Two Way Binding in WPF

Answer 1

You really need the INotifyPropertyChanged interface:

using System.ComponentModel;

namespace TestBindingApp
{
public class Class1
{
    // Singleton instance
    private static Class1 instance;
    private string _accountNumber;
    public Class1()
    {

    }

    // Singleton instance read-only property
    public static Class1 Instance
    {
    get
    {
        if (instance == null)
        {
        instance = new Class1();
        }
        return instance;
    }
    }

    public string AccountNumber
    {
    get
    {
        return _accountNumber;
    }
    set
    {
        if (value != _accountNumber)
        {
        _accountNumber = value;
            NotifyPropertyChanged("AccountNumber");
        }
    }
    }

        public event PropertyChangedEventHandler PropertyChanged;


    private void NotifyPropertyChanged(string property)
    {
        if(PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(property));
    }
}

Answer 2

You call NotifyPropertyChanged from a static member, but NotifyPropertyChanged itself isn't static.

Two ways to solve: Either make AccountNumber NOT static or provide an instance for your call to NotifyPropertyChanged (e.g. "Instance.NotifyPropertyChanged(...)")