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Text Block Bined to a Property in the View Model, and Implementing the INotifyPropertyChanged.

The Text does not update, even though the property value has changed. once I try to edit anything in the XAML of the TextBlock, the text appears.

I know that this may not be logical.

Here is the ViewModel Base Class :

    {
        public event PropertyChangedEventHandler PropertyChanged;
        public void OnpropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

Here is the View Model Binded to the WPF :

public class LoginViewModel : ViewModelBase
    {
         private string _errormessage = "";

         public string ErrorMessage
        {
            get
            {
                return _errormessage;                
            }
            set 
            {
                _errormessage = value;
                OnpropertyChanged(nameof(_errormessage));
            }
        }

public LoginViewModel()
        {
            LoginCommand = new ViewModelCommand(ExecuteLoginCommand, CanExecuteLoginCommand);
        }

private void ExecuteLoginCommand(object obj)
        {
            try
            {
                    ///Some Code that can return True or False
                    ///When the result is false the Error Message Property Should Display some text
                    bool valid = false;
                    if (valid)
                    {
                        ErrorMessage = "";
                    }
                    else if (!valid)
                    {
                    >>>>>>>>>    ErrorMessage = "Username or Password is Wrong";
                    }
                
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                ErrorMessage = "Couldn't Connect to Server";
            }
        }

As You can See in the code above, ErrorMessage takes a new value if the code is true or false.

Here Comes the Issue :

in the UI WPF View, the Text Does not appear.

Here is the XAML of the TextBlock

<TextBlock x:Name="txtError"
                               Text="{Binding ErrorMessage, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                               FontSize="15"
                               FontWeight="Normal"
                               Margin="0,10,0,0"
                               Foreground="Red"
                               HorizontalAlignment="Center"
                               VerticalAlignment="Center"
                               Visibility="Visible"/>

If I tried to change for Example the UpdateTriggerSource during debugging, suddenly the Text will appear.

can anyone locate the issue?

Mostafa Tarek Yassien
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    `OnpropertyChanged(nameof(_errormessage));` must be `OnpropertyChanged(nameof(ErrorMessage));`. You are notifying about a change of the property, not about the change of its backing field. – Clemens Feb 09 '23 at 12:31
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    Also note that both `Mode=TwoWay` and `UpdateSourceTrigger=PropertyChanged` on the Text Binding is pointless and should be removed. – Clemens Feb 09 '23 at 12:34
  • @Clemens I noticed that, and I will remove them eventually. Thank you – Mostafa Tarek Yassien Feb 09 '23 at 12:34

0 Answers0