Dictionaries are useful in situations where a set of values need to be associated with a set of keys (such as words and their definitions in an actual dictionary). For example, you might use a dictionary if you have a class full of students that each has a single grade for that class. In this case, the student's name would be the key and the student's grade would be the value.

// instantiate dictionary
Dictionary<string, double> studentGrades = new Dictionary<string, double>();

// add key value pairs
studentGrades.Add("John", 92.5);
studentGrades.Add("Jane", 97.6);
studentGrades.Add("Billy", 84.2);
studentGrades.Add("Sally", 78.0);

// access value for a specific key
double grade = studentGrades["John"];

// attempt to get value for key
if (studentGrades.TryGetValue("Billy", out grade))
{
	// do stuff with grade here
}

// see if dictionary contains a key
if (studentGrades.ContainsKey("Sally"))
{
	// do stuff with key here
}