Unable to declare Interface " async Task<myObject> MyMethod(Object myObj); " [duplicate]

Question

I'm unable to declare

interface IMyInterface
{
   async Task<myObject> MyMethod(Object myObj);
}

The compiler tells me:

• The modifier async isn't valid for this item
• The async modifier can only be used for methods that have a body

Is this something that should be implemented, or does the nature of async & await prohibit this from ever occurring?

Answer 1

Whether a method is implemented using async/await or not is an implementation detail. How the method should behave is a contract detail, which should be specified in the normal way.

Note that if you make the method return a Task or a Task<T>, it's more obvious that it's meant to be asynchronous, and will probably be hard to implement without being asynchronous.

From https://stackoverflow.com/a/6274601/4384

Answer 2

Whether or not your implementation is async, has no relevance to your interface. In other words, the interface cannot specify that a given method must be implemented in an asynchronous way.

Just take async out of your interface and it will compile; however, there is no way to enforce asynchronous implementation just by specifying an interface.

Answer 3

If you have an interface with two implementations (one that is truly async and the other that is synchronous) this is what it would look like for each implementation - with both returning a Task<bool>.

public interface IUserManager
{
    Task<bool> IsUserInRole(string roleName);
}

public class UserManager1 : IUserManager
{
    public async Task<bool> IsUserInRole(string roleName)
    {
        return await _userManager.IsInRoleAsync(_profile.Id, roleName);
    }
}

public class UserManager2 : IUserManager
{
    public Task<bool> IsUserInRole(string roleName)
    {
        return Task.FromResult(Roles.IsUserInRole(roleName));
    }
}

If it is a void method you need to return Task.CompletedTask; from the non async method (I think .NET 4.5 and later)

See also : Return Task<bool> instantly