C#: What is the difference between Task and ValueTask in C#?

Task — reference type. ValueTask — struct for performance.

Task:

• Reference type (class).
• Allocates on heap.
• Always asynchronous.

1public async Task<int> GetDataAsync() {
2    return await httpClient.GetIntAsync("url");
3}

ValueTask:

• Value type (struct).
• No allocation if result available synchronously.
• Reduces GC pressure.

1public async ValueTask<int> GetDataAsync() {
2    if (cache.TryGetValue("key", out int value)) {
3        return value;  // No allocation!
4    }
5    return await httpClient.GetIntAsync("url");
6}

When to use ValueTask:

• Frequently called methods.
• Often complete synchronously (cache).
• Performance-critical code.

Warning:

• Don't await multiple times.
• Don't store in field.