Some do and some don't! It totally depends on their location and training. They will protect your property against unknown intruders by alerting you. They might bark if they are concerned or worried and if they hear noises they are not used to they will bark. Some Dandies go outside and bark 'just in case' but others will go out quietly, sit and sniff the air, and assess the situation silently. It generally is within your gentle but firm control.

 Please do not use an electric collar on a Dandie. It should never come to that. Little tricks you can use if you do have a problem are one sheet of newspaper, loosely rolled up. Hit yourself, not the Dandie, on your arm or leg and the noise stops them. Say the word 'newspaper' as you do it and soon they will stop barking by you just using the word. Another way is with a hand water spray - the kind you mist plants with - a quick squirt with an instruction also does the trick and soon just the word 'spray' or whatever you choose to use will stop any incessant barking. 

note: Dandies do not yap! They have a magnificent deep bark worthy of a much larger dog.