Binding IsEnabled-property to a Model-property

Question

I am not using the mvvm-pattern. I directly bind my UI to a model.

My problem is that the binding to the model-properties works. The textbox gets a red border if the model-property MyStringValue is invalid and the MyBoolValue-property also changes. But my button doesn't update the IsEnabled-property.

My XAML looks like that

<Window x:Class="OkButtonTest.MainWindow"
        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:OkButtonTest"
        mc:Ignorable="d"
        Title="MainWindow">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition></RowDefinition>
            <RowDefinition></RowDefinition>
        </Grid.RowDefinitions>
        <TextBox Grid.Row="0" Text="{Binding MyStringValue, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
        <Button x:Name="MyExampleButton" Grid.Row="1" IsEnabled="{Binding MyBoolValue, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" Click="Button_Click" />
    </Grid>
</Window>

The DataContext is bound to the model at the csharp-code:

using System.Windows;

namespace OkButtonTest
{
    public partial class MainWindow : Window
    {
        ModelExample modelExample = new ModelExample();

        public MainWindow()
        {
            InitializeComponent();
            DataContext = modelExample;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("Button was clicked");
        }
    }
}

Notice that i have implemented Fody.PropertyChanged in my model, so i dont need to implement PropertyChanged for each property.

using System;
using System.ComponentModel;

namespace OkButtonTest
{
    public class ModelExample : INotifyPropertyChanged, IDataErrorInfo
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private string myStringValue = "";
        public string MyStringValue
        {
            get
            {
                return myStringValue;
            }
            set
            {
                myStringValue = value;
            }
        }

        private bool myBoolValue;
        public bool MyBoolValue
        {
            get
            {
                return myBoolValue;
            }
            set
            {
                myBoolValue = value;
                Console.WriteLine($"MyBoolValue is {MyBoolValue.ToString()}");
            }
        }

        public string Error
        {
            get
            {
                if(MyStringValue.Length < 5)
                {
                    MyBoolValue = false;
                    return "Invalid string value";
                }

                MyBoolValue = true;
                return string.Empty;
            }
        }

        public string this[string columnName]
        {
            get
            {
                return Error;
            }
        }
    }
}

Why doesnt my button update?

Answer 1

Only return a string from the Error property and set the MyBoolValue property in the setter of MyStringValue:

public class ModelExample : INotifyPropertyChanged, IDataErrorInfo
{
    public event PropertyChangedEventHandler PropertyChanged;

    private string myStringValue = "";
    public string MyStringValue
    {
        get
        {
            return myStringValue;
        }
        set
        {
            myStringValue = value;
            MyBoolValue = !string.IsNullOrEmpty(myStringValue) && myStringValue.Length > 4;
        }
    }

    private bool myBoolValue;
    public bool MyBoolValue
    {
        get
        {
            return myBoolValue;
        }
        set
        {
            myBoolValue = value;
        }
    }

    public string Error
    {
        get
        {
            if (MyStringValue.Length < 5)
            {
                return "Invalid string value";
            }
            return string.Empty;
        }
    }

    public string this[string columnName]
    {
        get
        {
            return Error;
        }
    }
}

A getter shouldn't set anything. Your current approach of setting the MyBoolValue in the getter of the Error property results in a StackoverflowExcpetion being thrown when I run it.

Answer 2

Seems you should be using following syntax to allow fody preprocess your viewmodel getters and setters.

[AddINotifyPropertyChangedInterface]
public class ModelExample : IDataErrorInfo
{
    ...
}

Also make sure you have this in your FodyWeavers.xml

<?xml version="1.0" encoding="utf-8" ?>
<Weavers>
  <PropertyChanged/>
</Weavers>