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)

Program.cs

using System;
using System.Collections.Generic;

Dictionary<string, int> scores = new Dictionary<string, int>
{
    { "Kogami Shinya", 85 },
    { "Tsunemori Akane", 92 },
    { "Ginoza Nobuchika", 78 }
};

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

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

// Check with ContainsKey() before adding a new entry.
string newKey = "Masaoka Tomomi";
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

Run the following command:

dotnet run
Tsunemori Akane's score: 92
No data for Masaoka. Value: 0
Added Masaoka Tomomi.
True
False

TryGetValue() is commonly used in cache read/write patterns. If the key exists, retrieve from the cache; otherwise, compute and store the result.

CacheSample.cs

using System;
using System.Collections.Generic;

Dictionary<string, string> cache = new Dictionary<string, string>();

string GetData(string key)
{
    if (cache.TryGetValue(key, out string cached))
    {
        Console.WriteLine($"Cache hit: {key}");
        return cached;
    }
    string value = $"Data:{key} (computed)";
    cache[key] = value;
    Console.WriteLine($"Cache miss: computed {key}.");
    return value;
}

Console.WriteLine(GetData("Kogami Shinya"));
Console.WriteLine(GetData("Tsunemori Akane"));
Console.WriteLine(GetData("Kogami Shinya")); // retrieved from cache on second call

Run the following command:

dotnet run
Cache miss: computed Kogami Shinya.
Data:Kogami Shinya (computed)
Cache miss: computed Tsunemori Akane.
Data:Tsunemori Akane (computed)
Cache hit: Kogami Shinya
Data:Kogami Shinya (computed)

Common Mistake: Accessing a Non-Existent Key with the Indexer

Using dict[key] to access a key that does not exist throws a KeyNotFoundException. When a key's existence is uncertain, use TryGetValue() instead.

using System;
using System.Collections.Generic;

// NG: accessing a non-existent key via the indexer
Dictionary<string, int> scores = new Dictionary<string, int>
{
    { "Kogami Shinya", 85 }
};

// int score = scores["Masaoka Tomomi"]; // KeyNotFoundException!

The same logic can also be written as:

using System;
using System.Collections.Generic;

// OK: use TryGetValue() to retrieve safely
Dictionary<string, int> scores = new Dictionary<string, int>
{
    { "Kogami Shinya", 85 }
};

if (scores.TryGetValue("Masaoka Tomomi", out int score))
{
    Console.WriteLine($"Score: {score}");
}
else
{
    Console.WriteLine("Data not found.");
}

Run the following command:

dotnet run
Data not found.

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.