C＃泛型并使用类型化方法的非泛型版本

Question

I'm using the RestSharp library to access a REST API.

I want all the API requests to go through the same method, so I can add headers, handle errors and do other stuff in a central place.

So I made a method that accepts a generic Func<> and that solves most of my problems, but I don't know how to handle the case where I don't have a return type.

private T PerformApiCall<T>(RestRequest restRequest, Func<RestRequest, IRestResponse<T>> restMethod)
{
    var response = restMethod.Invoke(restRequest);
    //handle errors
    ....

    return response.Data;
}

I call it like this:

var apples = PerformApiCall(new RestRequest('/api/apples'), req => Client.Execute<List<Apple>>(req));

But I came across a problem, a bunch of API calls don't have a return type because they don't return data. So I used Client.Execute(req) and I get the error saying the type arguments cannot be inferred, I tried to pass , but that failed because it couldn't convert the non-generic IRestResponse to the typed one.

Any ideas on how to tackle this in a nice way?

Answer 1

One thing you could try is to add an overload to your PerformApiCall function that takes a Func with a non-generic result type, and returns nothing:

// Notice the `Func` has `IRestResponse`, not `IRestResponse<T>`
public void PerformApiCall(RestRequest restRequest,
                           Func<RestRequest, IRestResponse> restMethod)
    ...

Then, depending on how complex your error checking/logic is, you could move it out to a separate method (which returns the response), and call it from both overloads of PerformApiCall :

private T PerformRequestWithChecks<T>(RestRequest restRequest,
                                      Func<RestRequest, T> restMethod)
    where T : IRestResponse
{
    var response = restMethod.Invoke(restRequest);
    // Handle errors...
    return response;
}

// You can use it from both versions of `PerformApiCall` like so:
//
//     // From non-generic version
//     var response =
//         PerformRequestWithChecks<IRestResponse>(restRequest, restMethod);
//
//     // From generic version
//     var response =
//         PerformRequestWithChecks<IRestResponse<T>>(restRequest, restMethod);
//     return response.Data;

You were getting a compiler error because it is sound to treat a subtype as if it was an instance of its supertype , but it is not sound to do it in the other direction (which is what was happening when you changed your calling code to just Client.Execute(req) , returning a non-generic).

Here's an ideone paste illustrating this: http://ideone.com/T2mQfl