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C# Dictionary

Dictionary in .NET

A C# Dictionary is a generic data structure added to the .NET Framework in version 2.0. It is a way to store key/value pairs. In contrast, a List (or ArrayList) can only store values.

There are different types of dictionary C# data structures available. This article will cover Dictionary and SortedDictionary.

Basic Dictionary Operations

The basic operations are the same for any generic dictionary class. Since dictionaries are generic, the types of the key and value must be specified when creating a dictionary, for example:

Dictionary<string, int> dictionary = new Dictionary<string, int>();

Adding data to a dictionary is similar to adding values to a List, except that it takes a key and value argument:

dictionary.Add("1", 1);

Once data is stored inside a C# dictionary, you use the key values as indices to retrieve values. To put it in context, a List uses numbers as keys (indices) to get its values:

int j = dictionary["1"];

Traversing a dictionary (going through each element) is tricky because the keys can be anything within its data type. The only way to access all the values in a C# dictionary is to know all the keys. That is when the Keys property comes in, which holds a collection of any key added. Here is an example on iterating through a C# dictionary:

List<string> keys = new List<string>(dictionary.Keys);
for (int i = 0; i < keys.Count; i++)
    int j = dictionary[keys[i]];

As mentioned before, the above operations work the same way on a SortedDictionary.

Dictionary vs. SortedDictionary

The difference between Dictionary and SortedDictionary start with the obvious. A Dictionary keeps elements in the order the were added. Meanwhile a SortedDictionary keeps elements sorted based on their comparer.

For example, adding the following key/value pairs to a Dictionary and then outputting the values in order would yield: (1, 1), (3, 3), (2, 2), (4, 4) -> 1, 3, 2, 4.

Adding the same input to a SortedDictionary would result in 1, 2, 3, 4. So elements are automatically sorted in a SortedDictionary.

The result is advantages and disadvantages for each data structure that have to be considered to choose the best one for a task.


Two things to consider: insertion time and searching time. Because elements are not automatically sorted, inserting elements is faster.

Similarly because elements are not sorted, it makes searching more complicated, meaning searching is slower.


In a SortedDictionary elements must be sorted when they are added, making insertion times slower.

But since elements are kept stored, searching can be done with binary search, meaning searching is faster.


While basic operations are written the same way for the types of C# Dictionary, insertion and searching times must be considered. Also consider what would happen if data being added was already sorted...

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