Question

Something like this:

Dictionary<int, string> myData = new Dictionary<int, string>();
myData.Add(1, "England");
myData.Add(2, "Canada");
myData.Add(3, "Australia");
myTreeView.Node[0].Tag = myData;

Then I want to get this object, how should I do it ?
Like:

string str = new string();
str = myTreeView.Node[0].Tag[2]; // "str" should be equal to "Canada"
myTreeView.Node[0].Tag[1] = "Spain";
str = myTreeView.Node[0].Tag[1]; // now "str" is equal to "Spain"

Second question – what will return this expression:

Dictionary<int, string> myData = new Dictionary<int, string>();
myData.Add(1, "England");
myData.Add(2, "Canada");
myData.Add(3, "Australia");

string str1 = new string();
str = myData[4]; // there isn't such a key as 4

Exception or null ?

Answer 1

Control.Tag is typed as object so you’ll need to cast it to access it as a Dictionary<int, string>:

Dictionary<int, string> dict = (Dictionary<int, string>)myTreeView.Node[0].Tag;
string str = dict[2];

And similarly to set a value:

var dict = (Dictionary<int, string>)myTreeView.Node[0].Tag;
dict[1] = "Spain";

If you try to access a non-existent key, a KeyNotFoundException will be thrown. You can check if the dictionary contains a given key using TryGetValue or ContainsKey:

if(dict.ContainsKey(key))
{
    var value = dict[key];
}
else
{
}

TryGetValue does the lookup and sets the given variable to the value (it it exists) in a single call, so is usually preferred.

string value;
if(dict.TryGetValue(key, out value))
{
    //use value
}
else { ... }