I wanted to create a Dictionary-like object and thought the correct way would be to implement the IDictionary<K,V>
interface, and use composition to include the underlying dictionary. I began with the below (K
=string
, V
=int
)
public class DictionaryLikeObject : IDictionary<string,int> { Dictionary<string,int> _backingDictionary = new Dictionary<string,int>(); }
Then I used Visual Studio's "Implement Interface" ability to stub out all the cover methods that I would need.
Three methods of IDictionary
do not seem to exist in Dictionary
:
void Add(KeyValuePair<string, int> item); void CopyTo(KeyValuePair<string, int>[] array, int arrayIndex); bool Remove(KeyValuePair<string, int> item);
Yet the Microsoft documentation clearly indicates that Dictionary
implements IDictionary
. So I would have expected these three methods to be available. To copy from the documentation, the definition of Dictionary<K,V>
[SerializableAttribute] [ComVisibleAttribute(false)] public class Dictionary<K, V> : IDictionary<K, V>, ICollection<KeyValuePair<K, V>>, IEnumerable<KeyValuePair<K, V>>, IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback
These three missing methods, I believe, are found in ICollection<>
. But so are other methods such as Clear()
that Dictionary
does have.
Question 1: How can C# get away without implementing these three, and why is this so? I suspect this is a compiler error (for my reasoning, see below). Question 2: Alternatively, what am I missing?
Here's why I think it might be a compiler error. Examine the following code:
Dictionary<string, int> dictionary1 = new Dictionary<string, int>(); IDictionary<string, int> dictionary2 = new Dictionary<string, int>(); KeyValuePair<string, int> item = new KeyValuePair<string, int>("test", 1); //dictionary1.Add(item); // compile error: No overload for method 'Add' takes 1 argument dictionary2.Add(item); // works like a charm Debug.WriteLine(@"dictionary2[""test""] = {0}", dictionary2["test"]); // outputs: dictionary2["test"] = 1
The method void Add(KeyValuePair<string, int> item)
appears not to be in Dictionary<string,int>
(since it doesn't compile), but it is in IDictionary<string,int>
, and somehow the compiler does properly find an implementation of it. Question 3: What's going on?
Note that the Microsoft documentation for Dictionary<K,V>
does not specify these three methods.
Lastly, in my actual implementation, I ended up using
IDictionary<string,int> _backingDictionary = new Dictionary<string,int>();
instead of
Dictionary<string,int> _backingDictionary = new Dictionary<string,int>();
so that all three methods could easily work.
Both Java and Python have the concept of a "string", C does not have the concept of a "string". C has character arrays which can come in "read only" or manipulatable. A character array is a sequence of contiguous characters with a unique sentinel character at the end (normally a NULL terminator '\0' ).
Because C comes after B The reason why the language was named “C” by its creator was that it came after B language. Back then, Bell Labs already had a programming language called “B” at their disposal.
As such, C programming does not provide direct support for error handling but being a system programming language, it provides you access at lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and set an error code errno.
This is due to the fact that C++ does not do bounds checking. Languages like Java and python have bounds checking so if you try to access an out of bounds element, they throw an error. C++ design principle was that it shouldn't be slower than the equivalent C code, and C doesn't do array bounds checking.
The Dictionary<TKey, TValue>
does implement these methods, it just does so explicitly. Hence you must access it via the IDictionary<TKey, TValue>
interface.
Dictionary<string, string> map = ...; KeyValuePair<string, string> pair = ...; map.Add(pair); // Compilation Error ((IDictionary<string, string>)map).Add(pair); // Works
Explicit implementation works by specifying precisely which interface method an instance method implements at the point of definition. For example
interface IFoo { void Method(); } class C1 : IFoo { // Implicitly implements IFoo.Method public void Method() { } } class C2 : IFoo { // Explicitly implements IFoo.Method void IFoo.Method() { } }
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