Asynchronous operation returning a result with method inheritance-CodePudding

I have a simple question about asynchronous operations. I have a composition class used to encapsulate an HttpClient instance. This class have an abstract base class to avoid code redundancy.

My problem occur when I'm trying to override a base method doing a part of the job in the derived method who is doing additional operations.

To be clearer I have:

Base method:

    /// <summary>
    /// Execute a Typed GET Request and returning <typeparamref name="TResponse"/> result.
    /// </summary>
    /// <typeparam name="TResponse">Response specified type.</typeparam>
    /// <param name="route">Uri string to request.</param>
    /// <param name="path">Path parameters to provide to the <paramref name="route"/>.</param>
    /// <param name="query">Query parameters to provide to the <paramref name="route"/>.</param>
    /// <returns>The <see cref="Task"/> that represents the asynchronous operation containing <typeparamref name="TResponse"/> response object.</returns>
    /// <exception cref="HttpRequestException">Raised if the Http request failed.</exception>
    public virtual async Task<TResponse?> GetAsync<TResponse>(string route, Dictionary<string, string>? path = null, Dictionary<string, string>? query = null)
    {
        using HttpResponseMessage response = await _httpClient.GetAsync(BuildUri(route, path, query));
        response.EnsureSuccessStatusCode();

        return await response.Content.ReadFromJsonAsync<TResponse>(new JsonSerializerOptions() { PropertyNameCaseInsensitive = true });
    }

Derived method:

    /// <inheritdoc/>
    public override async Task<TResponse?> GetAsync<TResponse>(string route, Dictionary<string, string>? path = null, Dictionary<string, string>? query = null)
    {
        var token = await GetBearerToken();
        base._httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
        return base.GetAsync<TResponse>(route, path, query);
    }

So the base class method have the http request responsibility and the derived method is adding additional pre-treatments to configure the httpClient with the required headers and others. The error occur on the return, I tried both:

return base.GetAsync<TResponse>(route, path, query);

with the error: Impossible to convert type Task<TResponse?> into TResponse?

and

return await base.GetAsync<TResponse>(route, path, query);

with the error: Impossible to convert type TResponse into TResponse?

I'm not understanding why, when I'm awaiting the result, a cast is done casting Nullable<TResponse> into TResponse in the derived method, knowing the fact that base method is returning Task<TResponse?>.

Thanks in advance.

Edit:

@Selvin and @Enigmativity got the right answer here adding a class constraint to the generic TResponse type in the method definition of both base and derived method resolve the issue, Thanks for your help!

CodePudding user response：

You method needs to constrain TResponse to a class:

public virtual async Task<TResponse?> GetAsync<TResponse>(
        string route,
        Dictionary<string, string>? path = null,
        Dictionary<string, string>? query = null)
    where TResponse : class