Using LINQ, how do I find an object with a given property value from a List?

Question

I have a class called Questions. This Questions has properties QuestionID and QuestionAnswer. While iterating through this List of Question in foreach, I have to find .QuestionID = 12. If I find .QuestionID = 12 then I have to immediately assign a value to .QuestionAnswer = "SomeText" of .QuestionID = 14.

I don't want iterate again inside .QuestionId = 12' to find.QuestionID = 14` again.

Is there any way I can go directly to .QuestionID = 14 using LINQ?.

For example:

For Each mQuestion As Question In _mQuestions
    If mQuestion.QuestionId = 12 Then
         'Find mQuestion.QuestionID= 14 and insert Somtext to 
          'mQuestion.QuestionAnswer="SomeText"
    End IF
Next

Answer 1

I think you're looking for something like this. If I have a moment, I'll translate it to VB, but I think you can follow.

if (_mQuestions.Any(q => q.QuestionID == 12)) 
{
   Question question14 = _mQuestions.FirstOrDefault(q => q.QuestionID == 14);
   if (question14 != null)
       question14.QuestionAnswer = "Some Text";
}

Answer 2

Unfortunately, your data structure (List) requires that you search again to find Question-14 once Question-12 is found. If your Question list is sorted by ID, then some improvements can be made, but in general, there is no way to directly access an element of a List or Array by only knowing the values of the element's property.

The data structure that is applicable to your problem is Dictionary as it allows indexing of objects via some value, as well as efficient direct-access to those objects without the need to iterate through the entire collection.

You can covert your list to a dictionary using Linq by calling the ToDictionary() extension method:

IDictionary<Question> questions = _mQuestions.ToDictionary(q => q.id);

This uses the ID of the Question object as the key, and the object as the value. Then in your code, you can do the following:

if (questions.ContainsKey(12) && questions.ContainsKey(14))
{
    questions[14].QuestionAnswer = "Some Text";
}

Note that ContainsKey and the indexer (operator[]) both execute in constant time.