dict.Add() / Remove() / Clear()

using System.Collections.Generic;

// Creates a Dictionary.
Dictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>();

// Adds a key-value pair. Throws ArgumentException if the key already exists.
dict.Add(TKey key, TValue value)

// Removes the element with the specified key. Returns true if the element was successfully removed.
dict.Remove(TKey key)

// Removes all elements.
dict.Clear()

using System;
using System.Collections.Generic;

// Creates a Dictionary<string, int> and adds elements.
Dictionary<string, int> scores = new Dictionary<string, int>();
scores.Add("Tanaka", 85);
scores.Add("Sato", 92);
scores.Add("Suzuki", 78);
Console.WriteLine(scores.Count); // 3

// Gets a value using the indexer.
Console.WriteLine(scores["Tanaka"]); // 85

// Overwrites a value using the indexer (unlike Add(), this does not throw on duplicate keys).
scores["Tanaka"] = 90;
Console.WriteLine(scores["Tanaka"]); // 90

// Creates a Dictionary with initial values.
Dictionary<string, string> capitals = new Dictionary<string, string>
{
    { "Japan", "Tokyo" },
    { "USA", "Washington D.C." },
    { "France", "Paris" }
};

// Removes an element with Remove().
bool removed = capitals.Remove("France");
Console.WriteLine(removed);         // True
Console.WriteLine(capitals.Count);  // 2

// Removes all elements with Clear().
scores.Clear();
Console.WriteLine(scores.Count);    // 0

Dictionary keys must be unique. Calling Add() twice with the same key throws an ArgumentException. If you are unsure whether a key already exists, use the indexer dict[key] = value instead, or check in advance with ContainsKey().

Accessing a non-existent key via the indexer throws a KeyNotFoundException. To retrieve a value safely, use TryGetValue().