CODEWITHSUNDEEP
c#
James Stevens
James Stevens

Reputation: 3

Preferred method to get sole key from a C# Dictionary that only has one key?

I'm writing a code block that prints the data in a dictionary, and I'm trying to format the display differently based on the number of keys.

I'd like to have output like:

Etc.

The code I have is something like:

Dictionary<string, string> dict = new Dictionary<string,string>();

//code that populates the dictionary

string output;
if(dict.Count == 0)
{
   output = "There's nothing in the dictionary."
}
else if(dict.Count ==1)
{
   string only_key;
   foreach (string key in dict.Keys)
   {
      //Surely there is a better way to do this
      only_key = key;
   }
   output = "Entry " + only_key + " has value " + dict[key] + ".";
}
else
{
   StringBuilder sb = new StringBuilder();
   sb.Append("The following entries are in the dictionary:");
   foreach (string key in dict.Keys)
   {
      sb.Append(Environment.NewLine + " * " + key + ": " + dict[key];
   }
   output = sb.ToString();
}

I don't like doing a foreach loop on the dictionary to get the key when I know it only has one key, but I'm not sure of a better method.

What would be a better way to do that?

Upvotes: 0

Views: 879

Answers (4)

Victor Zakharov
Victor Zakharov

Reputation: 26414

You could use LINQ's First extension method to get the first (and only) key/value pair:

else if (dict.Count == 1)
{
  var keyValuePair = dict.First();
  output = "Entry " + keyValuePair.Key + " has value " + keyValuePair.Value + ".";
}

Speaking about LINQ, you could use it to replace your else block also:

else
{
  var header = "The following entries are in the dictionary:" + Environment.NewLine;
  var kvPairs = dict.Select(p => " * " + p.Key + ": " + p.Value);
  output = header + string.Join(Environment.NewLine, kvPairs);
}

Upvotes: 0

Scott Hannen
Scott Hannen

Reputation: 29207

You're right - you don't need to loop through the dictionary keys if it only has one item.

If you know there's only one item then you can do this:

dict.Values.First()

or

dict.Values.Single()

The difference is that Single throws an exception if there's not exactly one item. Another answer suggested using this in case something gets added to the dictionary right after you checked that the count == 1. I disagree. If this is something multithreaded where another process could be modifying the dictionary then the next part where you iterate through the keys in the dictionary would also be at risk for the same reason. But unless you know that you're doing something multithreaded you don't need to worry about that. (And if it was you would use ConcurrentDictionary<string,string>.)

A good pattern to follow when handling a collection is to handle any number of items (1 or more) the same way. If you're creating a list of items from the dictionary and there's only one item, then create a list of one item. Something like

Your dictionary has (1) items:
Key: abc Value: xyz

Then you could choose to handle it differently if there are no items.

Upvotes: 0

Suraj
Suraj

Reputation: 36547

You can use LINQ:

string only_key = dict.Keys.Single();

An advantage of Single (besides eliminating the foreach) is that it will throw an exception if an object is added to the dictionary in-between the call to dict.Count == 1 and string only_key = dict.Keys.Single();

Upvotes: 1

CodingYoshi
CodingYoshi

Reputation: 27009

You do not need to index the dictionary when you are traversing it. You can access both the key and value when you are traversing it like this:

Dictionary<string, string> dict = new Dictionary<string, string>();
foreach (var thisItem in dict)
{
    output = $"Entry {thisItem.Key} has value {thisItem.Value}.";
}

Upvotes: 0

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