Wpf Mvvm Passing a string from ViewModel to a views label or textblock

Question

In my ViewModel I have made a property name MessageError, and in the View I make a binding. But somehow the message doesn't get to the view - leave the label/textblock blank.

No matter what I'm trying I cann't see where the problem is. When "run to cursor" I try to step into everything, but doesn't show me any kind of error. It set the message, but doesn't shop up in the view.

I hope one of you can see, where I make an error?

Best regards
Simsen :-)

My viewModel property:

private string _messageError;
        public string MessageError
        {
            get => _messageError;
            set
            {
                _messageError = value;
                OnPropertyChanged("MessageError");
            }
        }

In the viewmodel I have a method witch set the text (no errors when run to cursor)

    public void SaveChangesNew()
            {
                string name = CategoryName;
                bool global = CategoryIsGlobal;
                bool obsolete = CategoryIsObsolete;
                int projectId = ProjectId;
                MessageError = "Den er ikke gemt";
            }

And the view look likes this

<TextBlock x:Name="TblMessage" Text="{Binding MessageError, Mode=TwoWay}" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3" />
                <!--<Label x:Name="LblMessage" Content="{Binding Path=MessageError}" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3" />-->

Answer 1

Hi Anja,
your code works properly. Please, check assignment of DataContext and Immediate Window. Following demo with yours code works properly.

XAML:

<Window x:Class="WpfApp1.Window041"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp041"
        mc:Ignorable="d"
        Title="Window041" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <StackPanel>
    <Button Content="Click to check" Command="{Binding}"/>
    <TextBlock x:Name="TblMessage" Text="{Binding MessageError}"/>
  </StackPanel>
</Window>

And ViewModel:

using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Input;

namespace WpfApp041
{
  public class ViewModel : ICommand, INotifyPropertyChanged
  {
    private string _messageError;
    public string MessageError
    {
      get => _messageError;
      set
      {
        _messageError = value;
        OnPropertyChanged("MessageError");
      }
    }
    public void SaveChangesNew()
    {
      //string name = CategoryName;
      //bool global = CategoryIsGlobal;
      //bool obsolete = CategoryIsObsolete;
      //int projectId = ProjectId;
      MessageError = "Den er ikke gemt";
    }
    public void Execute(object parameter) => SaveChangesNew();

    public event EventHandler CanExecuteChanged;
    public bool CanExecute(object parameter) => true;

    #region INotifyPropertyChanged Members
    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(string propertyName) =>
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    #endregion
  }
}

Answer 2

I have now looked at 3 videos and can't get the Immediate Window. to show anything. Most of the time, it says that I can't make anything on a library.

But I have tried a lot of difference things and think that it could have something to do with the MyICommand I have made (again from a video) for the Command purpose.

Here I don't use the CanExecute command because I do not want it to grey out the buttons. So maybe it has with that to do?

My MyICommand

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Input;

namespace ViewModel
{
    public class MyICommand : ICommand
    {
        readonly Action _TargetExecuteMethod;
        readonly Func<bool> _TargetCanExecuteMethod;

        public MyICommand(Action executeMethod)
        {
            _TargetExecuteMethod = executeMethod;
        }

        public MyICommand(Action executeMethod, Func<bool> canExecuteMethod)
        {
            _TargetExecuteMethod = executeMethod;
            _TargetCanExecuteMethod = canExecuteMethod;
        }

        public void RaiseCanExecuteChanged()
        {
            CanExecuteChanged(this, EventArgs.Empty);
        }

        bool ICommand.CanExecute(object parameter)
        {

            if (_TargetCanExecuteMethod != null)
            {
                return _TargetCanExecuteMethod();
            }

            if (_TargetExecuteMethod != null)
            {
                return true;
            }

            return false;
        }

        // Beware - should use weak references if command instance lifetime 
        // is longer than lifetime of UI objects that get hooked up to command

        // Prism commands solve this in their implementation 
        public event EventHandler CanExecuteChanged = delegate { };

        void ICommand.Execute(object parameter)
        {
            _TargetExecuteMethod?.Invoke();
        }
    }
}

In my ViewModel I use it this way:

public CategoryViewModel()
        {
            LoadCategories();
            LoadProjects();            
            CmdSave = new MyICommand(SaveChanges);
            CmdSaveNew = new MyICommand(SaveChangesNew);
        }

and

public MyICommand CmdSaveNew { get; set; }
        public void SaveChangesNew()
        {
            if (CategoryName != null)
            {
                string name = CategoryName;
                bool global = CategoryIsGlobal;
                bool obsolete = CategoryIsObsolete;
                int projectId = ProjectId;
                MessageError = "This message I want to see in the view";

                string test = "hej";
            }
            else
            {
                MessageError = string.Empty;
            }
        }

Do I do it wrong this way?

Best regards
Simsen :-)