Returning null T

Question

I am having trouble returning null on this generic type T. I have tried marking it as Nullable, Nullable or T? without success...

This method is part of an abstract class and I need to make it as generic as possible so I can retrieve any type of object and use it from any derived class.

public T GetFromApi<T>(string apiRequest, string content)
{
    try
    {
        log.Debug("Requesting '" + apiRequest + "' from API with the following parameters: " + content);
        _httpClient.DefaultRequestHeaders.Accept.Clear();
        _httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        _httpClient.DefaultRequestHeaders.Add("Authorization", string.Format("{0} {1}", _token.token_type, _token.access_token));

        var httpContent = new StringContent(content);
        HttpResponseMessage response = _httpClient.GetAsync(apiRequest).Result;

        if (response.IsSuccessStatusCode)
        {
            //Returns requested Object (T) from API
            log.Info("Returning requested object " + typeof(T).ToString());                    
            return response.Content.ReadAsAsync<T>().Result;
        }
        else
        {
            log.Error("Error accessing API.");
            return null;
        }
    }
    catch (Exception ex)
    {
        log.Fatal("Error accessing API.", ex);
        throw ex;
    }
}

The return null on the else statement gives me the error:

Cannot convert null to type parameter 'T'

BTW: You should prefer `throw;` over `throw ex;` to keep the original exception's stack trace. — Uwe Keim, Jun 16 '16 at 21:19
The question has already been answered, but another viable approach is to throw a custom exception, rather than returning anything. Otherwise, how is the caller supposed to know something went wrong -- do they need to check every value against `null`? What if `T` is `int`, do they then need to check against `0`? What if `0` is a valid value as well? — Jeroen Mostert, Jun 16 '16 at 21:23

David L · Accepted Answer · 2016-06-16T21:21:55.293

6

Null may not be a viable value for T if the type passed in is a struct.

You can instead return the default type for T which for a reference type is null:

else
{
    log.Error("Error accessing API.");
    return default(T);
}

As noted in the below comment, you can alternatively return null if you restrict T to class.

public T GetFromApi<T>(string apiRequest, string content) where T : class
...
else
{
    log.Error("Error accessing API.");
    return null;
}

However, in the case where you DO need to potentially retrieve primitive types (bool, int, etc), you would no longer be able to consume this method.

edited Jun 16 '16 at 21:21

answered Jun 16 '16 at 21:18

David L

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4

Another option is to put a constraint on the generic type for class. – Derek Van Cuyk Jun 16 '16 at 21:19
awesome... so much to learn yet :) Thnks! – Nuno Jun 16 '16 at 21:21
@DerekVanCuyk great point! Updated. – David L Jun 16 '16 at 21:22
@DavidL only retrieving class objects. I guess I should use the second approach then. Thanks ;) – Nuno Jun 16 '16 at 21:33
@Nuno Happy to help! – David L Jun 16 '16 at 21:42

Returning null T

1 Answers1