Provides a value type that wraps a Task<TResult> and a TResult, only one of which is used.
generic <typename TResult>
public value class ValueTask : IEquatable<System::Threading::Tasks::ValueTask<TResult>>public readonly struct ValueTask<TResult> : IEquatable<System.Threading.Tasks.ValueTask<TResult>>public struct ValueTask<TResult> : IEquatable<System.Threading.Tasks.ValueTask<TResult>>type ValueTask<'Result> = structPublic Structure ValueTask(Of TResult)
Implements IEquatable(Of ValueTask(Of TResult))A ValueTask<TResult> instance may either be awaited or converted to a Task<TResult> using AsTask. A ValueTask<TResult> instance may only be awaited once, and consumers may not read Result until the instance has completed. If these limitations are unacceptable, convert the ValueTask<TResult> to a Task<TResult> by calling AsTask.
The following operations should never be performed on a ValueTask<TResult> instance:
.Result or .GetAwaiter().GetResult() when the operation hasn't yet completed, or using them multiple times.If you do any of the above, the results are undefined.
A method may return an instance of this value type when it's likely that the result of its operation will be available synchronously, and when it's expected to be invoked so frequently that the cost of allocating a new Task<TResult> for each call will be prohibitive.
There are tradeoffs to using a ValueTask<TResult> instead of a Task<TResult>. For example, while a ValueTask<TResult> can help avoid an allocation in the case where the successful result is available synchronously, it also contains multiple fields, whereas a Task<TResult> as a reference type is a single field. This means that returning a ValueTask<TResult> from a method results in copying more data. It also means, that if a method that returns a ValueTask<TResult> is awaited within an async method, the state machine for that async method will be larger, because it must store a struct containing multiple fields instead of a single reference.
For uses other than consuming the result of an asynchronous operation using await, ValueTask<TResult> can lead to a more convoluted programming model that requires more allocations. For example, consider a method that could return either a Task<TResult> with a cached task as a common result or a ValueTask<TResult>. If the consumer of the result wants to use it as a Task<TResult> in a method like WhenAll or WhenAny, the ValueTask<TResult> must first be converted to a Task<TResult> using AsTask, leading to an allocation that would have been avoided if a cached Task<TResult> had been used in the first place.
As such, the default choice for any asynchronous method should be to return a Task or Task<TResult>. Only if performance analysis proves it worthwhile should a ValueTask<TResult> be used instead of a Task<TResult>. The non generic version of ValueTask is not recommended for most scenarios. The CompletedTask property should be used to hand back a successfully completed singleton in the case where a method returning a Task completes synchronously and successfully.
Note
The use of the ValueTask<TResult> type is supported starting with C# 7.0, and is not supported by any version of Visual Basic.
Note
An instance created with the parameterless constructor or by the default(ValueTask<TResult>) syntax (a zero-initialized structure) represents a synchronously, successfully completed operation with a result of default(TResult).
Gets a value that indicates whether this object represents a canceled operation.
IsCompletedGets a value that indicates whether this object represents a completed operation.
IsCompletedSuccessfullyGets a value that indicates whether this object represents a successfully completed operation.
IsFaultedGets a value that indicates whether this object represents a failed operation.
ResultGets the result.
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