NULL error with FindByName

Question

Hi,

I am getting:

Object reference not set to an instance of an object

when trying FindByName this way:

StackLayoutPostDetails.FindByName("LabelPostGender").Text = "OK";

Here is my XAML:

Thanks,
Jassim

Answer

Hello,

Welcome to our Microsoft Q&A platform!

Get your control by FindByName<> in your DataTemplate code. It is not recommand way. You could get the control and set the value by Data Binding.

https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/data-binding/

Because they are in different NameScopes if you use this datatemplate in the listview or collectionview. The Xaml parser creates one NameScope per Xaml root object in your file, and then different NameScopes for DataTemplates. Here is a similar thread:https://stackoverflow.com/a/42854892

Do you want to achieve the function that click the Image, then text of LabelPostGender will be changed?

If so, I achieve simple code with data-binding.

First of all, I create a model called PlatformInfo.cs, it achieve the INotifyPropertyChanged for update at the runtime.

   public class PlatformInfo : INotifyPropertyChanged  
       {  
           private bool _isChecked;  
           private string _platformName;  
     
           public bool IsChecked  
           {  
               get { return _isChecked; }  
               set { _isChecked = value; NotifyPropertyChanged(); }  
           }  
     
           public string PlatformName  
           {  
               get { return _platformName; }  
               set { _platformName = value; NotifyPropertyChanged(); }  
           }  
     
           public event PropertyChangedEventHandler PropertyChanged;  
           public void NotifyPropertyChanged([System.Runtime.CompilerServices.CallerMemberName] string propertyName = "")  
           {  
               if (this.PropertyChanged != null)  
                   this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));  
           }  
       }

Then I achieve a viewModel, that I add binding data and command. ChangeTextCommmand will binding to the Image's TapGestureRecognizer command. If this command is invoked, PlatformName's text will be changed.

   public class ViewModel  
       {  
           public ViewModel()  
           {  
               this.GetContactsList();  
           }  
     
           public List PlatformsList { get; set; }  
           public ICommand ChangeTextCommmand { get; set; }  
     
           private void GetContactsList()  
           {  
               if (this.PlatformsList == null)  
                   this.PlatformsList = new List();  
     
               this.PlatformsList.Add(new PlatformInfo() { IsChecked = true, PlatformName = "Android" });  
               this.PlatformsList.Add(new PlatformInfo() { IsChecked = true, PlatformName = "iOS" });  
               this.PlatformsList.Add(new PlatformInfo() { IsChecked = false, PlatformName = "UWP" });  
     
               ChangeTextCommmand=new Command((Myobject) =>  
               {  
                   PlatformInfo  platformInfo= Myobject as PlatformInfo;  
     
                   platformInfo.PlatformName = "tes";  
               });  
           }  
       }

Here is test xaml layout.

Best Regards,

Leon Lu

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NULL error with FindByName

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