c#: Dictionary TryGetValue creating instance instead of getting reference

Question

I'm a total noob with c#, and cannot figure out why the same method works in different ways. I'm making a simple spreadsheet application, and am using a Dictionary of cells, where the key is a string name and the value is a Cell object:

public struct Cell
{
    private string Name { get; }
    public Object Content { get; set; }

    public Cell(string n, Object o)
    {
        Name = n;
        Content = o;
    }
}

Now, I'll need to be able to easily add/change the contents of the cell, so I've been doing this:

Dictionary<string, Cell> cells = new Dictionary<string, Cell>();

//  Assign new cell to 5.0 & print
cells.Add("a1", new Cell("a1", 5.0));
Console.WriteLine(cells["a1"].Content);     //  Writes 5

//  Assign cell to new content & print
cells.TryGetValue("a1", out Cell value);
value.Content = 10.0;
Console.WriteLine(cells["a1"].Content);     //  Writes 5
Console.ReadKey();

Of course, the dictionary creates the new cell just fine, but when I use TryGetValue, the new content for the cell doesn't make it to the actual object I'm trying to get. I was expecting the second print to be 10. In debug, it seems like it instantiates a new Cell instead of getting the reference of the cell at hand.

I've used a Dictionary before, and I've used TryGetValue to change properties of the existing object. So here's two questions: What am I doing wrong in this case, and what factors determine if the method returns a reference or not?

[structs](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/structs) are value types. So you will retrieve a copy instead of the reference — Link, Feb 16 '18 at 20:13

NetMage · Accepted Answer · 2018-02-16T20:25:54.033

Cell is a struct. It is not recommended that you use a struct for objects that can be modified. I think you just found out why.

When TryGetValue returns the struct, it copies it into a value, which is a different struct than the one in the Dictionary.

Imagine if you replaced struct by int - another value type - would you expect assigning to the int from TryGetValue to change the Dictionary entry int?

If other constraints require you use a struct, you will need to update the cells Dictionary with the new struct, just as you would with any other value type:

Dictionary<string, Cell> cells = new Dictionary<string, Cell>();

//  Assign new cell to 5.0 & print
cells.Add("a1", new Cell("a1", 5.0));
Console.WriteLine(cells["a1"].Content);     //  Writes 5

//  Assign cell to new content & print
cells.TryGetValue("a1", out Cell value);
value.Content = 10.0;
cells["a1"] = value;  // update cells Dictionary
Console.WriteLine(cells["a1"].Content);     //  Writes 5
Console.ReadKey();

Perfect, when I replaced 'struct' with 'class', it met my expectations. I guess the real problem here is that, per my professor's specifications, Cell is to be a struct. I suppose I could change the Dictionary to be (where Object is the cell's contents), but wouldn't that just eliminate the need for a Cell struct in the first place? Any insight? — crapolantern, Feb 16 '18 at 20:21

score 1 · Answer 2 · answered Feb 16 '18 at 20:19

1

You need to make your struct Cell into a class Cell.

That's because struct is a value type and it's content can't be changed by reference. If you want to go in detail, you can read about difference of value and reference types here.

answered Feb 16 '18 at 20:19

Glitch

42
9

c#: Dictionary TryGetValue creating instance instead of getting reference

2 Answers2