I am trying to build a dictionary where the key is a property of the value object. However I would like to construct the value object in the dictionary's add method. Is there a way to do this without using an intermediate variable?
For example I would like to do the following, but of course the key value isn't available when needed.
Dictionary<int,SomeComplexObject> dict = new Dicionary<int,SomeComplexObject>{
{someComplexObject.Key, new SomeComplexObject {Key = 1, Name = "FooBar"},
{someComplexObject.Key, new SomeComplexObject {Key = 2, Name = "FizzBang"}
};
Do I have to do it this ugly way:
Dictionary<int,SomeComplexObject> dict = new Dicionary<int,SomeComplexObject>();
SomeComplexObject value1 = new SomeComplexObject{Key=1,Name = "FooBar"};
dict.Add(value1.Key,value1);
SomeComplexObject value2 = new SomeComplexObject{Key=2,Name = "FizzBang"};
dict.Add(value.Key,value2);
I don't think this is the same question as
How to use an object's identity as key for Dictionary<K,V>
because I am not asking specifically about the key of a dictionary but if there is a way to have access to a objects property when the object is not being created until later in the methods parameter list.
I don't think an extension method (as proposed in comments) is really what you want here, as it's only a partial solution. I.e. you would have to write a new extension method for each dictionary value type you wanted to use, which negates the whole point of asking for a general solution.
Instead, it seems to me that you probably just want to subclass the Dictionary<TKey, TValue> type to add your own custom behavior. You can do this just once, in a general-purpose way, so that you can provide a delegate for each type you expect to have to use this way.
That would look something like this:
class KeyExtractorDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
private readonly Func<TValue, TKey> _extractor;
public KeyExtractorDictionary(Func<TValue, TKey> extractor)
{
_extractor = extractor;
}
public void Add(TValue value)
{
Add(_extractor(value), value);
}
}
You would use it something like this:
class Data
{
public int Key { get; }
public string Name { get; }
public Data(int key, string name)
{
Key = key;
Name = name;
}
}
class Program
{
static void Main(string[] args)
{
KeyExtractorDictionary<int, Data> dictionary =
new KeyExtractorDictionary<int, Data>(d => d.Key)
{
new Data(1, "FooBar"),
new Data(2, "FizzBang")
};
}
}
(I used Data as the value type type, instead of T as you seem to have used in your question, to avoid confusing the type name with a generic type parameter.)
In this way, you only have to write the class once, regardless of how many different types you might want to use for this type of dictionary. You can then pass the class constructor the key extractor delegate appropriate for the current value type of the dictionary.
Note that doing it this way, you also can take advantage of C#'s collection initializer syntax. Since your new type has an Add() method that takes just the value for each dictionary entry, the compiler will translate a collection initializer into the correct calls to add each object to the dictionary.
This allows for a dictionary in which you can still retrieve objects solely by the key value (using a custom comparer would require an instance of the value type with the same key you're looking for), while still addressing the broader concerns of not having to specify the key explicitly when adding objects, and of generality and reuse.
You can try an extension method, which is less invasive:
public static void AddByKey<TKey, T>(this Dictionary<TKey, T> dictionary, T item)
{
dictionary.Add(item.Key, item);
}
But to really do this correctly you also need an interface to protect you against types without the Key property:
public interface ItemWithKey<TKey>
{
TKey Key { get; }
}
public static void AddByKey<TKey, T>(this Dictionary<TKey, T> dictionary, T item)
where T : ItemWithKey<TKey>
{
dictionary.Add(item.Key, item);
}
I don't have a compiler in my hands right now, I cannot test this code so minor errors may have slipped in. I hope you get the idea and usefulness if you have those cases a lot in your code. Otherwise, I'd advise to go with the ugly working code you already have.
You can use the ToDictionary() extension method to solve this issue. Here is a complete example that can be run in LINQPad.
void Main()
{
Dictionary<int, SomeComplextObject> dict = new List<SomeComplextObject>{
{new SomeComplextObject {Key = 1, Name = "FooBar"}},
{new SomeComplextObject {Key = 2, Name = "FizzBangr"}}
}.ToDictionary(k =>k.Key);
//Dump Dictionary to LINQPad's result window.
