Develop Reference

InvalidCastException when invoking PropertyChangedEventHandler

Situation:
UWP app using MVVM and Xaml for UI
View Models are derived from a ModelBase class implementing the INotifyPropertyChanged interface
Problem:
when executing a specific UI test using the affected class, in some cases the application throws an InvalidCastException during the PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)) call. The propertyName is set correctly using the [CallerMemberName] which was verified using a debug log entry. The SetProperty method is called in the DependencyProperty with a backing field. A DebugConverter used in the Xaml element which is bound to the PropertyChanged event shows a valid conversion but the setting of the bound element fails. It looks like a Double to Double cast is not possible which makes no sense.
Question
Does anyone have an idea what the reason for this exception could be?
Code
Control.xaml
Maximum="{Binding TimeControlCanvasWidth, Converter={StaticResource DebugConverter}}" />
ModelBase.cs
public class ModelBase : INotifyPropertyChanged
{
/// <summary>
/// The event raised, when the property changed
/// </summary>
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName]string propertyName = null)
{
try
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
catch (System.InvalidCastException)
{
LogManager.Current.GetLogger(GetType()).LogCritical(() => $"#####{nameof(OnPropertyChanged)}" +
$" - InvalidCastException | propertyName={propertyName}");
}
}
protected virtual bool SetProperty<T>(ref T field, T value, [CallerMemberName]string propertyName = null)
{
if (field?.Equals(value) == true)
{
return false;
}
field = value;
// ReSharper disable once ExplicitCallerInfoArgument
OnPropertyChanged(propertyName);
return true;
}
}
}
ViewModel.cs
public class TimeControlViewModel : ModelBase
{
private double _timeControlCanvasWidth;
public double TimeControlCanvasWidth
{
get => _timeControlCanvasWidth;
set
{
if (SetProperty(ref _timeControlCanvasWidth, value))
{
// do sth.
}
}
}
}

Overriding RaisePropertyChanged to execute additional code

In my model i have properties which i want to execute some extra code for when the property gets changed. I want to add the new value and the property name to my database. I also want to keep a list of current alarms (value is equal to true).
public Boolean ActionAlarmLowLow
{
get
{
return _ActionAlarmLowLow;
}
set
{
if (value != this._ActionAlarmLowLow)
{
Boolean oldValue = _ActionAlarmLowLow;
_ActionAlarmLowLow = value;
RaisePropertyChanged("ActionAlarmLowLow", oldValue, value, true);
}
}
}
How can i do this properly?
I am wondering if i should add two lines of code to the property:
DB.Log.addLogItem("ActionAlarmLowLow", value);
AlarmList.UpdateItem("ActionAlarmLowLow", value);
Or if i can somehow extend/override the RaisePropertyChanged and do some extra stuff elsewhere for specific properties. I.e calling something called
RaisePropertyChangedWriteToDbUpdateAlarmList();`

Yes it is quite simple, all you need is to create a base class with INotifyPropertyChanged and call whatever you want inside.
public abstract class NotifyPropertyChangedBase: INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged<T>(Expression<Func<T>> expression)
{
var memberExpression = (MemberExpression) expression.Body;
var propertyName = memberExpression.Member.Name;
var handler = PropertyChanged;
if (handler != null)
{
// Do your common actions here, before property change notification is fired
handler(this, new PropertyChangedEventArgs(propertyName));
// Do your common actions here, after property change notification is fired
}
}
}
public class MyClass : NotifyPropertyChangedBase
{
public Boolean ActionAlarmLowLow
{
get
{
return _ActionAlarmLowLow;
}
set
{
if (value != this._ActionAlarmLowLow)
{
_ActionAlarmLowLow = value;
OnPropertyChanged(() => this.ActionAlarmLowLow);
}
}
}
}

Execute method in viewmodel when property changed

I have some ViewModel with string property Name
My ViewModel inherit from ViewModelBase : INotifyPropertyChanged
...
private string _name;
public string Name
{
get { return _name; }
set { SetField(ref _name, value, "Name"); }
}
...
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
protected bool SetField<T>(ref T field, T value, string propertyName)
{
if (EqualityComparer<T>.Default.Equals(field, value)) return false;
field = value;
OnPropertyChanged(propertyName);
return true;
}
}
I want to execute some logic when my property Name change its value (on user input). My ViewModel will have alot of such properies and i will have alot of ViewModels with same property processing.
How should i run method that process chaneged propery in a right way?
Should i subscribe on PropertyChanged event in my ViewModel, then use switch on string property name to detect actual changed property and then use it value?
Or should i just run my method from setter?
Is there any patterns for such interactions?

Since your SetField method returns true if the property has changed, I would call my method in the setter if true is returned.
...
public string Name
{
get { return _name; }
set
{
if (SetField(ref _name, value, "Name"))
MyMethod();
}
}
...

