I have a Dictionary called baseDictionary. The key is a string and the values are the properties of a class called myData. In partiular the properties are: "age" (as int), "nationality" (as string) and "income" (as double).
So baseDictionary has some string as key and for each key a series of properties related to a specific person.
I want at some point to make a deep copy of this dictionary so that I can then work with this new copy without modifying the content of the original dictionary.
I have found an answer in stackoverflow where the following code is suggested to do this deep copy:
public static Dictionary<TKey, TValue>
CloneDictionaryCloningValues<TKey, TValue>(
Dictionary<TKey, TValue> original) where TValue : ICloneable
{
Dictionary<TKey, TValue> ret = new Dictionary<TKey, TValue>(
original.Count, original.Comparer);
foreach (KeyValuePair<TKey, TValue> entry in original)
{
ret.Add(entry.Key, (TValue) entry.Value.Clone());
}
return ret;
}
The problem is that I cannot understand how I should modify it to make it work with my dictionary. For example I tried:
public static Dictionary<string, myData> CloneDictionaryCloningValues<TKey, TValue>
(Dictionary<string, myData> original) where TValue : ICloneable
{
Dictionary<string, myData> ret = new Dictionary<string, myData>(original.Count,
original.Comparer);
foreach (KeyValuePair<string, myData> entry in original)
{
ret.Add(entry.Key, (myData)entry.Value.Clone());
}
return ret;
}
But I get the following error and it doesn't work.
Error 3 'Project2.myData does not contain a definition for 'Clone' and no extension method 'Clone' accepting a first argument of type
'Project2.myDatacould be found (are you missing a using directive or
an assembly reference?)
How can I fix the problem?
You don't need to change the CloneDictionaryCloningValues method at all, if you make the myData class implement the ICloneable interface:
public class myData : ICloneable {
// your code
public object Clone() {
// whatever you need to create a copy, for example:
return new myData() {
age = this.age,
nationality = this.nationality,
income = this.income
};
}
}
You can also rewrite/overload the method to take a cloning method instead of demanding the IClonable interface:
public static Dictionary<TKey, TValue> CloneDictionaryCloningValues<TKey, TValue>
(Dictionary<TKey, TValue> original, Func<TValue, TValue> clone)
{
Dictionary<TKey, TValue> ret = new Dictionary<TKey, TValue>(original.Count, original.Comparer);
foreach (KeyValuePair<TKey, TValue> entry in original) {
ret.Add(entry.Key, clone(Value));
}
return ret;
}
Then you call the method with a function that creates a copy of an item:
myCopy = CloneDictionaryCloningValues(myOriginal, item => {
// whatever you need to create a copy, for example:
return new myData() {
age = item.age,
nationality = item.nationality,
income = item.income
};
});
You should not modify the CloneDictionaryCloningValues() routine, just use it as it is. Because its generic, it will work with your keys (strings) and value class (mydata).
BUT:
To make this routine work your class must have a public Clone() routine i.e. implement the ICloneable interface.
As the function specifies: you need to implement ICloneable on the type myData
Also:
Keep the original code and simply pass your dictionary.
Rename the myData class to something more descriptive and have it start with an uppercase character
Related
I have an IDictionary<string, MyEnum?> collection that needs to be passed to a class to wrap it in a IReadOnlyDictionary<string, MyEnum> (note MyEnum but not MyEnum?).
I have come up with two designs:
Delay the wrapping to IReadOnlyDictionary<string, MyEnum> until property access:
public class MyClass
{
private readonly IEnumerable<KeyValuePair<string, MyEnum?>> _kvps;
public MyClass(IEnumerable<KeyValuePair<string, MyEnum?>> kvps)
{
_kvps = kvps;
}
public IReadOnlyDictionary<string, MyEnum> Kvps
{
get
{
var filtered = from kvp in _kvps
where kvp.Value.HasValue
select kvp;
return new ReadOnlyDictionary<string, MyEnum>(
filtered.ToDictionary(kvp => kvp.Key, kvp => (MyEnum)kvp.Value);
}
}
}
Eagerly evaluate the collection in constructor
public class MyClass
{
public MyClass(IEnumerable<KeyValuePair<string, MyEnum?>> kvps)
{
Kvps = ToReadOnly(kvps);
}
public IReadOnlyDictionary<string, MyEnum> Kvps { get; }
private static IReadOnlyDictionary<string, MyEnum> ToReadOnly(
IEnumerable<KeyValuePair<string, MyEnum?>> kvps)
{
var filtered = from kvp in kvps
where kvp.Value.HasValue
select kvp;
return new ReadOnlyDictionary<string, MyEnum>(
filtered.ToDictionary(kvp => kvp.Key, kvp => (MyEnum)kvp.Value);
}
}
The constructor design section of the Framework Design Guidelines suggests that minimal work should be done in constructors so I am opting for the first approach. However, that means every call to MyClass.Kvps will trigger a copy of _kvps which is not ideal.