dict.Dump();
}
public class SomeComplextObject{
public int Key { get; set; }
public string Name {get;set;}
}
I would like to propose a different slightly different way of going about this, it's similar to #pid 's method but instead of an interface uses a linq expression. First built your list of objects, then use an extension method to add them to your dictionary in a single simple step. In my mind this is also a little more intuitive to read, your program would look like:
class Program
{
static void Main(string[] args)
{
List<SomeComplexObject> toAdd = new List<SomeComplexObject>() {
new SomeComplexObject(1,"FooBar"),
new SomeComplexObject(2,"FizzBang")
};
var dict = new Dictionary<int,SomeComplexObject>();
dict.AddByKey(toAdd, item => item.Key);
}
}
Where AddByKey is an extension method that uses linq to basically pass a reference to that property and would look like this:
using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Reflection;
public static class DictionaryExtensions
{
/// <summary>
/// This extension method was built for when you want to add a list of items to a dictionary as the values, and you want to use one of those
/// items' properties as the key. It uses LINQ to check by property reference.
/// </summary>
/// <typeparam name="TKey"></typeparam>
/// <typeparam name="TValue"></typeparam>
/// <param name="dict"></param>
/// <param name="targets"></param>
/// <param name="propertyToAdd"></param>
public static void AddByKey<TKey, TValue>(this Dictionary<TKey, TValue> dict, IEnumerable<TValue> targets, Expression<Func<TValue, TKey>> propertyToAdd)
{
MemberExpression expr = (MemberExpression)propertyToAdd.Body;
PropertyInfo prop = (PropertyInfo)expr.Member;
foreach (var target in targets)
{
var value = prop.GetValue(target);
if (!(value is TKey))
throw new Exception("Value type does not match the key type.");//shouldn't happen.
dict.Add((TKey)value, target);
}
}
}
And if you wanted to simplify the calling code even further, instead of returning void, you could have that extension method return the originally passed dictionary, and then your calling code could be collapsed to:
var dict = new Dictionary<int,SomeComplexObject>().AddByKey(toAdd, item => item.Key);
Related
I'd like to create a Dictionary object, with string Keys, holding values which are of a generic type. I imagine that it would look something like this:
Dictionary<string, List<T>> d = new Dictionary<string, List<T>>();
And enable me to add the following:
d.Add("Numbers", new List<int>());
d.Add("Letters", new List<string>());
I know that I can do it for a list of strings, for example, using this syntax:
Dictionary<string, List<string>> d = new Dictionary<string, List<string>>();
d.Add("Key", new List<string>());
but I'd like to do it for a generic list if possible...
2 questions then:
Is it possible?
What's the syntax?
EDIT: Now I've reread the question...
You can't do this, but a custom collection would handle it to some extent. You'd basically have a generic Add method:
public void Add<T>(string key, List<T> list)
(The collection itself wouldn't be generic - unless you wanted to make the key type generic.)
You couldn't extract values from it in a strongly typed manner though, because the compiler won't know which type you've used for a particular key. If you make the key the type itself, you end with a slightly better situation, but one which still isn't supported by the existing collections. That's the situation my original answer was responding to.
EDIT: Original answer, when I hadn't quite read the question correctly, but which may be informative anyway...
No, you can't make one type argument depend on another, I'm afraid. It's just one of the things one might want to express in a generic type system but which .NET's constraints don't allow for. There are always going to be such problems, and the .NET designers chose to keep generics relatively simple.
However, you can write a collection to enforce it fairly easily. I have an example in a blog post which only keeps a single value, but it would be easy to extend that to use a list.
Would something like this work?
public class GenericDictionary
{
private Dictionary<string, object> _dict = new Dictionary<string, object>();
public void Add<T>(string key, T value) where T : class
{
_dict.Add(key, value);
}
public T GetValue<T>(string key) where T : class
{
return _dict[key] as T;
}
}
Basically it wraps all the casting behind the scenes for you.
How about Dictionary<string, dynamic>? (assuming you're on C# 4)
Dictionary<string, dynamic> Dict = new Dictionary<string, dynamic>();
Source: https://stackoverflow.com/a/5038029/3270733
I prefer this way of putting generic types into a collection:
interface IList
{
void Add (object item);
}
class MyList<T> : List<T>, IList
{
public void Add (object item)
{
base.Add ((T) item); // could put a type check here
}
}
class Program
{
static void Main (string [] args)
{
SortedDictionary<int, IList>
dict = new SortedDictionary<int, IList> ();
dict [0] = new MyList<int> ();
dict [1] = new MyList<float> ();
dict [0].Add (42);
dict [1].Add ("Hello"); // Fails! Type cast exception.
