如何调试 Xamarin.Forms XAML

Question

I'm finding Xamarin.Forms XAML very frustrating.

If I use this syntax...

<ContentView.Resources>
    <local:MyConverter1 x:Key="MyConverter1"/>
</ContentView.Resources>

I will get a System.NullReferenceException from InitializeComponent(). Nothing in the stack trace or output window or anywhere else tells me what is wrong.
Note: this syntax works fine in WPF.

After a lot of struggle I discovered I need this syntax...

<ContentView.Resources>
    <ResourceDictionary>
        <local:MyConverter1 x:Key="MyConverter1"/>
    </ResourceDictionary>
</ContentView.Resources>

Likewise for ListView DataTemplate. This throws null reference exception...

            <ListView.ItemTemplate>
                <DataTemplate>
                    <Label Text="{Binding Converter={StaticResource MyConverter1}}"/>
                </DataTemplate>
            </ListView.ItemTemplate>

Because the proper syntax is this...

            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <Label Text="{Binding Converter={StaticResource MyConverter1}}"/>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>

Again this syntax works fine in WPF. I fully realize that Xamarin.Forms is not WPF but I getting weary of being sucker punched by null reference exceptions whenever I use a XAML construct that is valid in WPF.

What is the best way to debug Xamarin.Forms XAML issues?

Right now I am simply commenting stuff out until it starts working. This is akin to putting print statements in imperative code. Declarative code is supposed to be superior to imperative code.

What am I doing wrong?

Answer 1

I'm finding Xamarin Forms XAML very frustrating.

If I use this syntax...

<ContentView.Resources>
    <local:MyConverter1 x:Key="MyConverter1"/>
</ContentView.Resources>

I will get a System.NullReferenceException from InitializeComponent(). Nothing in the stack trace or output window or anywhere else tells me what is wrong. Note: this syntax works fine in WPF.

After a lot of struggle I discovered I need this syntax...

<ContentView.Resources>
    <ResourceDictionary>
        <local:MyConverter1 x:Key="MyConverter1"/>
    </ResourceDictionary>
</ContentView.Resources>

Likewise for ListView DataTemplate. This throws null reference exception...

            <ListView.ItemTemplate>
                <DataTemplate>
                    <Label Text="{Binding Converter={StaticResource MyConverter1}}"/>
                </DataTemplate>
            </ListView.ItemTemplate>

Because the proper syntax is this...

            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <Label Text="{Binding Converter={StaticResource MyConverter1}}"/>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>

Again this syntax works fine in WPF. I fully realize that Xamarin Forms is not WPF but I getting weary of being sucker punched by null reference exceptions whenever I use a XAML construct that is valid in WPF.

What is the best way to debug Xamarin Forms XAML issues? Right now I am simply commenting stuff out until it starts working. This is akin to putting print statements in imperative code. Declarative code is supposed to be superior to imperative code. What am I doing wrong?

Answer 2

I'm finding Xamarin Forms XAML very frustrating.

If I use this syntax...

<ContentView.Resources>
    <local:MyConverter1 x:Key="MyConverter1"/>
</ContentView.Resources>

I will get a System.NullReferenceException from InitializeComponent(). Nothing in the stack trace or output window or anywhere else tells me what is wrong. Note: this syntax works fine in WPF.

After a lot of struggle I discovered I need this syntax...

<ContentView.Resources>
    <ResourceDictionary>
        <local:MyConverter1 x:Key="MyConverter1"/>
    </ResourceDictionary>
</ContentView.Resources>

Likewise for ListView DataTemplate. This throws null reference exception...

            <ListView.ItemTemplate>
                <DataTemplate>
                    <Label Text="{Binding Converter={StaticResource MyConverter1}}"/>
                </DataTemplate>
            </ListView.ItemTemplate>

Because the proper syntax is this...

            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <Label Text="{Binding Converter={StaticResource MyConverter1}}"/>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>

Again this syntax works fine in WPF. I fully realize that Xamarin Forms is not WPF but I getting weary of being sucker punched by null reference exceptions whenever I use a XAML construct that is valid in WPF.

What is the best way to debug Xamarin Forms XAML issues? Right now I am simply commenting stuff out until it starts working. This is akin to putting print statements in imperative code. Declarative code is supposed to be superior to imperative code. What am I doing wrong?

Answer 3

I normally have 2 steps

1. add a try catch in code behind

   using System; using System.Diagnostics; using Xamarin.Forms;

   namespace TestApp.Views { public partial class AssetsPage : ContentPage { public WelcomePage() { try { InitializeComponent(); } catch (Exception ex) { Debug.WriteLine(ex); } } } }

2. Remove code and keep on checking if the problem disappears

not really a magic solution but it helps the most especially when you have large groups of complicated code