There are various approaches you can do to this,
personally I would go with one of the following:
If the .Net framework version you are using is less than version 4
if(String.IsNullOrEmpty(TextBox1.Text.Trim())
else go with:
if(String.IsNullOrWhiteSpace(TextBox1.Text ))
The latter approach does not require you to trim the string first as it will detect if the string is just whitespace characters such as space, carriage returns etc.