Updating a list using Linq

Question

I am creating two lists of objects. One "new" list, one "old list".

I want to take the value of one property from an object on the new list and set the property on the old list on the matching object the the new value.

//Original Solution
        foreach (RyderQuestion quest in myList)
        {
            //compare it to every question in ryder questions
            foreach (RyderQuestion oldQuestion in _ryderQuestions)
            {
                //if the question ids match, they are the same question, and should have the right selected option
                //selecting the option sets the checkbox of the MultipleChoideQuestionControl
                if (oldQuestion.QuestionID == quest.QuestionID)
                {
                    oldQuestion.SelectedOption = quest.SelectedOption;
                }
            }
        }

I am trying to convert it to LINQ to make it more effecient using joins, but how do i update the value directly?

        var x = from quest in myList
                join oldquest in _ryderQuestions
                on new { quest.QuestionID, quest.ShowOn, quest.QuestionOrder }
            equals new { oldquest.QuestionID, oldquest.ShowOn, oldquest.QuestionOrder }
                select oldquest.SelectedOption = quest.SelectedOption;

This query returns the values to the x list, but I want to actually update the object in the old list instead.

I think your best bet is what I proposed in another q. stackoverflow.com/a/28226427/779513 — Justin Pihony
– Justin Pihony, Commented Jan 30, 2015 at 14:29
If you want to update the old list, I see two options. Keep at least one loop , or build an entirely new "old list" and assign to that object. — ryanyuyu
– ryanyuyu, Commented Jan 30, 2015 at 14:29
Why are you joining in more properties in the linq statement then in the for loop? — Magnus
– Magnus, Commented Jan 30, 2015 at 14:31
@Magnus Im worried about duplicates, ill go back and fix the for loop one day. — DidIReallyWriteThat
– DidIReallyWriteThat, Commented Jan 30, 2015 at 14:54

Magnus · Accepted Answer · 2015-01-30 14:33:08Z

3

Linq is for querying, not updating. You can join the two lists to line up the object to update, but you'll still have to loop to make the changes:

var query = from quest in myList
            join oldquest in _ryderQuestions
            on new { quest.QuestionID, quest.ShowOn, quest.QuestionOrder }
        equals new { oldquest.QuestionID, oldquest.ShowOn, oldquest.QuestionOrder }
            select new {oldquest, quest};

foreach(var item in query}
    item.oldquest.SelectedOption = item.quest.SelectedOption

edited Jan 30, 2015 at 14:33

Magnus

47.3k8 gold badges88 silver badges128 bronze badges

answered Jan 30, 2015 at 14:30

D Stanley

153k12 gold badges190 silver badges257 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

DidIReallyWriteThat Over a year ago

the variables stay in scope outside of the linq query?

DidIReallyWriteThat Over a year ago

I dont have this available...?

D Stanley Over a year ago

@CalvinSmith No, oldquest and quest are properties of a new anonymous type created by the projection select new {oldquest, quest}

D Stanley Over a year ago

@CalvinSmith What do mean it's not available?

DidIReallyWriteThat Over a year ago

thank you, i didnt notice you seleted the questions, not the values

Florian Schmidinger · Accepted Answer · 2015-01-30 14:30:22Z

2

For example:

var x = from quest in myList
            join oldquest in _ryderQuestions
            on new { quest.QuestionID, quest.ShowOn, quest.QuestionOrder }
        equals new { oldquest.QuestionID, oldquest.ShowOn, oldquest.QuestionOrder }

            select new {quest , oldquest};

foreach(var item in x)
{
    item.quest.SelectedOption = item.oldquest.SelectedOption;
}

You mean this?

answered Jan 30, 2015 at 14:30

Florian Schmidinger

4,6822 gold badges19 silver badges28 bronze badges

Collectives™ on Stack Overflow

Updating a list using Linq

2 Answers 2

5 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related