}
}
But you do lose the type checks at compile time.
I came to a type safe implementation using ConditionalWeakTable.
public class FieldByType
{
static class Storage<T>
where T : class
{
static readonly ConditionalWeakTable<FieldByType, T> table = new ConditionalWeakTable<FieldByType, T>();
public static T GetValue(FieldByType fieldByType)
{
table.TryGetValue(fieldByType, out var result);
return result;
}
public static void SetValue(FieldByType fieldByType, T value)
{
table.Remove(fieldByType);
table.Add(fieldByType, value);
}
}
public T GetValue<T>()
where T : class
{
return Storage<T>.GetValue(this);
}
public void SetValue<T>(T value)
where T : class
{
Storage<T>.SetValue(this, value);
}
}
It can be used like this:
/// <summary>
/// This class can be used when cloning multiple related objects to store cloned/original object relationship.
/// </summary>
public class CloningContext
{
readonly FieldByType dictionaries = new FieldByType();
public void RegisterClone<T>(T original, T clone)
{
var dictionary = dictionaries.GetValue<Dictionary<T, T>>();
if (dictionary == null)
{
dictionary = new Dictionary<T, T>();
dictionaries.SetValue(dictionary);
}
dictionary[original] = clone;
}
public bool TryGetClone<T>(T original, out T clone)
{
var dictionary = dictionaries.GetValue<Dictionary<T, T>>();
if (dictionary == null)
{
clone = default(T);
return false;
}
return dictionary.TryGetValue(original, out clone);
}
}
See also this question where the type of the values is stored in as a generic parameter of the keys.
We're using lots of reflection to create an extensible administration tool. We needed a way to register items in the global search in the module definition. Each search would return results in a consistent way, but each one had different dependencies. Here's an example of us registering search for a single module:
public void ConfigureSearch(ISearchConfiguration config)
{
config.AddGlobalSearchCallback<IEmploymentDataContext>((query, ctx) =>
{
return ctx.Positions.Where(p => p.Name.Contains(query)).ToList().Select(p =>
new SearchResult("Positions", p.Name, p.ThumbnailUrl,
new UrlContext("edit", "position", new RouteValueDictionary(new { Id = p.Id }))
));
});
}
In the background during module registration, we iterate over every module and add the Func to a SearchTable with an instance of:
public class GenericFuncCollection : IEnumerable<Tuple<Type, Type, Object>>
{
private List<Tuple<Type, Type, Object>> objects = new List<Tuple<Type, Type, Object>>();
/// <summary>
/// Stores a list of Func of T where T is unknown at compile time.
/// </summary>
/// <typeparam name="T1">Type of T</typeparam>
/// <typeparam name="T2">Type of the Func</typeparam>
/// <param name="func">Instance of the Func</param>
public void Add<T1, T2>(Object func)
{
objects.Add(new Tuple<Type, Type, Object>(typeof(T1), typeof(T2), func));
}
public IEnumerator<Tuple<Type, Type, object>> GetEnumerator()
{
return objects.GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return objects.GetEnumerator();
}
}
Then when we finally call it, we do it with reflection:
var dependency = DependencyResolver.Current.GetService(search.Item1);
var methodInfo = search.Item2.GetMethod("Invoke");
return (IEnumerable<SearchResult>)methodInfo.Invoke(search.Item3, new Object[] { query, dependency });
I didn't find what I was looking for here but after reading I think it might be what is being asked for so an attempt to answer.
The problem is that when you use Dictionary it is a closed constructed type and all elements must be of the TValue type. I see this question in a number of places without a good answer.
Fact is that I want indexing but each element to have a different type and based on the value of TKey we already know the type. Not trying to get around the boxing but trying to simply get more elegant access something like DataSetExtensions Field. And don't want to use dynamic because the types are known and it is just not wanted.
A solution can be to create a non generic type that does not expose T at the class level and therefore cause the TValue part of the dictionary to be closed constructed. Then sprinkle in a fluent method to help initialization.
public class GenericObject
{
private object value;
public T GetValue<T>()
{
return (T)value;
}
public void SetValue<T>(T value)
{
this.value = value;
}
public GenericObject WithValue<T>(T value)
{
this.value = value;
return this;
}
}
class Program
{
static void Main(string[] args)
{
Dictionary<string, GenericObject> dict = new Dictionary<string, GenericObject>();
dict["mystring"] = new GenericObject().WithValue<string>("Hello World");
dict["myint"] = new GenericObject().WithValue<int>(1);
int i = dict["myint"].GetValue<int>();
string s = dict["mystring"].GetValue<string>();
}
}
Other posibility it's to use the variable dynamic.