I would like to know which is a better approach (or are there other ways) in terms of:
Memory efficiency (ideally only one copy of the collection is stored in MyClass)
Performance (property access should be fast and should not trigger a copy of the KeyValuePairs)
Out of the two requirements - don't copy the key value pairs and don't store two copies - you'll have to break one.
What causes us to look at this and think that there must be a solution is that we see TValue and TValue? and our minds want to see them as being of the same type. But they are not the same type.
It becomes clearer if you imagine that instead of TValue and TValue? that these are two different types, like an int and a string, and we want to project a collection of one to a collection of the other while filtering. For example,
List<string> GetStringsFromNonNegativeInts(List<int> ints)
{
return ints.Where(i=>i>-1).Select(i=>i.ToString()).ToList();
}
That's exactly the same scenario as trying to filter a set of TValue? to a set of TValue, even without the dictionary. It's just harder to see. TValue and TValue? code-blind us.
There are only two ways to do this. One is to copy each time, and the other is to keep two lists in synchronization.
EDIT: If you want the latest source values, best way is to implement your own class that implements IReadOnlyDictionary. Initialize this with a private field of ReadOnlyDictionary<string, MyEnum?>. Each call will do the lookup, and if the key exists AND HasValue, return the value.
Note that this implementation depends on the reference to the original values being passed in as an IReadOnlyDictionary to avoid having to copy values over.
public class MyReadOnlyDictionary<TKey, TValue> : IReadOnlyDictionary<TKey, TValue> where TValue : struct
{
// other methods to implement here...
public MyReadOnlyDictionary(IReadOnlyDictionary<TKey, TValue?> kvps)
{
_kvps = kvps;
}
private IReadOnlyDictionary<TKey, TValue?> _kvps;
new public TValue this[TKey key]
{
get
{
TValue? val = _kvps[key];
if (val.HasValue)
return val.Value;
throw new KeyNotFoundException();
}
}
}
I have a complex data container with multiple levels of nested Dictionaries.
But having Key and Value properties make it non-intuitive and hard to use.
Please suggest the simplest way to rename Key and Value properties in Dictionary<,> subclasses.
Update:
Patryk Ćwiek: If you implement IDictionary<TKey, TValue>, you also can't rename properties, because they're part of the contract.
You're right. My question was not correct. Usage of KeyValuePair in IDictionary restrict the pair properties to Key and Value. So if we want non Key/Value pair we had to implement IDictionary with custom KeyValuePair struct. Or may be there is some other tricky way?
PS. Maybe someone suggest an IDictionary code generation template?
Make your own interface with the property name(s) you want. Then, have your concrete class implement your custom interface.
To keep your code DRY, create a private Dictionary that you delegate all of your work to. You can even have your custom interface be Enumerable (or anything else that IDictionary implements) by delegating the required methods to your private variable.
Here is an example. You would just need change your code from using IDictionary to IComplexDataContainer.
interface IComplexDataContainer<TKey, TValue>
: IEnumerable<KeyValuePair<TKey,TValue>>
{
TValue this[TKey index] { get; set; }
}
class MyComplexDataContainer<TKey, TValue>
: IComplexDataContainer<TKey, TValue>
{
IDictionary<TKey, TValue> hiddenHelper { get; set; }
public MyComplexDataContainer()
{
hiddenHelper = new Dictionary<TKey, TValue>();
}
// delegate all of the work to the hidden dictionary
public TValue this[TKey index]
{
get
{
return hiddenHelper[index];
}
set
{
hiddenHelper[index] = value;
}
}
// Just delegate the IEnumerable interface to your hidden dictionary
// or any other interface you want your class to implement
public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
{
return hiddenHelper.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
Then you would just use like this:
IComplexDataContainer<string, int> myData = new MyComplexDataContainer<string,int>();
myData["tom"] = 18;
myData["dick"] = 22;
myData["harry"] = myData["tom"];
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Bidirectional 1 to 1 Dictionary in C#
Im curious if a datastructure exists in the standard .net libraries that can represent a 1-1 relationship, such as the following
1-a
4-b
6-c
5-d
Where I can say:
thisstructure[1] // returns "a"
thisstructure.GetKey["d"] // return 5
I understand all keys would have to be unique, does anything similar exist?
Thanks!
