If conditions with dates.

Use DateTime.TryParse[^] to avoid unnecessary exception.

This is worse than pulling teeth!
Spend some a lot of time learning about the DateTime[^] structure, and use it. You cannot use comparison operators on dates stored as strings, apart from simple string equality, and even then you can get the wrong answer.

Try following:

if(!String.IsNullOrEmpty(TextBox1.Text))
{
  if (Convert.ToDateTime(TextBox1.Text) < Convert.ToDateTime("01/01/2001"))
  {
    lblres.Text="value should be more than 01/01/2001";
  }
}

For more advanced you can follow the links:
http://msdn.microsoft.com/en-us/library/ch92fbc1.aspx[^]
http://msdn.microsoft.com/en-us/library/9h21f14e.aspx[^]

Let me know if you need further information.

Enjoy :)