C# ToDictionary

ToDictionary converts a collection into a Dictionary. It works on IEnumerable collections such as arrays and Lists. We can it to optimize performance—while retaining short and clear code. This method simplifies the demands of the code.

This C# tutorial shows how to use the ToDictionary extension from System.Linq.

Example

First, this is a simple example of using the ToDictionary method in LINQ. We have built many Dictionary instances before. You simply loop over each element in your array and add it to the dictionary if it isn't already there. LINQ gives us an extension method called ToDictionary, which can greatly simplify some code.

Program that uses ToDictionary [C#]

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
	// Example integer array.
	int[] values = new int[]
	{
	    1,
	    3,
	    5,
	    7
	};

	// First argument is the key, second the value.
	Dictionary<int, bool> dictionary = values.ToDictionary(v => v, v => true);

	// Display all keys and values.
	foreach (KeyValuePair<int, bool> pair in dictionary)
	{
	    Console.WriteLine(pair);
	}
    }
}

Output

[1, True]
[3, True]
[5, True]
[7, True]

ToDictionary has two arguments. There is a single argument and a two argument version of ToDictionary. The first argument uses lambda expressions to set the key, and the second for values. The Dictionary used here is an int Dictionary.

Lambda expressions. Look at the "v => v" style lines. Lambda can be expressed as "goes to," meaning each item in the array goes to itself. There is more detailed information on this site about lambda expressions.

Example 2

You can use this method with a List of strings. Dictionaries are most useful for strings and string lookups. This allows us to use a number—hash code—in place of a string, greatly speeding things up. Here we use the var keyword to simplify the syntax.

Program that uses ToDictionary and List [C#]

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
	// Example with strings and List.
	List<string> list = new List<string>()
	{
	    "cat",
	    "dog",
	    "animal"
	};
	var animals = list.ToDictionary(x => x, x => true);
	if (animals.ContainsKey("dog"))
	{
	    // This is in the Dictionary.
	    Console.WriteLine("dog exists");
	}
    }
}

Output

dog exists

LINQ

The benefit of LINQ is that it allows us to use fewer lines of boilerplate code to insert elements into a Dictionary. This reduces lines of code and typos. It is elegant and fits well with other LINQ code.

Negatives. LINQ methods are significantly slower, but will scale equally well. Many programmers may not understand lambda syntax—although they should learn. Sometimes, using ToDictionary may prevent you from combining the loop with another operation, also leading to inefficient code.

Summary

We looked at how you can use LINQ and the ToDictionary extension method to quickly transform one kind of collection such as an array or List into a Dictionary collection, which provides constant-time lookups. This is useful in some cases where performance is not critical and short code is more important. For an alternative to the ToDictionary method, please see the ToLookup method.