Check Whether a TextBox is empty or not

I think

string.IsNullOrEmpty(TextBox.Text)

or

string.IsNullOrWhiteSpace(TextBox.Text)

are your best options.

You should use String.IsNullOrEmpty() to make sure it is neither empty nor null (somehow):

if (String.IsNullOrEmpty(textBox1.Text))
{
    // Do something...
}

More examples here.

For practical purposes you might also consider using String.IsNullOrWhitespace() since a TextBox expecting whitespace as input probably negates any purpose, except in case of, say, letting the user pick a custom separator for stuff.