How does List<T>.Find work when T is a struct?

Question

I have a List<KeyValuePair<string, othertype>>. I need to do something along the lines of

list.Find(x=>x.Key=="foobar")

However, if that doesn't exist in the list, what will the behavior be? Usually it would return null, but structs can't be null.

Answer 1

My advice is to use FindIndex for non nullable types

int index = list.FindIndex(x => x.Key == "foobar");
if (index >= 0) {
    // found!
    UseResult(list[index]);
}

The default value default(T) is returned if Find() is not successful. For non nullable types, this result cannot be distinguished from regular entries having the default value. This is also true for nullable types when the list might contain regular null entries.

Answer 2

It will return the default(T) which will be the same as new KeyValuePair<string, othertype>>(), that is, a default initialized struct.

Basically, the default for reference types is always null, and for value types (including struct) it's the default (0 for numerics, false for bool, a struct with every field defaulted for structures, etc.)

So, for a default(KeyValuePair<string, othertype>>) you'd get back a KVP where the Key was null (default for string) and whatever the default(othertype) would be (as in the examples above)...

From the MSDN:

The first element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type T.

Using this, if you wanted to check and see if you got back the default, I'd recommend checking for yourResult.Key != null to see if you got a result, or you could use a different Find method such as FindIndex as Olivier suggests.