Check if key exists in dictionary of type [Type:Type?]

Actually your test dictionary[key] == nil can be used to check if a key exists in a dictionary. It will not yield true if the value is set to nil:

let dict : [String : Int?] = ["a" : 1, "b" : nil]

dict["a"] == nil // false,     dict["a"] is .Some(.Some(1))
dict["b"] == nil // false !!,  dict["b"] is .Some(.None)
dict["c"] == nil // true,      dict["c"] is .None

To distinguish between "key is not present in dict" and "value for key is nil" you can do a nested optional assignment:

if let val = dict["key"] {
    if let x = val {
        println(x)
    } else {
        println("value is nil")
    }
} else {
    println("key is not present in dict")
}

I believe the Dictionary type's indexForKey(key: Key) is what you're looking for. It returns the index for a given key, but more importantly for your proposes, it returns nil if it can't find the specified key in the dictionary.

if dictionary.indexForKey("someKey") != nil {
    // the key exists in the dictionary
}

Swift 3 syntax....

if dictionary.index(forKey: "someKey") == nil {
    print("the key 'someKey' is NOT in the dictionary")
}

You can always do:

let arrayOfKeys = dictionary.allKeys
if arrayOfKeys.containsObject(yourKey) {

}
else {
}

However I really dislike the idea of creating an NSDictionary which can contain optionals.