C# does generic contain empty collection

Question

An annoying behaviour of ProtoBuff.net is that empty collections are serialised as null. This can create some pretty hard to pin down bugs.

Retrieval of cached values from my application is done with the following function:

public T Get<T>(string cacheKey)
{
    var data = (byte[])Database.StringGet(cacheKey);
    if (data == null)
    {
        return default(T);
    }
    return Serialiser.Deserialise<T>(data);
}

If T is List<int> with zero values, this will return an empty list (because data == null it will return default(List<int>)).

If T is Dictionary<bool, Hashset<int>, where two keys true and false exist but there are no values in the corresponding hashset, the key exists but the value is null.

Is there any way to determine if T contains a collection, and if so return an empty collection instead of null if the collection is empty? Preferably, it would check for an empty collection anywhere in the object, not just if T itself is a collection that contains a collection.

The alternative (that I'm doing now) is to try to remember to check for nulls when I know the explicit type when I get from cache which isn't ideal.

Do you know the type you're expecting when you call Get? If so have an overload for this case. — Phil, Mar 07 '17 at 17:33

score 1 · Answer 1 · answered Mar 07 '17 at 17:27

1

You could use typeof operator.

if(typeof(T)== typeof(Dictioneary))
{
  return whatever you want;
}

answered Mar 07 '17 at 17:27

Luci

1,278
9
17

I'm after a more general approach that covers all cases – Tom Gullen Mar 07 '17 at 17:29
It helps if you will provide the default value as second argument to your method. – Luci Mar 07 '17 at 17:35
Like 'public T Get(string cacheKey, T defaultValue)' – Luci Mar 07 '17 at 17:36
Doesn't help as you don't know if a collection property of T is empty or not – Tom Gullen Mar 07 '17 at 21:42

Phil · Answer 2 · 2017-03-07T17:42:35.063

As per my comment, would something like this help?

void Main()
{
    Get<int>("cacheKey1").Dump();
    Get<List<string>, string>("cacheKey2").Dump();
    Get<Dictionary<string, string>, KeyValuePair<string,string>>("cacheKey3").Dump();
}

public T Get<T>(string cacheKey)
{
    ...
    return default(T);
}

public IEnumerable<S> Get<T, S>(string cacheKey) where T : IEnumerable<S>
{
    ...
    return Enumerable.Empty<S>();
}

C# does generic contain empty collection

2 Answers2