Two way databinding in winforms, Inotifypropertychanged implemented in base class

I use Winforms Databinding and I have derived classes, where the base class implements IPropertychanged :
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged(string propertyName) {
var handler = this.PropertyChanged;
if (handler != null) {
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
Each propertysetter calls:
protected void SetField<T>(ref T field, T value, string propertyName) {
if (!EqualityComparer<T>.Default.Equals(field, value)) {
field = value;
IsDirty = true;
this.RaisePropertyChanged(propertyName);
}
}
A typical Propertysetter:
public String LocalizationItemId {
get {
return _localizationItemId;
}
set {
SetField(ref _localizationItemId, value, "LocalizationItemId");
}
}
The way a property is bound to a textbox
private DerivedEntity derivedEntity
TextBoxDerivedEntity.DataBindings.Add("Text", derivedEntity, "Probenname");
If I programmatically assign text to the textbox, the textbox does not show it. But I can manually edit the textbox.

I know it is too late to answer, but this problem can be solved, if you set event when your binding should change value, if you set it on property value change event your problem will be solved. You can do this by this way
textBox.DataBindings.Add("textBoxProperty", entity, "entityProperty", true, DataSourceUpdateMode.OnPropertyChanged);

Binding source is updated on TextBox Validated event. TextBox validated event is called when user edit TextBox and then changes focus to other control.
Since you're changing TextBox text programmatically TextBox doesn't know that text were changed and therefore validation is not called and binding is not updated, so you need to update binding manually.
Initialize binding:
var entity;
textBox.DataBindings.Add("textBoxProperty", entity, "entityProperty");
Change TextBox.Text:
textBox.Text = "SOME_VALUE";
Update binding manually:
textBox.DataBindings["textBoxProperty"].WriteValue();
Binding.WriteValue() reads value from control and updates entity accordingly.
You could read about WriteValue at MSDN.

The subscriber isn't initialized. i.e.
private DerivedEntity derivedEntity
TextBoxDerivedEntity.DataBindings.Add("Text", derivedEntity, "Probenname");
derivedEntity is null.
Initialize it and you'll be fine.

I implemented the "INotifyPropertyChanged", but raise the PropertyChanged event only when the new value is different from the old value:
public class ProfileModel : INotifyPropertyChanged
{
private Guid _iD;
private string _name;
public event PropertyChangedEventHandler PropertyChanged;
public Guid ID
{
get => _iD;
set
{
if (_iD != value)
{
_iD = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("ID"));
}
}
}
public string Name
{
get => _name;
set
{
if (_name != value)
{
_name = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Name"));
}
}
}
}
Now just bind to the controls:
txtProfileID.DataBindings.Clear();
txtProfileID.DataBindings.Add("Text", boundProfile, "ID", true, DataSourceUpdateMode.OnPropertyChanged);

Binding property to control in Winforms

What is the best way to bind a property to a control so that when the property value is changed, the control's bound property changes with it.
So if I have a property FirstName which I want to bind to a textbox's txtFirstName text value. So if I change FirstName to value "Stack" then the property txtFirstName.Text also changes to value "Stack".
I know this may sound a stupid question but I'll appreciate the help.

You must implement INotifyPropertyChanged And add binding to textbox.
I will provide C# code snippet. Hope it helps
class Sample : INotifyPropertyChanged
{
private string firstName;
public string FirstName
{
get { return firstName; }
set
{
firstName = value;
InvokePropertyChanged(new PropertyChangedEventArgs("FirstName"));
}
}
#region Implementation of INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
public void InvokePropertyChanged(PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, e);
}
#endregion
}
Usage :
Sample sourceObject = new Sample();
textbox.DataBindings.Add("Text",sourceObject,"FirstName");
sourceObject.FirstName = "Stack";

A simplified version of the accepted answer that does NOT require you to type names of properties manually in every property setter like OnPropertyChanged("some-property-name"). Instead you just call OnPropertyChanged() without parameters:
You need .NET 4.5 minimum.
CallerMemberName is in the System.Runtime.CompilerServices namespace
public class Sample : INotifyPropertyChanged
{
private string _propString;
private int _propInt;
//======================================
// Actual implementation
//======================================
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
//======================================
// END: actual implementation
//======================================
public string PropString
{
get { return _propString; }
set
{
// do not trigger change event if values are the same
if (Equals(value, _propString)) return;
_propString = value;
//===================
// Usage in the Source
//===================
OnPropertyChanged();
}
}
public int PropInt
{
get { return _propInt; }
set
{
// do not allow negative numbers, but always trigger a change event
_propInt = value < 0 ? 0 : value;
OnPropertyChanged();
}
}
}
Usage stays the same:
var source = new Sample();
textbox.DataBindings.Add("Text", source, "PropString");
source.PropString = "Some new string";
Hope this helps someone.