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

Task is a reference type and always allocates on the heap, even when the async method completes synchronously. ValueTask is a struct that avoids allocation when the result is already available — useful for hot paths like cache lookups.

public ValueTask<User?> GetUserAsync(Guid id) {
    if (_cache.TryGetValue(id, out var u)) return new ValueTask<User?>(u);   // zero alloc
    return new ValueTask<User?>(LoadFromDbAsync(id));                        // wraps a Task
}

Rules of thumb:

• Use ValueTask only when synchronous completion is the common case (>50% of calls)
• Never await a ValueTask twice — it can be backed by a pooled object
• Stick with Task for I/O-heavy methods; the allocation cost is negligible compared to the I/O

In .NET 9 the JIT inlines many ValueTask paths so the gap has narrowed; use the simpler Task unless profiling shows allocation pressure.