C# issue with dictionarys

Question

I wrote the following code structure:

public Dictionary<string, Dictionary<string, Object>> file_data = new Dictionary<string, Dictionary<string, Object>>();

public Form1()
{
    InitializeComponent();

    Dictionary<string, Object> temporary = new Dictionary<string, Object>();
    temporary.Add("content", null);
    temporary.Add("droplist", false);
    temporary.Add("valid_file", false);
    file_data.Add("base_data", temporary);
    file_data.Add("supplier_data_1", temporary);
    file_data["base_data"]["droplist"] = true;
    MessageBox.Show(file_data["supplier_data_1"]["droplist"].ToString());
}

I just wanted to update ["base_data"]["droplist"], but the code updates the ["supplier_data_1"]["droplist"] value as well (messagebox shows true). Why is it doing that? How do I have to adapt the code that the file_data["base_data"]["droplist"] will be changed only? The Dictionary has to keep it's structure.

base_data and supplier_data_1 point to the same Dictionary. You can make supplier_data_1 point to a separate copy of the Dictionary by doing `file_data.Add("supplier_data_1", new Dictionary(temporary));` — Hans Kilian, Feb 20 '19 at 13:00

hardkoded · Accepted Answer · 2019-02-20T13:02:09.967

4

Object is a reference type. Microsoft Value Types post has a good explanation

A variable of a value type contains a value of the type. For example, a variable of the int type might contain the value 42. This differs from a variable of a reference type, which contains a reference to an instance of the type, also known as an object. When you assign a new value to a variable of a value type, that value is copied. When you assign a new value to a variable of a reference type, the reference is copied, not the object itself.

You should clone that dictionary. Like it's explained here. You could do something like this:

file_data.Add("supplier_data_1", CloneDictionary(temporary));

Or even better, create a new Dictionary based on the first one.

file_data.Add("supplier_data_1", new Dictionary<string, object>(temporary));

edited Feb 20 '19 at 13:02

answered Feb 20 '19 at 12:56

hardkoded

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I'd like to know, what makes you believe creating a new Dictionary based on the first one is *even better* than cloning it? – Rafalon Feb 20 '19 at 13:04
You'd have to clone all the `object` in the `Dictionary` or changes to any reference type in that one would be changed in both. – Mikael Dúi Bolinder Feb 20 '19 at 13:07
Basically, because you're using code built-in .NET instead of coding your own clone code. – hardkoded Feb 20 '19 at 13:07
2

That's true @MikaelDúiBolinder. But he's using only value types there. But good point. In that case, you would need a deep clone algorithm. – hardkoded Feb 20 '19 at 13:08
Then maybe using `using ValueTypeDictionary = Dictionary`? – Mikael Dúi Bolinder Feb 28 '19 at 07:27

score 0 · Answer 2 · answered Feb 20 '19 at 12:59

You're adding the same object twice:

file_data.Add("base_data", temporary);
file_data.Add("supplier_data_1", temporary);

The temporary variable points to an object in memory. Now two references in the dictionary also point to that object in memory. But there's still only one object in memory. Any update made to that object will be seen by any reference to that object.

In order to have two different objects in the dictionary, you need to create a second object. Which you might do manually by creating two variables in your code, populating each of them, and adding them to that resulting dictionary. Or you might also just create a new dictionary around the same values if they're all simple value types:

file_data.Add("supplier_data_1", temporary.ToDictionary(x => x.Key, x => x.Value));

Mikael Dúi Bolinder · Answer 3 · 2019-02-20T13:11:25.127

As @hardkoded said, it's a reference type.

// Here you define `temporal` object
Dictionary<string, Object> temporary = new Dictionary<string, Object>();

// Here you add a reference/pointer to the `temporal` object as values to these keys
file_data.Add("base_data", temporary);
file_data.Add("supplier_data_1", temporary);

// When you get the values for the "base_data" and "supplier_data_1" keys, you get a refence to the `temporal`object.
var baseData = file_data["base_data"];
var supplierData1 = file_data["supplier_data_1"];

// If you compare those variables, you'll find out that they are references to the same object!
if (baseData == supplierData1) // true
    Debug.WriteLine("They are the same!");

So, any changes made to file_data["base_data"] will apply to file_data["supplier_data_1"] as they are currently references/pointers to the same object!

C# issue with dictionarys

3 Answers3