I'm trying to implement an extension method for all dictionaries whose value is of a type that implement a certain interface.
In this case I would like to have a ToListSortedByValue() method that returns
List<KeyValuePair<string, IComparable>>
for any dictionary of type
Dictionary<string, IComparable>
that would be cool because it would allow me to use dictionaries instead of lists, but to be able to have them sorted when needed (for example when printing in files or at console).
This is what I tried, but it doesn't work, any idea why?
public static List<KeyValuePair<string, IComparable>> ToListSortedByValue(this Dictionary<string, IComparable> Dic)
{
return Dic.OrderBy(x => x.Value).ToList();
}
EDIT:
it's solved already, but for completeness sake this is the problem I got:
when trying to use the method I got the an error as if such method didn't exist. If instead of IComparable I use an actual comparable type, let's say int or a class implementing IComparable, than it would work.
Basically you need to make the method generic on the value type and then constrain that type to be IComparable<T>.
public static List<KeyValuePair<string, T>> ToListSortedByValue<T>(
this Dictionary<string, T> Dic) where T : IComparable<T>
{
return Dic.OrderBy(x => x.Value).ToList();
}
This has the added bonus of returning the values as there passed in type. You might even want to make the key type generic too so it's not limited to just string
public static List<KeyValuePair<TKey, TValue>> ToListSortedByValue<TKey, TValue>(
this Dictionary<TKey, TValue> Dic) where TValue : IComparable<TValue>
{
return Dic.OrderBy(x => x.Value).ToList();
}
You need to make your method generic, so that it extends your actual type instead of just IComparable:
public static List<KeyValuePair<string, T>> ToListSortedByValue<T>(this Dictionary<string, T> Dic) where T : IComparable<T>
Related
Say I have a Dictionary object which contains many instances of an object called SomeObject and each SomeObject has properties named Property1 and Property2. Now say I have a method which can return a new Dictionary sorted by Property1. However, I'd like to generalize that method such that I can tell it which property to sort by.
For example, to do this, I know I could easily have two separate methods such as:
public Dictionary<string, SomeObject> SortByProp1(Dictionary<string, SomeObject> dict) { ... }
public Dictionary<string, SomeObject> SortByProp2(Dictionary<string, SomeObject> dict) { ... }
But, I'm wondering if it's possible to combine these into one method in which I can give a parameter to identify which method to return? I don't know of a way to generalize a property like that...
Note: I do realize I could always use something like an if-statement which could be based on a string of the property name, but that doesn't seem like a very elegant, acceptable answer much better than just having two separate methods to begin with.
The method you seek is built into the IEnumerable<T> interface, and is OrderBy, though it might not work out as smothly for a Dictionary as you might like.
Dictionary<TKey, TValue> implements IEnumerable<KeyValuePair<TKey, TValue>>, so its OrderBy method takes a lambda expression which itself takes a KeyValuePair<TKey, TValue>, and returns a selector method that describes what you're sorting by.
For example, if you have Dictionary<int, Customer>, this will return to you an IEnumerable<KeyValuePair<int, Customer>> sorted by customer name:
myDict.OrderBy(kvp => kvp.Value.Name);
Then to get just the customers themselves in this sorted order, you'd say:
IEnumerable<Customer> orderedByName = myDict.OrderBy(kvp => kvp.Value.Name).Select(kvp => kvp.Value);
Can you try adding a generic type parameter, for the ordering key - and accept a predicate for the ordering?
Such as:
public Dictionary<string, SomeObject> SortByProp<TProp>(Dictionary<string, SomeObject> dict, Expression<SomeObject,TProp> orderPredicate)
{
return dict.OrderBy(orderPredicate);
}
// Usage:
SortByProp(apples, x => x.AppleColor);
SortByProp(oranges, x => x.OrangeType);
You can accept a delegate to select the property:
public Dictionary<string, SomeObject> Sort<TProperty>(Dictionary<string, SomeObject> dict, Func<SomeObject, TProperty> selector) { ... }
You can call it like this:
var sortedDict = Sort(dict, o => o.Property1);
I am working in the .NET 2.0 framework. I have some code working, just want it working a little more elegantly.
