Why does setting a style for StackLayout result in a NullReferenceException? - xamarin

Question

I want each Entry type in my in my StackLayout to have a margin. I'm trying to specify a style as follows:
<StackLayout IsVisible="{Binding IsLoggingIn, Converter={StaticResource BooleanConverter}}">
<StackLayout.Resources>
<Style TargetType="{x:Type Entry}">
<Setter Property="Margin" Value="0,0,0,20"/>
</Style>
</StackLayout.Resources>
<Entry ... />
<Entry ... />
</StackLayout>
Unfortunately, it results in a NullReferenceException:
04-13 16:26:15.406 E/mono (17167): Unhandled Exception:
04-13 16:26:15.406 E/mono (17167): System.NullReferenceException: Object reference not set to an instance of an object.
04-13 16:26:15.406 E/mono-rt (17167): [ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object.
With a bit more stack trace for reference:
Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object.
at MyApp.Views.Login.InitializeComponent () [0x00012] in C:\dev\PathTo\MyApp\obj\Debug\MyApp.Views.Login.xaml.g.cs:22
at MyApp.Views.Login..ctor () [0x00008] in C:\dev\PathTo\MyApp\Views\Login.xaml.cs:9
at MyApp.App.SetupLoginPage () [0x00001] in C:\dev\PathTo\MyApp\App.xaml.cs:94
at MyApp.App+<OnStart>d__4.MoveNext () [0x001ad] in C:\dev\PathTo\MyApp\App.xaml.cs:60
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/data/lanes/4009/3a62f1ea/source/mono/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143
at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>m__0 (System.Object state) [0x00000] in /Users/builder/data/lanes/4009/3a62f1ea/source/mono/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1018
at Android.App.SyncContext+<Post>c__AnonStorey0.<>m__0 () [0x00000] in /Users/builder/data/lanes/4009/3a62f1ea/source/xamarin-android/src/Mono.Android/Android.App/SyncContext.cs:35
at Java.Lang.Thread+RunnableImplementor.Run () [0x0000b] in /Users/builder/data/lanes/4009/3a62f1ea/source/xamarin-android/src/Mono.Android/Java.Lang/Thread.cs:36
at Java.Lang.IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this) [0x00009] in /Users/builder/data/lanes/4009/3a62f1ea/source/monodroid/src/Mono.Android/platforms/android-23/src/generated/Java.Lang.IRunnable.cs:81
at (wrapper dynamic-method) System.Object:d83972de-0492-4d07-a14d-24725ac1bfd1 (intptr,intptr)
What could be causing this, and how can I fix it? For the record, if I specify the margin inline e.g. <Entry margin="0,0,0,20"/>, it works fine.

Answer 1

I think your syntax is wrong. Shouldn't it be this;
<StackLayout.Resources>
<ResourceDictionary>
<Style TargetType="Entry">
<Setter Property="Margin" Value="0,0,0,20"/>
</Style>
</ResourceDictionary>
</StackLayout.Resources>
Note how I changed the TargetType and added the ResourceDictionary tag around it.