Problems emulating facebook search using Textbox Textchanged Event when text is empty

Question

I'm trying to emulate on how Facebook does it search on each time a key is being entered into their search box. The problem I'm currently facing is that, is when I want to clear the search box or when I try to erase all the text manually. It stills fires the default text (Search) as a search entry. Much help is appreciated, thanks!

private void SearchTB_TextChanged(object sender, TextChangedEventArgs e)
  {
    int keyword_length = ((TextBox)sender).Text.Length;


    if (keyword_length == 0)
    {
      SearchTB.Text = "Search";
      myButton.Text = "Cancel";
     }

     //  When user starts to enter in text
     if (keyword_length > 0)
       {
         SearchTB.Text = "";
         myButton.Text = "Clear";
       }

     // Search when 3 chars has been entered into Searchbox
     if (keyword_length > 2)
       {
         keyword = ((TextBox)sender).Text;
         SearchBook(this.keyword);
        }
    }

private void myButton_Tap(object sender, System.Windows.Input.GestureEventArgs e)
    {
        if (myButton.Text == "Clear")
        {
           SearchTB.Text = "Search";
           myButton.Text = "Cancel";
        }
        else
        {
            if (cancelButton.Text == "Cancel")
            {
              Debug.WriteLine("This is cancel button");
            }
        }
    }

Answer 1

As I can see, you are using WPF (no tags, but namespaces belong to WPF), so you may do that in XAML:

<Grid>
    <TextBox x:Name="SearchTB" />
    <TextBlock x:Name="SearchBlock" Text="Search" IsHitTestVisible="False"
               VerticalAlignment="Center" />
</Grid>

And in code

if (keyword_length == 0)
{
  SearchBlock.Visibility = Visibility.Visible;
  myButton.Text = "Cancel";
}
else
{
  SearchBlock.Visibility = Visibility.Hidden;
  myButton.Text = "Clear";
}

So, you are hiding textblock, which not clickable because of IsHitTestVisible="False" and visible only when search text is empty.