Should I Throw ArgumentNullException if a string is blank?

Question

I'm working on a method that does something given a string parameter. A valid value for the string parameter is anything other than null or string.Empty. So my code looks like this.

private void SomeMethod(string someArgument) {     if(string.IsNullOrEmpty(someArgument))         throw new ArgumentNullException("someArgument");      // do some work } 

Nothing too exciting there. My question is, is it okay to throw an ArgumentNullException even if the string is equal to string.Empty? Because technically it isn't null. If you believe it should not throw ArgumentNullException, what exception should be thrown?

Answer 1

ArgumentException should be thrown for the String.Empty case. This would indicate an issue other than it being null. To avoid a NullReferenceException I check for null first, then I trim and check for the empty case to prevent any whitespace from passing.

private void SomeMethod(string someArgument) {     if(someArgument == null)         throw new ArgumentNullException("someArgument");      if (someArgument.Trim() == String.Empty)         throw new ArgumentException("Input cannot be empty", "someArgument");      // do some work } 

As of .NET 4.0 you can use the String.IsNullOrWhiteSpace method to perform these checks in one go. By doing so you forgo the ability to specify a granular exception type, so I would opt for the ArgumentException and update the message accordingly.

Answer 2

You should throw an ArgumentException if an empty string is not an accepted input for your method. It may be very confusing to clients if you throw an ArgumentNullException while they didn't provide a null argument.

It is simply another use case. You may also have methods that do not accept null input values but that do accept empty strings. It's important to be consistent across your entire application.