For example:
Dictionary<string, List<dynamic>> d = new Dictionary<string, List<dynamic>>();
d.Add("Key", new List<dynamic>());
the variable dynamic resolve the type on runtime.
No, but you can use object instead of generic type.
Long answer:
The current version of C# will not allow you to make entries of generic type in a dictionary. Your options are either a) create a custom class that is the same as a dictionary except allow it to accept generic types, or b) make your Dictionary take values of type object. I find option b to be the simpler approach.
If you send lists of specific types, then when you go to process the lists you will have to test to see what kind of list it is. A better approach is to create lists of objects; this way you can enter integers, strings, or whatever data type you want and you don't necessarily have to test to see what type of object the List holds. This would (presumably) produce the effect you're looking for.
Here is a short console program that does the trick:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace dictionary
{
class Program
{
static void Main(string[] args)
{
Dictionary<string, object> dic = new Dictionary<string, object>();
var lstIntList = new List<object>();
var lstStrings = new List<object>();
var lstObjects = new List<object>();
string s = "";
lstIntList.Add(1);
lstIntList.Add(2);
lstIntList.Add(3);
lstStrings.Add("a");
lstStrings.Add("b");
lstStrings.Add("c");
dic.Add("Numbers", lstIntList);
dic.Add("Letters", lstStrings);
foreach (KeyValuePair<string, object> kvp in dic)
{
Console.WriteLine("{0}", kvp.Key);
lstObjects = ((IEnumerable)kvp.Value).Cast<object>().ToList();
foreach (var obj in lstObjects)
{s = obj.ToString(); Console.WriteLine(s);}
Console.WriteLine("");
}
Console.WriteLine("");
Console.WriteLine("press any key to exit");
Console.ReadKey();
}//end main
}
}
One of the way is to create a Dictionary value with type "object" like:
Dictionary<string, object> d = new Dictionary<string, object>();
So, here object datatype is used as a generic datatype, you can put anything in this as a value.
Or it's possible to use generic Type like this
public static void SafeUpdateInDictionary<T, L>(T DictionaryToBeUpdated, string Key, L Value) where T : Dictionary<string, L>
{
if (DictionaryToBeUpdated != null)
{
if(Value != null)
{
if (!DictionaryToBeUpdated.ContainsKey(Key))
DictionaryToBeUpdated.Add(Key, Value);
else
DictionaryToBeUpdated[Key] = Value;
}
}
}
I'm looking for a collection.
I need to be able to add elements as if using a 2D integer key, for example .Add(3, 4, element). If I add outside the range of the collection I need the collection to expand, this include negatively, although it can have a limit, for example the range of an Int16 would be good. Every element in the collection can have the same type as each other but I need to specify what that is, for example Set<type> s;
I also need to avoid slow operations such as searching when looking up an element, performance is less important when adding to the collection.
Does anyone have any ideas about what approach to use or best could provide the class in there answer.
If you want a compound key, you can use the Tuple<T1,T2> class in a : Dictionary<Tuple<T1,T2>, TItem>.
var coll = new Dictionary<Tuple<int,int>, AnyClass>();
coll.Add(new Tuple<int,int>(2, 3), new AnyClass("foo"));
coll.Add(new Tuple<int,int>(4, 2), new AnyClass("bar"));
var foo = coll[new Tuple<int,int>(2,3)];
var bar = coll[new Tuple<int,int>(4,2)];
If the syntax is too weird, you may wrap the class like this :
public class Dictionary2d<TKey1, TKey2, TItem> : Dictionary<Tuple<TKey1, TKey2>,TItem>
{
public void Add(TKey1 k1, TKey2, TItem item) {
this.Add(Tuple.Create(k1,k2), item);
}
public TItem this[TKey1 k1, TKey2 k2] {
get { return this[Tuple.Create(k1,k2)]; }
}
}
public class Program
{
static void Main() {
var coll = new Dictionary2d<int,int, AnyClass>();
coll.Add(2, 3, new AnyClass("foo"));
coll.Add(4, 2, new AnyClass("bar"));
var foo = coll[2,3];
var bar = coll[4,2];
}
}
The benefits of using Tuple class, is that the equality and hashcode comparison is natively handled, so even if it's a class, two differents instances of tuple with same values will be considered equals.