I have a need to effectively "mirror" a Dictionary object such that if we start with an object like this
Dictionary<TKey,TValue> StartDictionary;
We can Mirror it like this
Dictionary<TValue,TKey> MirroredDictionary = MirrorDictionary(StartDictionary);
And we would end up with a new dictionary with the values and keys being swapped for each KeyValuePair
Before anyone asks me why: the source dictionary is fairly large and loaded once from reflection calls when my program loads. I don't want to run the same reflection calls a second time to load the mirrored dictionary. Creating a mirrored Dictionary and populating its values and keys the way I came up with seemed to me to be much less costly.
So being the kind of person that hates to rewrite things, I decided to write a Generic method in a helper class I have to do the Mirror using Generics.
Now mind you I've written simple Generic methods before for normal scalar types
Here's what I came up with
public static TOutDic MirrorDictionary<TInDic, TOutDic>(TInDic InDictionary)
where TInDic : IDictionary
where TOutDic : IDictionary
{
Type[] KVPTypes = typeof(TInDic).GetGenericArguments();
Type TKey = KVPTypes[0];
Type TValue = KVPTypes[1];
Type TDic = typeof(Dictionary<,>).MakeGenericType(TValue, TKey);
IDictionary OutDic = (IDictionary)Activator.CreateInstance(TDic);
foreach (DictionaryEntry DE in (IDictionary)InDictionary) OutDic.Add(DE.Value, DE.Key);
return (TOutDic)OutDic;
}
A little bit there but it works, Loads up the Types of the Keys and Values and creates an instance of the mirrored Dictionary
Then just looping through the base DictionaryEntries of the InDictionary it adds the items to the OutDic and returns it casting it to the Type expected
Compiles just fine
Now when i go to call it I would think just like when i call a Generic method for a scalar type I could just using our code snippits above say
Dictionary<TValue,TKey> MirroredDictionary = MirrorDictionary(StartDictionary);
But that does not compile gives me
The type arguments for method MirrorDictionary(TInDic)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
So If I call it instead like this
Dictionary<TValue, TKey> MirrorDic = MirrorDictionary<Dictionary<Tkey, TValue>, Dictionary<TValue,TKey>>(StringDic);
It compiles and works like a charm.
Now the question becomes how do I make it properly infer the Type being passed into this method when the Type being passed in and the Type being passed out are complex types like in this example?
You can make life much easier for the compiler by telling it the key and value types thus:
public static Dictionary<TValue, TKey> MirrorDictionary<TKey, TValue>
(Dictionary<TKey, TValue> source)
{
Dictionary<TValue, TKey> destination = new Dictionary<TValue, TKey>();
foreach (KeyValuePair<TKey, TValue> kvp in source)
{
destination.Add(kvp.Value, kvp.Key);
}
return destination;
}
I don't think you need reflection here at all.
Sample usage:
static void Main(string[] args)
{
Dictionary<int, string> source = new Dictionary<int, string>();
source.Add(3, "foo");
source.Add(4, "bar");
DumpDic(source);
DumpDic(MirrorDictionary(source));
Console.ReadLine();
}
where DumpDic is:
public static void DumpDic<TK, TV>(Dictionary<TK, TV> dic)
{
foreach (KeyValuePair<TK, TV> keyValuePair in dic)
{
Console.WriteLine("{0} => {1}", keyValuePair.Key, keyValuePair.Value);
}
}
Here's a 3.5 solution (you can also use it in 2.0 with VS2008 and LinqBridge)
IDictionary<TValue, TKey> MirrorDictionary<TKey, TValue>(IDictionary<TKey, TValue> dict)
{
return dict.ToDictionary(kvp => kvp.Value, kvp => kvp.Key);
}
And a pure 2.0 solution
IDictionary<TValue, TKey> MirrorDictionary<TKey, TValue>(IDictionary<TKey, TValue> dict)
{
Dictionary<TValue, TKey> newDict = new Dictionary<TValue, TKey>();
foreach(KeyValuePair<TKey, TValue> kvp in dict)
{
newDict.Add(kvp.Value, kvp.Key);
}
return newDict;
}
Type inference should work fine with both solutions (as they have the same signature)
You could define the Out dictionary as an out parameter. Type inference does not look the type of the variable you're assigning to, only the types of the parameters. That's the reason this doesn't compile.
