What is ValueTask?
ValueTask is a structure introduced in .NET for scenarios requiring micro-optimizations in asynchronous programming. It provides an alternative to Task with better performance for specific use cases, such as when the result of an asynchronous operation is already available or can be synchronously produced. Key Characteristics: ValueTask can be used for both asynchronous and synchronous results, while Task is inherently asynchronous. Unlike Task, which always allocates a heap object, ***ValueTask* **avoids allocation when the result is immediately available. It’s particularly useful in high-performance scenarios to reduce unnecessary allocations. Common Use Cases: Operations that are frequently completed synchronously, where allocation overhead from Task would be significant. APIs implementing IAsyncEnumerable, which often return ValueTask for efficiency. Precautions When Using ValueTask Avoid Awaiting the Same Instance Multiple Times Unlike Task, a ValueTask is not reusable. Awaiting it multiple times can lead to undefined behavior. ValueTask task = SomeMethodAsync(); await task; // Valid await task; // Undefined behavior Avoid .GetAwaiter().GetResult() If the Operation Isn’t Completed Avoid Using the Instance After It’s Awaited Once awaited, the state of ValueTask can no longer be relied upon, unlike Task, which can be awaited multiple times. Avoiding excessive bouncing For methods that are called many times in a loop, you can avoid the cost of repeatedly bouncing to a UI message loop by calling ConfigureAwait. This forces a task not to bounce continuations to the synchronization context, cutting the overhead closer to the cost of a context switch When to Use ValueTask? Use Task for the majority of asynchronous methods. It is simpler, more versatile, and less error-prone. Use ValueTask when: You need to eliminate allocations in highly optimized scenarios. The operation frequently completes synchronously. You understand the constraints and risks associated with ValueTask. GitHub code: https://github.com/stevsharp/ValueTaskVsTask Microsoft Documentation ValueTask Structure: https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.valuetask?view=net-7.0 Provides an in-depth overview of the ValueTask struct, its properties, and when to use it. Performance Considerations: https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/async-scenarios#valuetask Discusses the scenarios where ValueTask can improve performance and best practices. IAsyncEnumerable and ValueTask: Books Concurrency in C# Cookbook by Stephen Cleary: https://www.amazon.com/dp/B0D3LBH7XK/ref=syn_sd_onsite_desktop_0?ie=UTF8&psc=1&pf_rd_p=d77a94d7-221a-4129-af34-3c16ad136bb7&pf_rd_r=G7CDS7WJY28HD4V75DSS&pd_rd_wg=1B1RT&pd_rd_w=CV9FH&pd_rd_r=061c7653-1306-4284-bac1-291bdf9f8c8f&aref=40Jzhu2vTB Contains practical examples and explanations on using ValueTask effectively in asynchronous programming. Pro .NET Memory Management by Konrad Kokosa: https://www.amazon.com/Pro-NET-Memory-Management-Performance/dp/148424026X
ValueTask is a structure introduced in .NET for scenarios requiring micro-optimizations in asynchronous programming. It provides an alternative to Task with better performance for specific use cases, such as when the result of an asynchronous operation is already available or can be synchronously produced.
Key Characteristics:
- ValueTask can be used for both asynchronous and synchronous results, while Task is inherently asynchronous.
- Unlike Task, which always allocates a heap object, ***ValueTask* **avoids allocation when the result is immediately available.
- It’s particularly useful in high-performance scenarios to reduce unnecessary allocations.
Common Use Cases:
- Operations that are frequently completed synchronously, where allocation overhead from Task would be significant.
- APIs implementing IAsyncEnumerable, which often return ValueTask for efficiency.
Precautions When Using ValueTask
Avoid Awaiting the Same Instance Multiple Times
- Unlike Task, a ValueTask is not reusable. Awaiting it multiple times can lead to undefined behavior.
ValueTask task = SomeMethodAsync();
await task; // Valid
await task; // Undefined behavior
- Avoid .GetAwaiter().GetResult() If the Operation Isn’t Completed
Avoid Using the Instance After It’s Awaited
Once awaited, the state of ValueTask can no longer be relied upon, unlike Task, which can be awaited multiple times.Avoiding excessive bouncing
For methods that are called many times in a loop, you can avoid the cost of
repeatedly bouncing to a UI message loop by calling ConfigureAwait. This forces
a task not to bounce continuations to the synchronization context, cutting the
overhead closer to the cost of a context switch
When to Use ValueTask?
Use Task for the majority of asynchronous methods. It is simpler, more versatile, and less error-prone.
Use ValueTask when:
- You need to eliminate allocations in highly optimized scenarios.
- The operation frequently completes synchronously.
- You understand the constraints and risks associated with ValueTask.
GitHub code:
https://github.com/stevsharp/ValueTaskVsTask
Microsoft Documentation
ValueTask Structure:
https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.valuetask?view=net-7.0
Provides an in-depth overview of the ValueTask struct, its properties, and when to use it.
Performance Considerations:
https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/async-scenarios#valuetask
Discusses the scenarios where ValueTask can improve performance and best practices.
IAsyncEnumerable and ValueTask:
Books
Concurrency in C# Cookbook by Stephen Cleary:
https://www.amazon.com/dp/B0D3LBH7XK/ref=syn_sd_onsite_desktop_0?ie=UTF8&psc=1&pf_rd_p=d77a94d7-221a-4129-af34-3c16ad136bb7&pf_rd_r=G7CDS7WJY28HD4V75DSS&pd_rd_wg=1B1RT&pd_rd_w=CV9FH&pd_rd_r=061c7653-1306-4284-bac1-291bdf9f8c8f&aref=40Jzhu2vTB
Contains practical examples and explanations on using ValueTask effectively in asynchronous programming.
Pro .NET Memory Management by Konrad Kokosa:
https://www.amazon.com/Pro-NET-Memory-Management-Performance/dp/148424026X