It sounds like you want a Dictionary<int, T>.
You can implement this Set<T> by storing its data in a private variable of type Dictionary<int, Dictionary<int, T>>.
You can then store using
public void Add(int key1, int key2, T value)
{
_storage[key1][key2] = value;
}
There's an XML scheme which says something like this:
<ExtraFields>
<ExtraField Type="Int">
<Key>Mileage</Key>
<Value>500000 </Value>
</ExtraField>
<ExtraField Type="String">
<Key>CarModel</Key>
<Value>BMW</Value>
</ExtraField>
<ExtraField Type="Bool">
<Key>HasAbs</Key>
<Value>True</Value>
</ExtraField>
</ExtraFields>
I want to store this info in the class and I want its field to be of the specified type. I thought of a generic approach
static class Consts
{
public const string Int32Type = "int32";
public const string StringType = "string";
public const string BoolType = "bool";
}
public class ExtraFieldValue<TValue>
{
public string Key;
public TValue Value;public static ExtraFieldValue<TValue> CreateExtraField(string strType, string strValue, string strKey)
{
IDictionary<string, Func<string, object>> valueConvertors = new Dictionary<string, Func<string, object>> {
{ Consts.Int32Type, value => Convert.ToInt32(value)},
{ Consts.StringType, value => Convert.ToString(value)},
{ Consts.BoolType, value => Convert.ToBoolean(value)}
};
if (!valueConvertors.ContainsKey(strType))
return null;
ExtraFieldValue<TValue> result = new ExtraFieldValue<TValue>
{
Key = strKey,
Value = (TValue)valueConvertors[strType](strValue)
};
return result;
}
}
But the problem with this approach is that I need a list of ExtraFields and each of them can have different type in a list.
I can only think of two options so far:
1) using a dynamic keyword for this field but this approach seems to have limits
2) using an object type for the field and casting its dynamic type to the necessary type. But anyhow if I will need some object specific calls, I will have to make a static cast.
I would be glad to read your thoughts/proposals
Just use a name/value collection. If you don't even know the property names until runtime, using dynamic, or dynamically building a type at runtime isn't going to help you because you won't be able to write source that accesses those properties.
So, just use a name/value collection like something that implements IDictionary<string, object>.
I need to add key/object pairs to a dictionary, but I of course need to first check if the key already exists otherwise I get a "key already exists in dictionary" error. The code below solves this but is clunky.
What is a more elegant way of doing this without making a string helper method like this?
using System;
using System.Collections.Generic;
namespace TestDictStringObject
{
class Program
{
static void Main(string[] args)
{
Dictionary<string, object> currentViews = new Dictionary<string, object>();
StringHelpers.SafeDictionaryAdd(currentViews, "Customers", "view1");
StringHelpers.SafeDictionaryAdd(currentViews, "Customers", "view2");
StringHelpers.SafeDictionaryAdd(currentViews, "Employees", "view1");
StringHelpers.SafeDictionaryAdd(currentViews, "Reports", "view1");
foreach (KeyValuePair<string, object> pair in currentViews)
{
Console.WriteLine("{0} {1}", pair.Key, pair.Value);
}
Console.ReadLine();
}
}
public static class StringHelpers
{
public static void SafeDictionaryAdd(Dictionary<string, object> dict, string key, object view)
{
if (!dict.ContainsKey(key))
{
dict.Add(key, view);
}
else
{
dict[key] = view;
}
}
}
}
Just use the indexer - it will overwrite if it's already there, but it doesn't have to be there first:
Dictionary<string, object> currentViews = new Dictionary<string, object>();
currentViews["Customers"] = "view1";
currentViews["Customers"] = "view2";
currentViews["Employees"] = "view1";
currentViews["Reports"] = "view1";
Basically use Add if the existence of the key indicates a bug (so you want it to throw) and the indexer otherwise. (It's a bit like the difference between casting and using as for reference conversions.)
If you're using C# 3 and you have a distinct set of keys, you can make this even neater:
var currentViews = new Dictionary<string, object>()
{
{ "Customers", "view2" },
{ "Employees", "view1" },
{ "Reports", "view1" },
};
That won't work in your case though, as collection initializers always use Add which will throw on the second Customers entry.
