Tutorial :C# using LINQ and Nullable Boolean



Question:

I have the below linq query which takes a text field which may be Y, N or DBnull and populates a boolean? parameter with either True, False or null depending on the value of the field.

var dset = from i in tbdc.Talkbacks         where i.talkback_id == id         select new Talkback(                 i.talkback_id, i.acad_period, i.reference,                 i.staff_member, i.date_received, i.no_talkers,                 i.gender_id, i.names, i.type_id,                 i.method_id, i.area_id, i.site_id,                 i.category_id, i.date_closed, i.expenddate,                 i.acknowledgementtarget,                 (i.targetmet == "Y") ? true :                      ((i.targetmet == "N") ? false : null),                     (i.acknowledgementtargetmet != "N") ? true : false  

The problematic line is

(i.targetmet == "Y") ? true : ((i.targetmet == "N") ? false : null)  

After reading up I found some documentation which states that the 2nd and 3rd arguments of the inline if need to be of the same type or be implicitly convertible to one another.

My question is, how do I get around this limitation to achieve my desired result?

I am relatively new to C# so am not familiar with all its quirks/capabilities yet.


Solution:1

My suggestion would be to replace it with :

(i.targetmet != null) ? (bool?)(i.targetmet == "Y") : null;  

the reason why the compiler does not agree without the cast is that even if you store it into a nullable structure, the ternary operation checks if the results are compatible through an implicit conversion.

Results true and false are treated as bool literals, not bool?, thus not implicitly convertible to null. Casting either results to bool? will make them comparable. The one I suggested has an implicit convertion between bool? and null, and this works too:

(i.targetmet != null) ? (i.targetmet == "Y") : (bool?)null;  

Which is an implicit conversion between a bool and bool?. I arbitrary prefer the first one..


Solution:2

You can explicitly convert one or more of the expressions to a bool?:

(i.targetmet == "Y") ? true : ((i.targetmet == "N") ? (bool?)false : (bool?)null)  

Note:If u also have question or solution just comment us below or mail us on toontricks1994@gmail.com
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