dict.TryGetValue() / ContainsKey()

using System.Collections.Generic;

// Returns true and the value if the key exists; returns false and the default value if it does not.
dictionary.TryGetValue(TKey key, out TValue value)

// Returns true if the specified key exists in the dictionary.
dictionary.ContainsKey(TKey key)

// Returns true if the specified value exists in the dictionary (unlike key lookup, this is an O(n) operation).
dictionary.ContainsValue(TValue value)

using System;
using System.Collections.Generic;

Dictionary<string, int> scores = new Dictionary<string, int>
{
    { "Alice", 85 },
    { "Bob", 92 },
    { "Carol", 78 }
};

// Safely retrieve a value using TryGetValue().
if (scores.TryGetValue("Bob", out int bobScore))
{
    Console.WriteLine($"Bob's score: {bobScore}"); // Bob's score: 92
}

// Returns false if the key does not exist.
if (!scores.TryGetValue("Dave", out int daveScore))
{
    Console.WriteLine($"No data for Dave. Value: {daveScore}"); // Value: 0 (default)
}

// Check with ContainsKey() before adding a new entry.
string newKey = "Dave";
if (!scores.ContainsKey(newKey))
{
    scores.Add(newKey, 70);
    Console.WriteLine($"Added {newKey}.");
}

// Check whether a value exists using ContainsValue().
Console.WriteLine(scores.ContainsValue(92)); // True
Console.WriteLine(scores.ContainsValue(100)); // False

Accessing a dictionary with dictionary[key] throws a KeyNotFoundException if the key does not exist. When you need to check for a key before retrieving its value, a single call to TryGetValue() is more efficient than the two-step approach of using if + ContainsKey().

For retrieving and iterating over dictionary keys and values, see Keys / Values / foreach.