What's wrong with...
dict[key] = view;
It'll automatically add the key if it's non-existent.
simply
dict[key] = view;
From the MSDN documentation of Dictionary.Item
The value associated with the
specified key. If the specified key is
not found, a get operation throws a
KeyNotFoundException, and a set
operation creates a new element with
the specified key.
My emphasis
As usual John Skeet gets in there with lighting speed with the right answer, but interestingly you could also have written your SafeAdd as an Extension Method on IDictionary.
public static void SafeAdd(this IDictionary<K, T>. dict, K key, T value)...
Although using the indexer is clearly the right answer for your specific problem, another more general answer to the problem of adding additional functionality to an existing type would be to define an extension method.
Obviously this isn't a particularly useful example, but something to bear in mind for the next time you find a real need:
public static class DictionaryExtensions
{
public static void SafeAdd<TKey, TValue>(this Dictionary<TKey, TValue> dict,
TKey key, TValue value)
{
dict[key] = value;
}
}
Is there anything built into the core C# libraries that can give me an immutable Dictionary?
Something along the lines of Java's:
Collections.unmodifiableMap(myMap);
And just to clarify, I am not looking to stop the keys / values themselves from being changed, just the structure of the Dictionary. I want something that fails fast and loud if any of IDictionary's mutator methods are called (Add, Remove, Clear).
No, but a wrapper is rather trivial:
public class ReadOnlyDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
IDictionary<TKey, TValue> _dict;
public ReadOnlyDictionary(IDictionary<TKey, TValue> backingDict)
{
_dict = backingDict;
}
public void Add(TKey key, TValue value)
{
throw new InvalidOperationException();
}
public bool ContainsKey(TKey key)
{
return _dict.ContainsKey(key);
}
public ICollection<TKey> Keys
{
get { return _dict.Keys; }
}
public bool Remove(TKey key)
{
throw new InvalidOperationException();
}
public bool TryGetValue(TKey key, out TValue value)
{
return _dict.TryGetValue(key, out value);
}
public ICollection<TValue> Values
{
get { return _dict.Values; }
}
public TValue this[TKey key]
{
get { return _dict[key]; }
set { throw new InvalidOperationException(); }
}
public void Add(KeyValuePair<TKey, TValue> item)
{
throw new InvalidOperationException();
}
public void Clear()
{
throw new InvalidOperationException();
}
public bool Contains(KeyValuePair<TKey, TValue> item)
{
return _dict.Contains(item);
}
public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
{
_dict.CopyTo(array, arrayIndex);
}
public int Count
{
get { return _dict.Count; }
}
public bool IsReadOnly
{
get { return true; }
}
public bool Remove(KeyValuePair<TKey, TValue> item)
{
throw new InvalidOperationException();
}
public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
{
return _dict.GetEnumerator();
}
System.Collections.IEnumerator
System.Collections.IEnumerable.GetEnumerator()
{
return ((System.Collections.IEnumerable)_dict).GetEnumerator();
}
}
Obviously, you can change the this[] setter above if you want to allow modifying values.
As far as I know, there is not. But maybe you can copy some code (and learn a lot) from these articles:
Immutability in C# Part One: Kinds of Immutability
Immutability in C# Part Two: A Simple Immutable Stack
Immutability in C# Part Three: A Covariant Immutable Stack
Immutability in C# Part Four: An Immutable Queue
Immutability in C# Part Five: LOLZ
Immutability in C# Part Six: A Simple Binary Tree
Immutability in C# Part Seven: More on Binary Trees
Immutability in C# Part Eight: Even More On Binary Trees
Immutability in C# Part Nine: Academic? Plus my AVL tree implementation
Immutability in C# Part 10: A double-ended queue
Immutability in C# Part Eleven: A working double-ended queue
With the release of .NET 4.5, there is a new ReadOnlyDictionary class. You simply pass an IDictionary to the constructor to create the immutable dictionary.
Here is a helpful extension method which can be used to simplify creating the readonly dictionary.
I know this is a very old question, but I somehow found it in 2020 so I suppose it may be worth noting that there is a way to create immutable dictionary now:
https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutabledictionary.toimmutabledictionary?view=netcore-3.1
Usage:
using System.Collections.Immutable;
public MyClass {
private Dictionary<KeyType, ValueType> myDictionary;
public ImmutableDictionary<KeyType, ValueType> GetImmutable()
{
return myDictionary.ToImmutableDictionary();
}
}
Adding onto dbkk's answer, I wanted to be able to use an object initializer when first creating my ReadOnlyDictionary. I made the following modifications:
private readonly int _finalCount;
/// <summary>
/// Takes a count of how many key-value pairs should be allowed.
