Change theme exception

[HUGO VLADIMIR FELIX ANAYA]

I try to use dark and light themes in Xamarin Forms, but when I change theme the UI don't change, searching and reading documentation here I found a partial solution, when try to change from dark to light or light to dark the application crash.

I share my error.

SfScheduleMapping.HeaderStyle_PropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e)System.

NullReferenceException: Object reference not set to an instance of an object.

Stack traces

• Syncfusion.SfSchedule.XForms.Droid
  SfScheduleMapping.HeaderStyle_PropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e)
• Xamarin.Forms
  BindableObject.OnPropertyChanged (System.String propertyName) D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:229
• Xamarin.Forms
  Element.OnPropertyChanged (System.String propertyName) D:\a\1\s\Xamarin.Forms.Core\Element.cs:353
• Xamarin.Forms
  BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindableObject+BindablePropertyContext context, System.Object value, System.Boolean currentlyApplying, Xamarin.Forms.Internals.SetValueFlags attributes, System.Boolean silent) D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:461
• Xamarin.Forms
  BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, Xamarin.Forms.Internals.SetValueFlags attributes, Xamarin.Forms.BindableObject+SetValuePrivateFlags privateAttributes) D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:397
• Xamarin.Forms
  BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, Xamarin.Forms.Internals.SetValueFlags attributes) D:\a\1\s\Xamarin.Forms.Core\BindableObject.cs:343
• Xamarin.Forms
  Element.OnResourceChanged (Xamarin.Forms.BindableProperty property, System.Object value) D:\a\1\s\Xamarin.Forms.Core\Element.cs:590
• Xamarin.Forms
  Element.OnResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:446
• Xamarin.Forms
  Element.OnParentResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:398
• Xamarin.Forms
  Element.OnParentResourcesChanged (System.Object sender, Xamarin.Forms.Internals.ResourcesChangedEventArgs e) D:\a\1\s\Xamarin.Forms.Core\Element.cs:393
• Xamarin.Forms
  Element.OnResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:424
• Xamarin.Forms
  Element.OnParentResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:398
• Xamarin.Forms
  VisualElement.OnParentResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\VisualElement.cs:899
• Xamarin.Forms
  Element.OnParentResourcesChanged (System.Object sender, Xamarin.Forms.Internals.ResourcesChangedEventArgs e) D:\a\1\s\Xamarin.Forms.Core\Element.cs:393
• Xamarin.Forms
  Element.OnResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:424
• Xamarin.Forms
  Element.OnParentResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:398
• Xamarin.Forms
  VisualElement.OnParentResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\VisualElement.cs:899
• Xamarin.Forms
  Element.OnParentResourcesChanged (System.Object sender, Xamarin.Forms.Internals.ResourcesChangedEventArgs e) D:\a\1\s\Xamarin.Forms.Core\Element.cs:393
• Xamarin.Forms
  Element.OnResourcesChanged (System.Collections.Generic.IEnumerable`1[T] values) D:\a\1\s\Xamarin.Forms.Core\Element.cs:424
• Xamarin.Forms
  Element.OnResourcesChanged (System.Object sender, Xamarin.Forms.Internals.ResourcesChangedEventArgs e) D:\a\1\s\Xamarin.Forms.Core\Element.cs:415
• Xamarin.Forms
  ResourceDictionary.OnValuesChanged (System.Collections.Generic.KeyValuePair`2[System.String,System.Object][] values) D:\a\1\s\Xamarin.Forms.Core\ResourceDictionary.cs:327
• Xamarin.Forms
  ResourceDictionary.MergedDictionaries_CollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) D:\a\1\s\Xamarin.Forms.Core\ResourceDictionary.cs:124
• System.Collections.ObjectModel
  ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) /Users/builder/jenkins/workspace/archive-mono/2019-10/android/release/external/corefx/src/System.ObjectModel/src/System/Collections/ObjectModel/ObservableCollection.cs:263
• System.Collections.ObjectModel
  ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedAction action, System.Object item, System.Int32 index) /Users/builder/jenkins/workspace/archive-mono/2019-10/android/release/external/corefx/src/System.ObjectModel/src/System/Collections/ObjectModel/ObservableCollection.cs:338
• System.Collections.ObjectModel
  ObservableCollection`1[T].InsertItem (System.Int32 index, T item) /Users/builder/jenkins/workspace/archive-mono/2019-10/android/release/external/corefx/src/System.ObjectModel/src/System/Collections/ObjectModel/ObservableCollection.cs:196

