TryGetValue on a null dictionary

Question

I am trying to use TryGetValue on a Dictionary as usual, like this code below:

Response.Context.Skills[MAIN_SKILL].UserDefined.TryGetValue("action", out var actionObj)

My problem is the dictionary itself might be null. I could simply use a "?." before UserDefined but then I receive the error:

"cannot implicitly convert type 'bool?' to 'bool'"

What is the best way I can handle this situation? Do I have to check if UserDefined is null before using TryGetValue? Because if I had to use Response.Context.Skills[MAIN_SKILL].UserDefined twice my code could look a little messy:

if (watsonResponse.Context.Skills[MAIN_SKILL].UserDefined != null && 
    watsonResponse.Context.Skills[MAIN_SKILL].UserDefined.TryGetValue("action", out var actionObj))
{
    var actionName = (string)actionObj;
}

Check for null before you make a call. This way you do not have to use TryGetvalue. — Smitha Kalluz, Jan 06 '22 at 19:53

score 6 · Accepted Answer · answered Jan 06 '22 at 19:52

6

Add a null check (?? operator) after the bool? expression:

var dictionary = watsonResponse.Context.Skills[MAIN_SKILL].UserDefined;
if (dictionary?.TryGetValue("action", out var actionObj)??false)
{
    var actionName = (string)actionObj;
}

answered Jan 06 '22 at 19:52

Nigel

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Thanks! That actually solved my problem. But I still receive a ```Use of unassigned local variable 'actionObj'```... is my only option here to declare that object before the ```if``` statement? – Alexandre Paiva Jan 06 '22 at 22:03
1

@AlexandrePaiva Yes you would have to do so. If you think about the logic you will see why that error is thrown – Charlieface Jan 06 '22 at 22:07
@Charlieface yeah, I'll declare the object before then. Just for feature reference, I found a question with a interesting discussion regarding the use of local variables inside the TryGetValue: https://stackoverflow.com/questions/56779825/why-compiler-throw-error-cs0165-use-of-unassigned-local-variable – Alexandre Paiva Jan 06 '22 at 22:17

Charlieface · Answer 2 · 2022-01-07T00:45:45.750

Another option is to compare to true.

It looks slightly weird, but it works with three-valued logic and says: is this value true but not false or null

if (watsonResponse.Context.Skills[MAIN_SKILL]
    .UserDefined?.TryGetValue("action", out var actionObj) == true)
{
    var actionName = (string)actionObj;
}

You can do the opposite logic with != true: is this value not true, so either false or null

if (watsonResponse.Context.Skills[MAIN_SKILL]
    .UserDefined?.TryGetValue("action", out var actionObj) != true)
{
    var actionName = (string)actionObj;
}

TryGetValue on a null dictionary

2 Answers2