Tips and Tricks #68: Freeze Collections for Thread-Safe Read Access

Use FrozenDictionary and FrozenSet for immutable, highly-optimized read-only collections.

Code Snippet

using System.Collections.Frozen;

// Create once at startup
private static readonly FrozenDictionary _lookup = 
    new Dictionary
    {
        ["alpha"] = 1,
        ["beta"] = 2,
        ["gamma"] = 3
    }.ToFrozenDictionary();

// Ultra-fast lookups, no locking needed
public int GetValue(string key) => 
    _lookup.TryGetValue(key, out int val) ? val : -1;

Why This Helps

• Optimized internal structure for read performance
• Thread-safe without locks
• Lower memory overhead than concurrent collections

How to Test

• Benchmark lookup times vs Dictionary
• Verify thread safety under concurrent access

When to Use

Configuration data, lookup tables, any collection populated once and read many times.

Performance/Security Notes

Available in .NET 8+. Creation is slower than regular collections but reads are faster.

References

• https://learn.microsoft.com/en-us/dotnet/api/system.collections.frozen

Try this tip in your next project and share your results in the comments!