Why can't I add a new pair into a SortedDictionary?

Question

I have this SortedDictionary:

var signature_parameters = new SortedDictionary<string, string>
{
    { "oauth_callback", SocialEngine.twitter_aggrega_redirect_uri },
    { "oauth_consumer_key", SocialEngine.twitter_aggrega_consumer_key },
};

now I tried this:

var header_parameters = signature_parameters.Add("oauth_signature", Uri.EscapeDataString(oauth_signature));

but it says I can't assign a void variable to a implicit method.

Where am I wrong?

What would you like to assign to `header_parameters`? A new dictionary? — Adam Houldsworth, Oct 09 '13 at 10:24
@AdamHouldsworth: I assume he wants to have the `KeyValuePair` of the last inserted item. (_"Why can't I add a new pair..."_) — Tim Schmelter, Oct 09 '13 at 10:30
@TimSchmelter Others assume he wants a new dictionary with that item added. Hence my question. I don't like assumptions. — Adam Houldsworth, Oct 09 '13 at 10:31
@AdamHouldsworth: I don't like `var` in this case. But method chaining is a known pattern. http://stackoverflow.com/questions/1119799/method-chaining-in-c-sharp Also, markzzz is a fan of `KeyValuePairs` http://stackoverflow.com/q/19251781/284240 ;) — Tim Schmelter, Oct 09 '13 at 10:35
@TimSchmelter I know it is, but there are still assumptions about what is being returned to be chained. Now there is an accepted answer I suppose we know what was wanted :-) — Adam Houldsworth, Oct 09 '13 at 10:38

score 2 · Accepted Answer · answered Oct 09 '13 at 10:23

Where am I wrong?

You're calling the Add method, but expecting it to return something. It doesn't.

If you're trying to create a new dictionary with the same entries and one new one, you probably want:

var headerParameters = new SortedDictionary<string, string>(signatureParameters)
{
    "oauth_signature", Uri.EscapeDataString(oauth_signature)
};

The constructor will create a copy of the existing dictionary, then the collection initializer will add the key/value pair just to the new one.

score 1 · Answer 2 · answered Oct 09 '13 at 10:21

Add method adds item to dictionary, but returns nothing, so you can't assign result of Add method to variable.

signature_parameters.Add("oauth_signature", Uri.EscapeDataString(oauth_signature))

You can assign it later, but because it is a reference type, you'll just copy the reference.

var header_parameters = signature_parameters;

score 0 · Answer 3 · answered Oct 09 '13 at 10:22

Add is a void method since it adds an item to the dictionary but it does not return something.

So change this

var header_parameters = signature_parameters.Add("oauth_signature", Uri.EscapeDataString(oauth_signature));

to

var header_parameters = new KeyValuePair<string, string>("oauth_signature", Uri.EscapeDataString(oauth_signature));
signature_parameters.Add(header_parameters.Key, header_parameters.Value);

( assuming that you want to have the last inserted KeyValuePair )

Why can't I add a new pair into a SortedDictionary?

3 Answers3