There is a tag which is of type 2 ("required, empty if unknown"), with value representation Integer String which I would like to leave empty. I have tried creating the attribute like so:
var attribute = new DicomIntegerString(DicomTag.SeriesNumber, string.Empty);
The storing of the file works. When I read the file again, the result of the following call returns null:
var result = dicomDataset.GetString(DicomTag.SeriesNumber); // <-- this is null
How can I set the element to be correctly "zero-length", or "empty, if unknown"?
Thanks.
As already mentioned in the comments, the code to set an empty string in the dataset is correct:
dataset.AddOrUpdate(new DicomIntegerString(DicomTag.SeriesNumber, string.Empty));
Note that you could also write a null value:
dataset.AddOrUpdate(new DicomIntegerString(DicomTag.SeriesNumber, (string)null));
Writing out the dataset will create an empty tag for SeriesNumber in exactly the same way in both cases, as both cases are equivalent in DICOM.
The code to read the tag back is also correct, and the fact that it returns null is due to this equivalence, which creates an ambiguity in the interpretation of an empty string tag in DICOM. As the number of values in a DICOM string tag is defined only by the string itself (and the number of backslashes it contains), there is no difference between a tag with no values (which usually is represented by a null value), and a tag with an empty string (which would be represented by a "" value). For consistence with other tags it makes sense to return null for a tag with VM 0 - for non-string VRs there is no ambiguity here, as the length of a value is defined. For string values it could also make sense to return an empty string instead - both approaches have pros and cons, so in the end it is a design decision of the library creators.
In fo-dicom, it is probably best to handle both cases (e.g. using something like string.IsNullOrEmpty). If you read the value from a file dataset, you always get null, but if you are writing an empty string to a tag in a dataset (first version shown above) and read it back, you will get the same empty string back.
As an aside: in pydicom (the common Python DICOM library) there was a discussion about this same design decision, and in the end a configuration entry was added that defines the behavior (e.g. return None or an empty string for such values).