Is it possible to track string change?

.NET has provided two Interfaces used in specifying the monitoring of changes to Properties since version 2.0: INotifyPropertyChanging, and INofifyPropertyChanged.

Beginning in .NET 4.5 a new feature was added for avoiding having to hard code the name of the property changing/changed into the call:

.NET 2.0 to .NET 3.5:

using System.ComponentModel;

namespace YourNameSpace
{
    public class PropertyNotificationDemo : INotifyPropertyChanging, INotifyPropertyChanged
    {
        public event PropertyChangingEventHandler PropertyChanging;
        public event PropertyChangedEventHandler PropertyChanged;

        private void NotifyPropertyChanging(string data)
        {
            if (PropertyChanging != null) PropertyChanging(this, new PropertyChangingEventArgs(data));
        }

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

        // property that implements PropertyChanging
        private string someString;
    
        public string SomeString
        {
            get { return someString; }
    
            set
            {
                if (value != someString)
                {
                    NotifyPropertyChanging("SomeString");
                    someString = value;
                }
            }
        }

        // property that implements PropertyChanged
        private string anotherString;

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

You use these Event declarations that implement the Property Changing/Changed facilities by creating instances of the Class and subscribing to the Events like you would any other custom Event:

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace YourNameSpace
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        // declare in Form Class scope
        PropertyNotificationDemo pnd = new PropertyNotificationDemo();
    
        private void Form1_Load(object sender, EventArgs e)
        {
            // initialize somewhere like in the Form Load EventHandler
            pnd.PropertyChanging += pnd_PropertyChanging;
            pnd.PropertyChanged += pnd_PropertyChanged;
            pnd.SomeString = "hello";
            pnd.AnotherString = "goodbye";
        }
    
        // define the EventHandlers
        private void pnd_PropertyChanging(object sender, PropertyChangingEventArgs e)
        {
            MessageBox.Show(string.Format("{0} is changing", e.PropertyName));
        }

        private void pnd_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            MessageBox.Show(string.Format("{0} is changed", e.PropertyName));
        }
    }
}

Beginning with .NET 4.5: the 'CallerMemberName Attribute was added: now the Class that uses the new Property Changed/Changing requires:

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace YourNameSpace
{
    public class PropertyNotificationDemo : INotifyPropertyChanging, INotifyPropertyChanged
    {
        public event PropertyChangingEventHandler PropertyChanging;
        public event PropertyChangedEventHandler PropertyChanged;

        private void NotifyPropertyChanging([CallerMemberName] string propertyName = "")
        {
            if (PropertyChanging != null) PropertyChanging(this, new PropertyChangingEventArgs(propertyName));
        }

        private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
        {
            if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

        // property that implements PropertyChanging
        private string someString;
    
        public string SomeString
        {
            get { return someString; }
    
            set
            {
                if (value != someString)
                {
                    NotifyPropertyChanging();
                    someString = value;
                }
            }
        }

        // property that implements PropertyChanged
        private string anotherString;
    
        public string AnotherString
        {
            get { return anotherString; }
    
            set
            {
                if (value != anotherString)
                {
                    anotherString = value;
                    NotifyPropertyChanged();
                }
            }
        }
    }
}

And, the EventHandlers in the Form that creates an instance of, and uses, this Class look like this:

private void pnd_PropertyChanging(object sender, PropertyChangingEventArgs e)
{
    MessageBox.Show(string.Format("{0} is changing", e.PropertyName));
}

private void pnd_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    MessageBox.Show(string.Format("{0} is changed", e.PropertyName));
}