Yes- it's called KeyedCollection. It is intended to be subclassed and provides indexed access as well as access by a property derived from the added item. I usually make a generic subclass:
public class GenericKeyedCollection<TKey, TValue> : KeyedCollection<TKey, TValue> {
private readonly Func<TValue, TKey> _keyGenerator;
public GenericKeyedCollection(Func<TValue, TKey> keyGenerator) {
_keyGenerator = keyGenerator;
}
protected override int GetKeyForItem(TValue item)
{
return _keyGenerator(item);
}
}
To use it:
var myCollection = new GenericKeyedCollection<String, Car>(c=>c.Model);
myCollection.Add(new Car("Ford", "Mustang"));
var byIndex = myCollection[0];
var byModel = myCollection["Mustang"];
The only caveat is that the derived property (the "key") mustn't change after the item has been added.
If your key is not a property of the value, then you can use a Tuple<T1, T2> to combine the key and value:
var myCollection = new GenericKeyedCollection<String, Tuple<String, Car>>(t=>t.Item1);
myCollection.Add(new Tuple<String, Car>("Foo", Car("Ford", "Mustang")));
var byIndexCar = myCollection[0].Item2;
var byItem1Car = myCollection["Foo"].Item2;
Could this method fit your needs?
public static class Extensions
{
public static TKey GetKey<TKey, TValue>(this Dictionary<TKey, TValue> dict, TValue value)
{
int index = dict.Values.ToList().IndexOf(value);
if (index == -1)
{
return default(TKey); //or maybe throw an exception
}
return dict.Keys.ToList()[index];
}
}
You could then use it like so:
Dictionary<int, char> dict = new Dictionary<int, char>();
dict.Add(1, 'a');
dict.Add(4, 'b');
dict.Add(6, 'c');
dict.Add(5, 'd');
Console.WriteLine(dict.GetKey('d')); //5
The Dictionary....or IDictionary interface is the closest I can think of to what you want. It doesn't have quite so simple a searching operation, in that searching on a value can return the key, but I do know you can search on a key to get a value. providing functionality for the reverse in a custom extended class wouldn't be difficult at all.
MSDN IDictionary page
I am building the following class to manage a dictionary.
public class EnumDictionary<TKey, TValue>
{
private Dictionary<TKey, TValue> _Dict;
public EnumDictionary(Dictionary<TKey, TValue> Dict)
{
this._Dict = Dict;
}
public TKey GetValue(TValue value)
{
foreach (KeyValuePair<TKey, TValue> kvp in _Dict)
{
if (kvp.Value == value)
return kvp.Key;
}
throw new Exception("Undefined data type: " + value);
}
}
But I am getting an error "Operator '==' cannot be applied to operands of type 'TValue' and 'TValue'".
BTW, I am making this custom collection is because my dictionary has unique value, but I can't get key by value from a dictionary.
Any help is appreciated. Thank you.
Did you try using the Equals method?
if (kvp.Value.Equals(value))
I think this restriction is due to the fact that the == operator can't be used with all types. Take the following for instance:
struct Test
{
public int Value;
}
Given the above struct, the following code will not compile:
Test a, b;
a = b = new Test();
bool areEqual = a == b; // Operator '==' cannot be applied to
// operands of type 'Test' and 'Test'
However, all types have the Equals method, so calling that will work:
Test a, b;
a = b = new Test();
bool areEqual = a.Equals(b);
Fredrik is right; you need to use Equals as you can't presume to be able to use == for all types, since the operator isn't defined for every type.
Depending on your scenario, it might also make sense to add
where TValue : IEquatable<TValue>
as a generic type constraint on your class. The reason for this is that object.Equals accepts another object as a parameter, which means that if TValue is a value type it will be boxed. If it can be known to implement IEquatable<TValue>, on the other hand, then Equals can be resolved to IEquatable<TValue>.Equals*, which takes a TValue as a parameter and therefore won't require value types to be boxed.
I might also recommend that you rethink the internal structure of this class. As it currently stands, there's no reason you need this class at all, as you could easily add an extension method to IDictionary<TKey, TValue> to find a key by value via enumeration over the values. What I would do instead is store two dictionaries: a Dictionary<TKey, TValue> and a Dictionary<TValue, TKey>, so that two-way lookup is possible in O(1).
*By the way, in case you're curious, the reason you can't use IEquatable<T> (or any interface for that matter) to ensure that a type has implemented the == operator is that operators are static, and interfaces cannot provide static methods (and thus can't provide operators).
When you use generic comparsions I think you should implement a (x)CompareTo(Y) or a comparable class. Please correct me if im wrong.
you can use if (kvp.Value.Equals(value)) instead of ==.
