SingleOrDefault（）引发异常序列包含多个匹配元素

Question

i have dictionary and there are few element has been stored like

    Dictionary<string, string> dService = new Dictionary<string, string>();
    dService.Add("UPS Express Plus", "001");
    dService.Add("UPS Express Plus", "054");
    dService.Add("UPS Express", "007");
    dService.Add("UPS Express Saver", "065");
    dService.Add("UPS Expedited", "008");
    dService.Add("UPS Express Plus", "001");

this way i am trying to get key based on value

dService.SingleOrDefault(x => x.Value == ("001")).Key

this code is throwing error. i search google and got solution that people saying not to use SingleOrDefault() rather use FirstOrDefault()

but i got no good explanation why & when SingleOrDefault() throw error ? should i presume that dictionary store multiple value that is why SingleOrDefault() will not work?

looking for explanation when why & when SingleOrDefault() throw error....please guide me sample situation. thanks

Answer 1

First, your sample-data above does not throw since it doesn't contain multiple equal values which are "001" . It would if you'd add two values with Value="001" .

If you enumerate a dictionary you get an IEnumerable<KeyValuePair<TKey, TValue>> . You are asking for the first and only value in the dictionary which is "001" . Of course a dictionary can contain multiple equal Values (but only unique keys).

So you could use FirstOrDefault instead which doesn't throw on multiple results. Another way is to find all with Where , then Select the key:

IEnumerable<string> all001ValKeys = dService
    .Where(kv => kv.Value == "001")
    .Select(kv => kv.Key);

SingleOrdefault (or Single ) is handy if you want to ensure a business rule like:

var record = someTable.Single(obj => obj.ID == 123); // ID must be unique

This makes your code more readable and also follows the "failing fast"-rule.

Answer 2

FirstOrDefault is used if you want first element in list , if no data in list , it will return you default value (which is null) else it will return you first element of list.

SingleOrDefault is used when you are sure only one element in list , if it is not in list , it will return default (which is null), but if it has more then single value , it will throw you exectption.

I hope this explanation is use full for you.

Answer 3

Note that FirstOrDefault on an Dictionary returns a KeyValuePair containing the default values for the defined Key and Value types. Since you're using string as Key and Value, both will be null. So the result of your LINQ expression will be null and working with the key may result in a NullReferenceException later in your code.