Checking if field is null

You need to distinguish between a null string reference and an empty string.

If you have this:

  string s = null;

then it means that the variable 's' doesn't currently refer to a string instance.

However, if you have this:

  string s = "";

it means that 's' does refer to a string instance but that instance has no characters i.e. it's an empty string.

There is a convenience method that can be used to check for both:

if (String.IsNullOrEmpty(fieldName.Text))
{
   // do something
}