You need to tell it what TValue and TKey are. Unless they are defined up in the signature of the method calling this code, they don't have any specific types. You need to give it something like:
Dictionary<string, int> MirroredDictionary = MirrorDictionary(StartDictionary);
I'm getting a "cannot convert from 'int' to 'TValue'" in the following code. How can I fix this? I understand why there is an issue, but I'm wonder the best way around this. Do I need to make a decision to specific type the ReturnValuesDict, and not leave it generic?
public class ReturnValuesDict<TKey, TValue> : CloneableDictionary<TKey, TValue>
{
public static ReturnValuesDict<TKey, TValue> CreateEmptyClone(ReturnValuesDict<TKey, TValue> current)
{
var newItem = new ReturnValuesDict<TKey, TValue>();
foreach (var curr in current)
{
newItem.Add(curr.Key, 0); // ERROR on the 2nd parameter here
}
return newItem;
}
}
The reason this does not compile is that 0 (an int) cannot, in general, be converted to the dictionary-values' type (TValue), which, as far as the compiler is concerned, is some arbitrary type. (where TValue : int wouldn't work, but that's another matter)
I assume you're trying to construct a dictionary with the same keys as the original, but with 'empty' values?
If you are ok with what .NET considers to be the 'default' value of a type, you could try changing the line to:
newItem.Add(curr.Key, default(TValue));
This will use the default-value of the dictionary-values' type. For example:null for reference-types, zero for numeric-value types.
If, on the other hand, the intention is to write a method that only works with dictionarys having intvalues, you could make it more restrictive (place this in another class):
public static ReturnValuesDict<TKey, int> CreateEmptyClone<TKey>(ReturnValuesDict<TKey, int> current)
{
var newItem = new ReturnValuesDict<TKey, int>();
foreach (var curr in current)
{
newItem.Add(curr.Key, 0);
}
return newItem;
}
Note that the method is now a generic method that takes an unconstrained genericTKeyparameter.
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);
}
}
I'm trying to write an extension method to insert data into a dictionary of dictionaries defined as follows:
items=Dictionary<long,Dictionary<int,SomeType>>()
What I have so far is:
public static void LeafDictionaryAdd<TKEY1,TKEY2,TVALUE>(this IDictionary<TKEY1,IDictionary<TKEY2,TVALUE>> dict,TKEY1 key1,TKEY2 key2,TVALUE value)
{
var leafDictionary =
dict.ContainsKey(key1)
? dict[key1]
: (dict[key1] = new Dictionary<TKEY2, TVALUE>());
leafDictionary.Add(key2,value);
}
but the compiler doesn't like it. The statement:
items.LeafDictionaryAdd(longKey, intKey, someTypeValue);
gives me a type inference error.
For the statement:
items.LeafDictionaryAdd<long, int, SomeType>(longKey, intKey, someTypeValue);
I get "...does not contain a definition for... and the best extension method overload has some invalid arguments.
What am I doing wrong?
Some inventive generic usage ;-p
class SomeType { }
static void Main()
{
var items = new Dictionary<long, Dictionary<int, SomeType>>();
items.Add(12345, 123, new SomeType());
}
public static void Add<TOuterKey, TDictionary, TInnerKey, TValue>(
this IDictionary<TOuterKey,TDictionary> data,
TOuterKey outerKey, TInnerKey innerKey, TValue value)
where TDictionary : class, IDictionary<TInnerKey, TValue>, new()
{
TDictionary innerData;
if(!data.TryGetValue(outerKey, out innerData)) {
innerData = new TDictionary();
data.Add(outerKey, innerData);
}
innerData.Add(innerKey, value);
}
Try to use a concrete type:
public static void LeafDictionaryAdd<TKEY1,TKEY2,TVALUE>(this IDictionary<TKEY1, Dictionary<TKEY2,TVALUE>> dict,TKEY1 key1,TKEY2 key2,TVALUE value)
see the Dictionary<TKEY2,TVALUE> instead of IDictionary<TKEY2,TVALUE>
I'm guessing that this is a covariance / contravariance issue. Your method signature is expecting an IDictionary of IDcitionaries, but you're passing it an IDictionary of Dictionary. Try using a concrete Dictionary instead in your method signature, for the inner Dictionary.
If you specify an IDictionary on your Parameter list for the Extension method,
then your items will not match that.
Either change your Extension to
public static void LeafDictionaryAdd<TKEY1,TKEY2,TVALUE>(
this IDictionary<TKEY1, Dictionary<TKEY2,TVALUE>> dict,
TKEY1 key1,
TKEY2 key2,
TVALUE value)
OR Try and cast your items to
((IDictionary<long, IDictionary<int, YourType>>)items).LeafDictionaryAdd(l, i, o);