Coding Bolt

As a .NET developer, you’ve probably worked a lot with Task for handling asynchronous operations. While Task is great, it isn’t always the most efficient option. Enter ValueTask! This nifty little feature can help you boost performance in certain scenarios.

What is ValueTask?

ValueTask is a struct that can be used to represent asynchronous operations. Unlike Task, which is a reference type, ValueTask can avoid heap allocations in certain situations, making it more efficient for high-performance scenarios.

When to use ValueTask

ValueTask is particularly useful when:

• The result of an asynchronous operation is often already available.
• You need to avoid the overhead of allocating a Task object for frequently occurring operations.

An edge case that you should not use it, is when you are awaiting a ValueTask method twice. I haven’t found a reason as to why you would do that but keep it in mind.

Practical Example

Let’s consider a practical example where we have a method that retrieves data from a cache. If the data is available in the cache, it returns the result immediately. Otherwise, it performs an asynchronous operation to fetch the data.

public class DataService
{
    private readonly Dictionary<int, string> _cache = new Dictionary<int, string>();

    public ValueTask<string> GetDataAsync(int id)
    {
        if (_cache.TryGetValue(id, out var cachedData))
        {
            return new ValueTask<string>(cachedData);
        }
        else
        {
            return new ValueTask<string>(FetchDataAsync(id));
        }
    }

    private async Task<string> FetchDataAsync(int id)
    {
        await Task.Delay(1000); // Simulate an async operation
        var data = $"Data for {id}";
        _cache[id] = data;
        return data;
    }
}

If the data is already present in our cache, we get a small performance benefit. If you don’t get many cache misses, ValueTask is something you should use.

Benchmarking Task vs. ValueTask

To understand the performance implications, let’s create a benchmark to compare Task and ValueTask.

[MemoryDiagnoser]
public class TaskVsValueTaskGood
{
    private static Test _test = new Test();

    [Benchmark]
    public async Task<Test> GetTestTask()
    {
        return await GetTest();
    }

    [Benchmark]
    public async ValueTask<Test> GetTestValueTask()
    {
        return await GetTestValue();
    }

    private async Task<Test> GetTest()
    {
        await Task.CompletedTask;

        return _test;
    }

    private async ValueTask<Test> GetTestValue()
    {
        await ValueTask.CompletedTask;

        return _test;
    }
}

public class Test
{
    public string Name { get; set; }
}

Conclusion

ValueTask can offer performance benefits in scenarios where the result is often readily available or when you need to avoid the overhead of allocating a Task. By benchmarking Task and ValueTask, you can make informed decisions about which type to use based on your specific use case, ,as you should in all cases when it comes to performance.

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