CODEWITHSUNDEEP
c#entity-frameworklinq
B--rian
B--rian

Reputation: 5880

How to change the value of a variable within a child of a nested list

Being new rather new to entity framework, LINQ and C#, I am not sure how to change the value of a variable in a nested list. I am aware that LINQ is not meant to update objects, but I read somewhere that you can do it somehow and that's what I am trying here. I am happy with any solution.

Assume we have two lists, myData with data of the current month, and myDataBefore with the same list from the previous month. I want to update myData. More specifically I want to change (in SQL you would say UPDATE) the valuePrevMonth and deltaPercentage for all 1st-level children inside that list - everything else should remain untouched. I tried the following, but this does not have the desired effect.

var myChildren = myData.SelectMany(x => x.children).ToList();
foreach (var child in myChildren)
  {
     var valuePrevMonth = myDataBefore.SelectMany(x => x.children)
                           .Where(x => (x.PositionId == child.PositionId && 
                                        x.SubPositionId == child.SubPositionId))
                           .Select(x => x.Value).FirstOrDefault();
     // now comes the write-back step which I am probably messing up
     myData.SelectMany(x => x.children)
            .Where(x => (x.PositionId == child.PositionId &&
                         x.SubPositionId == child.SubPositionId))
            .Select(x => { 
                 x.previousValue = valuePrevMonth;
                 x.deltaPercentage = (x.Value - valuePrevMonth) / valuePrevMonth; // probably some IF-NULL check would not hurt
                 return x;
               }).ToList();
  }

The last .Select(..) is my current understanding how to update these two values of a given child object.

Some background

My nested list List<myData> is of following format (please allow me to use some kind of pseudocode for the definition):

myData 
    {
        int? PositionId 
        string PositionName 
        int SubPositionId 
        string SubPositionName
        myObject children 
        decimal? Value
        decimal? previousValue 
        decimal? deltaPercentage 
    }

References

Upvotes: 1

Views: 424

Answers (1)

Ben Leedom
Ben Leedom

Reputation: 13

My first thought is that your second statement beginning with .SelectMany(x => x.Children) may be unnecessary; if I'm reading it right, the thing you want to update is already in the child variable, so you would just do:

child.previousValue = valuePrevMonth;
...

That said, it may matter whether your myObject type is a struct or a class; if it's a struct, updating its fields will not do what you want, because you would be operating on a copy. To update elements of a List of structs, you have to get the updated struct value that you want (e.g., by setting fields on a copy of the value), and then update the entire list element with list[x] = updatedValue;.

Upvotes: 1

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