异步任务并在C＃中等待

Question

I'm working on Async webservice call and was playing with this Task and await construction:

private static async Task<RSAParameters> GetPublicSecretKey(ICoreIdentityService identityChannel)
{
        Object state = null;
        var t = Task<RSAParameters>.Factory.FromAsync(
            identityChannel.BeginGetPublicKey,
            identityChannel.EndGetPublicKey,
                null, state, TaskCreationOptions.None);
        return await t;
}

//Methods definition:
//IAsyncResult BeginGetPublicKey(AsyncCallback callback, object asyncState)
//RSAParameters EndGetPublicKey(IAsyncResult result)

Building the code I get The type arguments for method .... cannot be inferred from usage. Am I missing something?

Thank's in advance. Cheers, inoel

Modified compiled code:

var t = Task<RSAParameters>.Factory.FromAsync(
    identityChannel.BeginGetPublicKey,
    identityChannel.EndGetPublicKey,
    TaskCreationOptions.None);

Answer 1

It appears that you are calling the FromAsync() method with your parameters in an unexpected order.

The error message itself suggests explicitly naming your parameters, so your code would look something along the lines of this:

var t = Task<RSAParameters>.Factory.FromAsync(
                asyncResult: identityChannel.BegineGetPublicKey,
                endMethod: identityChannel.EndGetPublicKey,
                creationOptions: TaskCreationOptions.None,
                scheduler: state);

Alternatively, you could correct the order of your parameters, and this should solve the problem. The closest overload I can find is this:

public Task<TResult> FromAsync<TArg1, TArg2>(Func<TArg1, TArg2, AsyncCallback, object, IAsyncResult> beginMethod, Func<IAsyncResult, TResult> endMethod, TArg1 arg1, TArg2 arg2, object state, TaskCreationOptions creationOptions);

So assuming you intend to use this one, your code will need to be modified slightly to pass in the types of arg1, and arg2, then pass in an additional parameter:

Object state = null;
var t = Task<RSAParameters>.Factory.FromAsync<TArg1, TArg2>(
            beginMethod: identityChannel.BeginGetPublicKey,
            endMethod: identityChannel.EndGetPublicKey,
            arg1: null, // Either arg1, or arg2 is missing
            arg2: null, // from your code
            state: state,
            creationOptions: TaskCreationOptions.None);

return t;

I've left the named parameters here for clarity, but you should be able to remove these if you prefer.