C# SortedDictionary in-place update

Question

I have a SortedDictionary defined as:

public SortedDictionary<DateTime,RosterLine> RosterLines = new SortedDictionary<DateTime,RosterLine>();

RosterLine itself is a simple struct:

struct RosterLine {
    public string RosCd;
    public string ActCd;
    public double Hrs;
}

I can .Add(dt, rosterLine) no problems, and iterate through the dictionary fine too.

My problem is trying to update the RosterLine values given a specified date eg.

DateTime currDt = new DateTime(2013,12,02);
RosterLines[currDt].ActCd = "SO"; // error here

It tells me: Cannot modify the return value (dictionary def here) because it is not a variable. My goal is to do this with an iterating loop (which I thought might be the problem), but it won't work outside the loop on its own either (as above).

My question is: how do I update a SortedDictionary with a given key (date)?

Thanks L.B! That did the trick. I'm wondering why though? When I tested a new SortedDictionary that had as its value a simple string, it worked too, so I'm figuring it's something to do with the struct or the way I access it. If you make your comment an answer I'll make it the answer. Thanks again. — Pete855217, Dec 02 '13 at 07:49

Markus · Accepted Answer · 2013-12-02T09:16:11.127

The reason for the error message is that RosterLine is a struct and by that a value type. The error I get in ideone is:

Cannot modify a value type return value of `System.Collections.Generic.SortedDictionary.this[System.DateTime]'. Consider storing the value in a temporary variable

For value types, the dictionary stores a copy of the value and not a reference to the object on the heap. Also, when retrieving the value (as in dict[DateTime.Today]), it is copied again. Therefore, changing a property in the way you do in your sample only works on the copy of the value type. The compiler prevents misunderstandings by the error message - if it wouldn't one would wonder why the value in the dict has not been changed.

    var dict = new SortedDictionary<DateTime, RosterLine>();
    dict.Add(DateTime.Today, new RosterLine());
    // Does not work as RosterLine is a value type
    dict[DateTime.Today].ActCd = "SO";
    // Works, but means a lot of copying
    var temp = dict[DateTime.Today];
    temp.ActCd = "SO";
    dict[DateTime.Today] = temp;

In order to solve this, you could make RosterLine a class or you can work with temp variables as the error message suggests.

Thanks Markus, that explains the situation well. Using the struct, I'm accessing copied values, not the reference to the object itself. I guess making it a class forces me to use the object itself (it's made via a new statement with its inherent pointer). Is that right? — Pete855217, Dec 02 '13 at 09:23
@Pete855217: yes, that's right. When using a reference type (a class), the reference is stored and handed out and you work on the object on the heap itself. — Markus, Dec 02 '13 at 09:26

C# SortedDictionary in-place update

1 Answers1