bool Dictionary foreach List Property String Literal

`NameValueCollection`

This C# type allows many values for one key. It is found in System.Collections Specialized, and it does not provide the best performance.

Other collections are likely to be faster than NameValueCollection. Unless you need to use NameValueCollection, you should consider Dictionary or List.

Example

We add 4 string key-value pairs to a NameValueCollection. The keys can occur more than once, but map to the same array of values. You can have one key pointing to two values.

Part 1 We create a new NameValueCollection. The collection maps two keys to 4 values.

Part 2 We access keys by using the AllKeys property. We use it in a foreach-loop to see all the keys in the collection.

Part 3 The collection returns null if the key is not found. If more than one value is found, it returns a string value joined by commas.

Part 4 With the HasKeys method, NameValueCollection can test whether there are keys.

Part 5 GetKey with an index to gets the key at that index. This method always receives an int, not a string.

Part 6 Get returns the value at that index. This method also receives an int (like GetKey).

using System;
using System.Collections.Specialized;

class Program
{
    static void Main()
    {
        // Part 1: create new NameValueCollection.
        var collection = new NameValueCollection();
        collection.Add("cat", "orange");
        collection.Add("dog", "brown");
        collection.Add("dog", "white");
        collection.Add("cat", "black");

        // Part 2: loop over keys.
        foreach (string key in collection.AllKeys)
        {
            Console.WriteLine(key);
        }

        // Part 3: get values.
        Console.WriteLine(collection["cat"]);
        Console.WriteLine(collection["x"] == null);

        // Part 4: use HasKeys.
        Console.WriteLine(collection.HasKeys());

        // Part 5: use GetKey.
        Console.WriteLine(collection.GetKey(0));

        // Part 6: get the first value.
        string value = collection.Get(0);
        Console.WriteLine(value);
    }
}cat
dog
orange,black
True
True
cat
orange,black

Benchmark

Does the NameValueCollection have competitive performance to the Dictionary? In some programs, we can use a NameValueCollection instead of a Dictionary.

Version 1 We access a value by passing a key to the NameValueCollection in this version.

Version 2 We use the Dictionary and access a value associated with a key. We use the same key as in the other version.

Result In 2024, NameValueCollection lookup speed on even a small collection is poor. Its performance is likely much worse in many situations.

#define VERSION1

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Diagnostics;

class Program
{
    public static void Main()
    {
        var collection = new NameValueCollection();
        collection.Add("cat", "orange");
        collection.Add("dog", "brown");

        var dictionary = new Dictionary<string, string>();
        dictionary.Add("cat", "orange");
        dictionary.Add("dog", "brown");

        const int _max = 10000000;
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
#if VERSION1
            string value = collection["dog"];
#endif
#if VERSION2
            string value = dictionary["dog"];
#endif
        }
        s1.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }
}89.20 ns    NameValueCollection
10.83 ns    Dictionary

ASP.NET

The NameValueCollection is prevalent in ASP.NET, and it is used in appSettings, QueryString, and Headers collections. In many projects it is used with appSettings.

Summary

NameValueCollection is located in the System.Collections Specialized namespace. It allows you to associate one string key with multiple string values.