Binding DataContext Value that affect by other property

Question

I have create a class for the Data Context

public class Amt
{            
    private double _Amount = 0;
    public double Amount
    {
        get { return _Amount; }
        set { _Amount = value; }
    }   

    private double _RedeemAmount = 0;
    public double RedeemAmount
    {
        get { return _RedeemAmount; }
        set
        {
            _RedeemAmount = value;            
        }
    }

    private double _BalAmount = 0;
    public double BalAmount
    {
        get { return Amount - RedeemAmount; }
    }
}

And I have bind it to TextBox at XAML

<TextBox Name="txtAmount" Text="{Binding Amount}" FontSize="16" Margin="0,0,0,2"/>
<TextBox Name="txtRedeemAmount" Text="{Binding RedeemAmount,Mode=TwoWay}" FontSize="16" Margin="0,0,0,2" TextChanged="TxtRedeemAmount_TextChanged"/>
<TextBox Name="txtBalAmount" Text="{Binding BalAmount}" FontSize="16" Margin="0,0,0,2"/>

Then, what I want is when I change the Redeem Amount in TextChanged event, I expect the Balance amount should change too, I have do some research and try some code on it, but it just not working. Did I do this in wrong way? Any other ways to do this better?

private void TxtRedeemAmount_TextChanged(object sender, EventArgs e)
  {
    double amt = 0;
    double.TryParse(txtRedeemAmount.Text, out amt);
    Amt.RedeemAmount = amt;
   }

Thanks for the help anyway!!!

Answer 1

To accomplish what you want, you need to implement the INotifyPropertyChanged interface. Whenever a property of the class is changed you should fire the PropertyChanged event.

using System.ComponentModel;

public class Amt : INotifyPropertyChanged
{
    private double _Amount = 0;
    public double Amount
    {
        get { return _Amount; }
        set 
        { 
            _Amount = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Amount)));
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(BalAmount)));
        }
    }

    private double _RedeemAmount = 0;
    public double RedeemAmount
    {
        get { return _RedeemAmount; }
        set
        {
            _RedeemAmount = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(RedeemAmount)));
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(BalAmount)));
        }
    }

    private double _BalAmount = 0;

    public double BalAmount
    {
        get { return Amount - RedeemAmount; }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}