Nullable nested type in extension method in C#

Question

I am trying to make a super cool extension for IDictionary - GetValue with a default value that is null if not set. Here is the code I came up with (doesn't work):

public static TValue GetValue<TKey, TValue> (this IDictionary<TKey,
    TValue> dictionary, TKey key, TValue defaultValue = null)
{
    TValue value;
    return dictionary.TryGetValue(key, out value)
        ? value
        : defaultValue;
}

How to make this for nullables only? (like, don't include int , etc).

Answer 1

You mean for reference types only. Add where T: class as follows:

public static TValue GetValue<TKey, TValue> (this IDictionary<TKey,
    TValue> dictionary, TKey key, TValue defaultValue = null)
    where TValue: class
{

However you can make this work with value types too, by using default(TValue) to specify the default:

public static TValue GetValue<TKey, TValue>(this IDictionary<TKey,
    TValue> dictionary, TKey key, TValue defaultValue = default(TValue))
{
    TValue value;
    return dictionary.TryGetValue(key, out value)
        ? value
        : defaultValue;
}

Of course, only do this if you actually WANT it to work with all possible types, rather than just with reference types.

Answer 2

You can use constraints on your type parameters ( MSDN Type Constraints ). What you want here is the class constraint, like so:

public static TValue GetValue<TKey, TValue> (this IDictionary<TKey,
TValue> dictionary, TKey key, TValue defaultValue = null) where TValue : class

This works for reference types, which is what you really want. Nullable would imply something like int? working as well.

Answer 3

Use the class constraint :

public static TValue GetValue<TKey, TValue> (this IDictionary<TKey,
    TValue> dictionary, TKey key, TValue defaultValue = null) where TValue : class
{
    TValue value;
    return dictionary.TryGetValue(key, out value)
        ? value
        : defaultValue;
}