Question

I want a typed lookup helper function for a heterogenous collection: It should return a struct or class, else null if the item is not found.
Below is an example using a trivial collection lookup, but it could be a database call or whatever.

Is there any way to achieve this with a single method signature?

    public T GetClass<T>(string key)  where T : class
    {
        object o;
        if (Contents.TryGetValue(key, out o))
        {
            return o as T;
        }
        return null;
    }

    public T? GetStruct<T>(string key) where T : struct
    {
        object o;
        if (Contents.TryGetValue(key, out o))
        {
            if (o is T)
            {
                return (T?) o;
            }
        }
        return null;
    }

What I’ve already tried:

• I understand that generic restrictions can’t be used to disambiguate overloads. So I can’t simply give these two methods the same name.
• Returning (Default) T isn’t an option, since 0 is a valid int value.
• I have tried calling with <int ?> as the type, but as discussed, Nullable<T> isn’t a reference type.

Is there some way to indicate that I’m going to return a boxed int?

Answer 1

The following method works for both classes and nullable structures:

public static T GetValue<T>(string key)
{
    object o;
    if (Contents.TryGetValue(key, out o))
    {
        if (o is T)
        {
            return (T)o;
        }
    }
    return default(T);
}

Usage:

int?   result1 = GetValue<int?>("someInt");
string result2 = GetValue<string>("someString");

Notice how the ? is part of generic type argument and not defined by the method on the return type.