Question

I’ve got the following LINQ expression:

var ageEntry = from entry in args
                           where (entry.Key == "Age")
                           select entry.Value;

//I want age as Int16:
Int16 age = Convert.ToInt16(ageEntry); 

However I get the following exception:

Unable to cast object of type
‘WhereSelectEnumerableIterator2[System.Collections.Generic.KeyValuePair2[System.String,System.String],System.String]’
to type ‘System.IConvertible’.

which isn’t the clearest exception I’ve seen, can someone please explain this to me?

Answer 1

First, ageEntry is a sequence, being the result of a query. It looks like you expect your query to have a single result. Thus, you can say:

short age = Convert.ToInt16(ageEntry.Single());

which isn’t the clearest exception I’ve seen, can someone please explain this to me?

Actually, it’s pretty clear. It’s telling you that the type of ageEntry is WhereSelectEnumerableIterator2[System.Collections.Generic.KeyValuePair2[System.String,System.String],System.String] and that it can’t convert it. This is your major clue that ageEntry isn’t representing a number or something that can be converted to a number like you think it should be.

Rather, it represents a query that filters and projects a certain sequence (in your case, args). Basically, ageEntry knows how to filter through a sequence of KeyValuePair<string, string> and project to string. This is why you have to use Enumerable.Single. You’re saying “give me the one result from the query that results from doing all the filtering and projecting that you know how to do (by the way, throw an exception if there isn’t a single result).”