Use the "where" condition on your generic types
class Dictionary<TKey,TVal>
where TKey: IComparable, IEnumerable
where TVal: MyI
{
public void Add(TKey key, TVal val)
{
}
}
from http://msdn.microsoft.com/en-us/library/6b0scde8%28VS.80%29.aspx
Don't create a new class. Create a extension method:
public static class DictionaryHelper
{
public static TKey GetKeyFromValue<TKey, TValue>(this IDictionary<TKey, TValue> instance, TValue value)
{
foreach (var kvp in instance)
{
if (kvp.Value.Equals(value))
return kvp.Key;
}
return default(TKey);
}
}
public class Example
{
public static void Main(string[] argv)
{
Dictionary<string, string> test = new Dictionary<string, string> { { "Mykey", "MyValue" }, { "Key1", "Value2" } };
string key = test.GetKeyFromValue("MyValue");
}
}
If you want this to be general purpose, then you will want the definition of equality to be configurable, just as it is in the dictionary for keys.
Have a property of type IEqualityComparer<TValue>, which is set in the constructor.
Then have a version of the constructor that makes the default EqualityComparer<TValue>.Default. This will work by calling Equals on the type in question.
public class EnumDictionary<TKey, TValue>
{
private Dictionary<TKey, TValue> _Dict;
private readonly IEqualityComparer<TValue> _cmp;
public EnumDictionary(Dictionary<TKey, TValue> Dict, IEqualityComparer<TValue> cmp)
{
this._Dict = Dict;
_cmp = cmp;
}
public EnumDictionary(Dictionary<TKey, TValue> Dict)
:this(Dict, IEqualityComparer<TValue>.Default){}
public TKey GetValue(TValue value)
{
foreach (KeyValuePair<TKey, TValue> kvp in _Dict)
{
if (cmp.Equals(kvp.Value, value))
return kvp.Key;
}
throw new Exception("Undefined data type: " + value);
}
}
Lot of times I have to create a Dictionary<KeyType, List<ValueType>>
Before I can start using the dictionary I have to first verify that List has been created for that key.
//Can i remove these two lines?
if(!dict.ContainsKey(key))
dict[key]= new List<ValueType>;
//now use the key
dict[key].Add(value);
I know its only "2 lines" of code but it annoys me and I think it can be removed.
I can extend dictionary in someway but before I do it, I want to know if someone has found a clever way to remove the above if statement.
Basically i want to create a Dictionary<KeyType, Collection<ValueType>> and start using it right away like dict[key].Add(value).
You could create something like Google Java Collection's Multimap... or you could just add an extension method like this:
public static void AddValue<TKey, TValue>
(this IDictionary<TKey, List<TValue>> dictionary, TKey key, TValue value)
{
List<TValue> values;
if (!dictionary.TryGetValue(key, out values))
{
values = new List<TValue>();
dictionary.Add(key, values);
}
values.Add(value);
}
As Bevan says, Lookup can help as well - but you can only create one with the ToLookup method, and you can't modify it afterwards. In many cases that's a thoroughly good thing, but if you need a mutable map then you'll ned something like the above.
Have a look at the LookUp class introduced with Linq in .NET 3.5 - it might be just what you're looking for: a Dictionary like class that supports multiple items per key.
Perhaps the only significant downside is that you have to have all your elements available in one batch, as LookUp is immutable.
The ConcurrentDictionary<T,K>.GetOrAdd method is amazingly useful.
private ConcurrentDictionary<string, ICollection<int>> _dictionary;
private static ICollection<int> CreateEmptyList(string dummyKey)
{
return new List<int>();
}
private void AddValue(string key, int value)
{
ICollection<int> values = _dictionary.GetOrAdd(key, CreateEmptyList);
values.Add(value);
}
Edit: Here's an example of how to implement the feature as an extension method for IDictionary<T,K> (C# 3):
Note that IDictionary<TKey, TValue> is generally not thread safe, so if you want thread safety with this extension method you'll have to manually implement it just as for other operations.
public static TValue GetOrAdd<TKey, TValue>(
this IDictionary<TKey, TValue> dictionary,
TKey key,
Func<TKey, TValue> valueFactory)
{
TValue value;
if (!dictionary.TryGetValue(key, out value))
{
value = valueFactory(key);
dictionary.Add(key, value);
}
return value;
}
To add to the answers, you can also add a more general extension which accepts a delegate for instantiation:
public static TValue GetOrCreate<TKey, TValue>
(this IDictionary<TKey, TValue> dict,
TKey key,
Func<TKey, TValue> getValue)
{
TValue value;
if (!dict.TryGetValue(key, out value))
{
dict.Add(key, getValue(key));
}
return value;
}
and then you can provide whatever instantiation method you like:
Dictionary<int, string> dict = new Dictionary<int, string>();
string result = dict.GetOrCreate(5, i => i.ToString());