter x:Key="s2iConverter" />
  </ResourceDictionary>
</ContentPage.Resources>

file2.xml

xmlns:cv="clr-namespace:XF_IValueConverterSample.Converters;assembly=XF_IValueConverterSample"
xmlns:vm="clr-namespace:XF_IValueConverterSample.ViewModel;assembly=XF_IValueConverterSample"

<ContentPage.BindingContext>
  <vm:CommonViewModel />
</ContentPage.BindingContext>

<ContentPage.Resources>
  <ResourceDictionary>
    <cv:StringCaseConverter x:Key="scConverter" />
    <cv:StringToLengthConverter x:Key="s2lConverter" />
  </ResourceDictionary>
</ContentPage.Resources>

file3.txt

var listdata = new List<Words> {
    new Words { word = "the" },
    new Words { word = "quick" },
    new Words { word = "brown" },
    new Words { word = "fox" },
    new Words { word = "jumps" },
    new Words { word = "over" },
    new Words { word = "the" },
    new Words { word = "lazy" },
    new Words { word = "dog" }
};

this.BindingContext = listdata;

file3.xml

<StackLayout>
  <Editor Text="{Binding Message}" />
  <Label Text="{Binding Message}" />
  <Label Text="{Binding Message, 
                Converter={StaticResource scConverter},
                ConverterParameter=True}" />
  <Label Text="{Binding Message, 
                Converter={StaticResource s2lConverter},
                StringFormat='{0} letters'}" />
</StackLayout>

file4.txt

string _message;

public string Message
{
    get { return _message; }
    set
    {
        if (_message != value)
        {
            _message = value;
            OnPropertyChanged("Message");
        }
    }
}

file5.cs

BindingContext = new ViewModel.CommonViewModel();

var editor = new Editor { Text = "" };
editor.SetBinding(Editor.TextProperty, "Message");

var sclabel = new Label { Text = "" };
sclabel.SetBinding(Label.TextProperty,
    new Binding("Message",
        converter: new Converters.StringCaseConverter(),
        converterParameter: "True"));

var sllabel = new Label { Text = "" };
sllabel.SetBinding(Label.TextProperty,
    new Binding("Message",
        converter: new Converters.StringToLengthConverter(),
        stringFormat: "{0} letters"));

Content = new StackLayout
{
    Children = {
        editor,
        sclabel,
        sllabel
    }
};

file6.cs

if (value == null)
    return 0;

file7.xml

<ListView ItemsSource="{Binding}">
  <ListView.ItemTemplate>
    <DataTemplate>
      <TextCell Text="{Binding word, 
                       Converter={StaticResource scConverter},
                       ConverterParameter=True}"
                Detail="{Binding word, 
                         Converter={StaticResource s2lConverter},
                         StringFormat='{0} letters'}" />
    </DataTemplate>
  </ListView.ItemTemplate>
</ListView>

file8.cs

var listdata = new List<Words> {
    new Words { word = "the" },
    new Words { word = "quick" },
    new Words { word = "brown" },
    new Words { word = "fox" },
    new Words { word = "jumps" },
    new Words { word = "over" },
    new Words { word = "the" },
    new Words { word = "lazy" },
    new Words { word = "dog" }
};

this.BindingContext = listdata;