Quick Answer

Handle missing dictionary keys.

Understanding the Issue

Occurs when trying to access a non-existent key in a dictionary collection.

The Problem

This code demonstrates the issue:

Csharp Error 
var dict = new Dictionary<string, int>();
var value = dict["missing"]; // Throws

The Solution

Here's the corrected code:

Csharp Fixed 
// Solution 1: Check key existence
if (dict.ContainsKey("key")) {
    value = dict["key"];
}

// Solution 2: TryGetValue
if (dict.TryGetValue("key", out var val)) {
    // Use val
}

// Solution 3: Dictionary extensions
public static TValue GetValueOrDefault<TKey, TValue>(
    this Dictionary<TKey, TValue> dict, 
    TKey key, 
    TValue defaultValue = default) {
    return dict.TryGetValue(key, out var value) ? value : defaultValue;
}

// Solution 4: ConcurrentDictionary
var concurrentDict = new ConcurrentDictionary<string, int>();
value = concurrentDict.GetOrAdd("key", k => 0);

// Solution 5: Default value pattern
value = dict.ContainsKey("key") ? dict["key"] : 0;

Key Takeaways

Always check for key existence or use TryGetValue for safe dictionary access.