## Why is this an issue?

When using the `ConcurrentDictionary`, there are many overloads of the `GetOrAdd` and `AddOrUpdate` methods that take both a `TKey` argument and a lambda that expects a `TKey` parameter. This means that the right side of the lambda can be written using either the lambda’s parameter or the method’s argument. However, using the method’s argument leads to the lambda capturing it, and the compiler will need to generate a class and instantiate it before the call. This means memory allocations, as well as more time spend during Garbage Collection.

### What is the potential impact?

We measured a significant improvement both in execution time and memory allocation. For more details see the `Benchmarks` section from the `More info` tab.

## How to fix it

When you are using the `ConcurrentDictionary` methods `GetOrAdd` or `AddOrUpdate`, reference the key by using the lambda’s parameter instead of the method’s one.

### Code examples

#### Noncompliant code example

    int UpdateValue(ConcurrentDictionary<int, int> dict, int key) =>
        dict.GetOrAdd(key, _ => key + 42);

#### Compliant solution

    int UpdateValue(ConcurrentDictionary<int, int> dict, int key) =>
        dict.GetOrAdd(key, x => x + 42);

## Resources

### Documentation

- ConcurrentDictionary<TKey,TValue>.GetOrAdd
- ConcurrentDictionary<TKey,TValue>.AddOrUpdate

### Benchmarks

Method | Runtime | Mean | Standard Deviation | Allocated
--- | --- | --- | --- | ---
Capture | .NET 7.0 | 68.52 ms | 4.450 ms | 88000063 B
Lambda | .NET 7.0 | 39.29 ms | 3.712 ms | 50 B
Capture | .NET Framework 4.6.2 | 74.58 ms | 5.199 ms | 88259787 B
Lambda | .NET Framework 4.6.2 | 42.03 ms | 2.752 ms | -

#### Glossary

- Mean
- Standard Deviation
- Allocated

The results were generated by running the following snippet with BenchmarkDotNet:

    private ConcurrentDictionary<int, string> dict;
    private List<int> data;

    [Params(1_000_000)]
    public int N { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        dict = new ConcurrentDictionary<int, string>();
        data = Enumerable.Range(0, N).OrderBy(_ => Guid.NewGuid()).ToList();
    }

    [Benchmark(baseline=true)]
    public void Capture()
    {
        foreach (var guid in data)
        {
            dict.GetOrAdd(guid, _ => $"{guid}"); // "guid" is captured
        }
    }

    [Benchmark]
    public void Lambda()
    {
        foreach (var guid in data)
        {
            dict.GetOrAdd(guid, x => $"{x}"); // no capture
        }
    }

Hardware configuration:

    BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2846/22H2/2022Update)
    11th Gen Intel Core i7-11850H 2.50GHz, 1 CPU, 16 logical and 8 physical cores
    .NET SDK=7.0.203
      [Host]               : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
      .NET 7.0             : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
      .NET Framework 4.6.2 : .NET Framework 4.8.1 (4.8.9139.0), X64 RyuJIT VectorSize=256