Prior to C# 7.0, async methods had to return either void, Task, or Task<T>. As Task is a reference type, returning such an object from async methods can impact performance because it allocates an object into memory even though it returns a cached object or runs asynchronously.

To overcome this, C# 7.0 introduces the ValueTask type, which is set to prevent the allocation of a Task<T> object when the result of the async operation is already available. Using it, the async methods can return types other than Task, Task<T>, and void:

  public async ValueTask<long> GetValue() 
  { 
    return await Task.Run<long>(() => 5000); 
  } 

If you receive an error accessing ValueTask in C# 7.0, you must explicitly reference System.Threading.Tasks.Extensions from the NuGet package library. To install the package, either open the NuGet package manager or the NuGet package manager console; or you can simply click the Install package 'System.Threading.Tasks.Extensions' menu item from the tooltip, as shown in the following screenshot. This is the simplest way to download and install the package:

Alternatively, you can find the package here: http://www.nuget.org/packages/System.Threading.Tasks.Extensions/.

Once you have installed the package, you can see that the DLL reference to System.Threading.Tasks.Extensions has already been added to your project: