Manage all textchanged events with one declare

OK you already know how to add a event handler to the TextChanged event and I'm guessing you also know that if you specify the same method name in this case txt_TextChanged then all the then they would use the same method to process the TextChanged event, such as;

void txt_TextChanged(object sender, EventArgs e)
{
    //This becomes a reference to the TextBox that raised the TextChanged event
    TextBox textBox = sender as TextBox;

}

If you want to use a separate class to process all TextChanged events you can also do that.
If you create a class like the following (remember to add using System.Windows.Forms; to the class);

public class TextChangeHandler
{
    public void TextChanged(object sender, EventArgs e)
    {
        //This becomes a reference to the TextBox that raised the TextChanged event
        TextBox textBox = sender as TextBox;
    }
}

you can then use the following to handle the TextChanged event;

//This needs to be global for the form
TextChangeHandler txtChangeHandler = new TextChangeHandler();

//These go in the form constructor
textBox1.TextChanged += new EventHandler(txtChangeHandler.TextChanged);
textBox2.TextChanged += new EventHandler(txtChangeHandler.TextChanged);
textBox3.TextChanged += new EventHandler(txtChangeHandler.TextChanged);

You can also make the class static and use;

public static class TextChangeHandler
{
    public static void TextChanged(object sender, EventArgs e)
    {
        //This becomes a reference to the TextBox that raised the TextChanged event
        TextBox textBox = sender as TextBox;
    }
}

you can then use the following to handle the TextChanged event;

textBox1.TextChanged += new EventHandler(TextChangeHandler.TextChanged);
textBox2.TextChanged += new EventHandler(TextChangeHandler.TextChanged);
textBox3.TextChanged += new EventHandler(TextChangeHandler.TextChanged);

Try this:

foreach (Control c in this.Controls)
{
    ((TextBox)c).TextChanged += new EventHandler(Form1_TextChanged);
}