/// Dictionary can be modified to add up to that many pairs, but no
/// pair can be modified or removed after it is added. Intended to be
/// used with an object initializer.
/// </summary>
/// <param name="count"></param>
public ReadOnlyDictionary(int count)
{
_dict = new SortedDictionary<TKey, TValue>();
_finalCount = count;
}
/// <summary>
/// To allow object initializers, this will allow the dictionary to be
/// added onto up to a certain number, specifically the count set in
/// one of the constructors.
/// </summary>
/// <param name="key"></param>
/// <param name="value"></param>
public void Add(TKey key, TValue value)
{
if (_dict.Keys.Count < _finalCount)
{
_dict.Add(key, value);
}
else
{
throw new InvalidOperationException(
"Cannot add pair <" + key + ", " + value + "> because " +
"maximum final count " + _finalCount + " has been reached"
);
}
}
Now I can use the class like so:
ReadOnlyDictionary<string, string> Fields =
new ReadOnlyDictionary<string, string>(2)
{
{"hey", "now"},
{"you", "there"}
};
The open-source PowerCollections library includes a read-only dictionary wrapper (as well as read-only wrappers for pretty much everything else), accessible via a static ReadOnly() method on the Algorithms class.
I don't think so. There is a way to create a read-only List and read only Collection, but I don't think there's a built in read only Dictionary. System.ServiceModel has a ReadOnlyDictinoary implementation, but its internal. Probably wouldn't be too hard to copy it though, using Reflector, or to simply create your own from scratch. It basically wraps an Dictionary and throws when a mutator is called.
One workaround might be, throw a new list of KeyValuePair from the Dictionary to keep the original unmodified.
var dict = new Dictionary<string, string>();
dict.Add("Hello", "World");
dict.Add("The", "Quick");
dict.Add("Brown", "Fox");
var dictCopy = dict.Select(
item => new KeyValuePair<string, string>(item.Key, item.Value));
// returns dictCopy;
This way the original dictionary won't get modified.
"Out of the box" there is not a way to do this. You can create one by deriving your own Dictionary class and implementing the restrictions you need.
I've found an implementation of an Inmutable (not READONLY) implementation of a AVLTree for C# here.
An AVL tree has logarithmic (not constant) cost on each operation, but stills fast.
http://csharpfeeds.com/post/7512/Immutability_in_Csharp_Part_Nine_Academic_Plus_my_AVL_tree_implementation.aspx
You could try something like this:
private readonly Dictionary<string, string> _someDictionary;
public IEnumerable<KeyValuePair<string, string>> SomeDictionary
{
get { return _someDictionary; }
}
This would remove the mutability problem in favour of having your caller have to either convert it to their own dictionary:
foo.SomeDictionary.ToDictionary(kvp => kvp.Key);
... or use a comparison operation on the key rather than an index lookup, e.g.:
foo.SomeDictionary.First(kvp => kvp.Key == "SomeKey");
In general it is a much better idea to not pass around any dictionaries in the first place (if you don't HAVE to).
Instead - create a domain-object with an interface that doesn't offer any methods modifying the dictionary (that it wraps). Instead offering required LookUp-method that retrieves element from the dictionary by key (bonus is it makes it easier to use than a dictionary as well).
public interface IMyDomainObjectDictionary
{
IMyDomainObject GetMyDomainObject(string key);
}
internal class MyDomainObjectDictionary : IMyDomainObjectDictionary
{
public IDictionary<string, IMyDomainObject> _myDictionary { get; set; }
public IMyDomainObject GetMyDomainObject(string key) {.._myDictionary .TryGetValue..etc...};
}
Since Linq, there is a generic interface ILookup.
Read more in MSDN.
Therefore, To simply get immutable dictionary you may call:
using System.Linq;
// (...)
var dictionary = new Dictionary<string, object>();
// (...)
var read_only = dictionary.ToLookup(kv => kv.Key, kv => kv.Value);
There's also another alternative as I have described at:
http://www.softwarerockstar.com/2010/10/readonlydictionary-tkey-tvalue/
Essentially it's a subclass of ReadOnlyCollection>, which gets the work done in a more elegant manner. Elegant in the sense that it has compile-time support for making the Dictionary read-only rather than throwing exceptions from methods that modify the items within it.