My code:

 ICollection<ResourceDictionary> mergedDictionaries = this.Resources.MergedDictionaries;
                if (ThemeHelper.CurrentTheme == Enums.Theme.Dark)
                {
                    var darkTheme = mergedDictionaries.OfType<DarkTheme>().FirstOrDefault();
                    if (darkTheme != null) return;

                    var lightTheme = mergedDictionaries.OfType<LightTheme>().FirstOrDefault();
                    if (lightTheme != null)
                    {
                        mergedDictionaries.Remove(lightTheme);
                    }
                    mergedDictionaries.Add(new DarkTheme());
                }
                else if (ThemeHelper.CurrentTheme == Theme.Light)
                {
                    var lightTheme = mergedDictionaries.OfType<LightTheme>().FirstOrDefault();
                    if (lightTheme != null) return;

                    var darkTheme = mergedDictionaries.OfType<DarkTheme>().FirstOrDefault();
                    if (darkTheme != null)
                    {
                        mergedDictionaries.Remove(darkTheme);
                    }
                    mergedDictionaries.Add(new LightTheme());
                }

[SaiGanesh Sakthivel]

Hi Hugo, 

 

Thank you for the contacting Syncfusion support. 

 

We have checked the reported query “Change theme exception” from our end. We would like to inform you that you can achieved your requirement of changing themes at run time by removing the ark themes and add the light themes with of button clicked. Please refer to the following code snippet for your reference. 

 

Code Snippet 

private void Theme_Clicked(object sender, EventArgs e) 

{ 

    ICollection<ResourceDictionary> mergedDictionaries = Application.Current.Resources.MergedDictionaries; 

    var darkTheme = mergedDictionaries.OfType<DarkTheme>().FirstOrDefault(); 

    var lightTheme = mergedDictionaries.OfType<LightTheme>().FirstOrDefault(); 

    if(darkTheme != null) 

    { 

        mergedDictionaries.Remove(darkTheme); 

        mergedDictionaries.Add(new LightTheme()); 

    } 

    else 

    { 

        mergedDictionaries.Remove(lightTheme); 

        mergedDictionaries.Add(new DarkTheme()); 

    } 

} 

 

 

We have prepared the sample as per your requirement and attached in the following link for your reference. 

Link: https://www.syncfusion.com/downloads/support/directtrac/general/ze/SCHEDU~1968224958 

 

We hope this helps. Please let us know if you need any further assistance. 

 

Regards, 

SaiGanesh Sakthvel 

[HUGO VLADIMIR FELIX ANAYA]

Hi SaiGanesh Sakthivel, thanks for reply me, I tested your sample and work fine! but when I use navigation with Shell control in tabs and moved into tabs and back to page when is the Schedule control and try to change the theme with your code (or mine, I tried some things without a great result) the application crash, the exception is a NullReferenceException and I tried a lot of things, I don't know how to solve this issue, in this moment I don't know if I should change the control with some another.

I hope that you could help me.

Thanks, regards.

[SaiGanesh Sakthivel]

Hi Hugo, 

 

Thank you for the patience. 

   

We have checked, and we can replicate the reported issue “Change theme exception”. We have logged issue report for the same, we will fix this issue and include the issue fix our upcoming 2020 Volume 2 SP release which is planned to roll out in the month of July, 2020. We appreciate your patience until then. 

 

You can track the status of this report through the following feedback link, 

Link: https://www.syncfusion.com/feedback/15115/change-in-theme-of-sfschedule-control-inside-tab-view-cause-null-reference 

  

Note: The provided feedback link is private, you need to login to view this feedback.   

  

Regards,  

SaiGanesh Sakthivel

[SaiGanesh Sakthivel]

Hi Hugo, 

  

Thank you for the patience, 

   

We have fixed the reported issue “Change theme exception” and included the issue fix in our latest Weekly NuGet release update version 18.2.0.54 which is available for download (nuget.org). 

 

We thank you for your support and appreciate your patience in waiting for this update. Please get in touch with us if you would require any further assistance. 

 

Regards, 

SaiGanesh Sakthivel