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OnPlatform(
	(Device.iOS, () => CustomiOSAction()), 
	(Device.Android, () => CustomAndroidAction())
);

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static void OnPlatform(params (string, Action)[] platformActions) =>  
    platformActions.ToList().ForEach(pa => {  
        if (pa.Item1 == Device.RuntimePlatform)  
            pa.Item2();  
    });

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double width2 = OnPlatform(200.0, (Device.iOS, 200), (Device.Android, 150), (Device.UWP, 120));

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static T OnPlatform<T>(T defValue, params (string, T)[] platformValues) {  
    foreach (var pv in platformValues) {  
        if (pv.Item1 == Device.RuntimePlatform)  
            return pv.Item2;  
    }
    return defValue;  
}

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double width;  
switch (Device.RuntimePlatform) {  
    case Device.iOS:  
    case Device.Android:  
        width = 200;  
        break;  
    case Device.UWP:  
        width = 120;  
        break;  
}

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<Button Text="Ok">  
    <Button.WidthRequest>  
        <OnPlatform x:TypeArguments="x:Double">  
            <On Platform="iOS,Android">200</On>  
            <On Platform="UWP">120</On>  
        </OnPlatform>  
    </Button.WidthRequest>  
</Button>

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<Button Text="OK">  
    <Button.WidthRequest>  
        <OnPlatform x:TypeArguments="x:Double">  
            <On Platform="iOS">200</On>  
            <On Platform="Android">150</On>  
        </OnPlatform>  
    </Button.WidthRequest>  
</Button>

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<ListView.HeaderTemplate>
    <DataTemplate>
        <ContentView>
            <Label
                Margin="16"
                HorizontalOptions="CenterAndExpand"
                Text="{Binding PhoneNumber, StringFormat='Meu número: {0}'}" />
        </ContentView>
    </DataTemplate>
</ListView.HeaderTemplate>

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<ListView
    ItemsSource="{Binding Items}"
    Header="{Binding}"
    Footer="{Binding}">
    ...

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/// <summary>
/// Obtém os itens da lista.
/// </summary>
public IList<Contact> Items { get; private set; }

/// <summary>
/// Obtém o número de telefone atual.
/// </summary>
public string PhoneNumber { get; private set; } = "+55 (11) 1111-1111";