按值获取字典键

我如何得到一个字典键值在c# ?

Dictionary<string, string> types = new Dictionary<string, string>()
{
{"1", "one"},
{"2", "two"},
{"3", "three"}
};

我想要这样的东西:

getByValueKey(string value);

getByValueKey("one")必须返回"1"。

最好的方法是什么?也许是哈希表或排序列表?

767243 次浏览
小开

你可以这样做:

  1. 通过循环遍历字典中的所有KeyValuePair<TKey, TValue>(如果字典中有大量条目，这将对性能造成相当大的影响)
  2. 使用两个字典，一个用于值到键映射，一个用于键到值映射(这会占用两倍的内存空间)。

如果不考虑性能，请使用方法1;如果不考虑内存，请使用方法2。

此外，所有键都必须是唯一的，但值不要求是唯一的。您可以有多个具有指定值的键。

小开
最佳答案

值不一定是唯一的，所以必须进行查找。你可以这样做:

var myKey = types.FirstOrDefault(x => x.Value == "one").Key;

如果值是唯一的，并且插入的频率低于读取，那么创建一个反向字典，其中值是键，键是值。

小开

我有一个很简单的方法。这对我来说很完美。

Dictionary<string, string> types = new Dictionary<string, string>();


types.Add("1", "one");
types.Add("2", "two");
types.Add("3", "three");


Console.WriteLine("Please type a key to show its value: ");
string rLine = Console.ReadLine();


if(types.ContainsKey(rLine))
{
string value_For_Key = types[rLine];
Console.WriteLine("Value for " + rLine + " is" + value_For_Key);
}
小开 
types.Values.ToList().IndexOf("one");

values . tolist()将您的字典值转换为对象列表。 IndexOf("one")搜索你的新列表，寻找"one"，并返回与字典中Key/Value对的索引相匹配的Index

这个方法并不关心字典中的键，它只是返回您正在寻找的值的索引。

请记住，在你的字典中可能有不止一个“1”值。这就是没有“get key”方法的原因。

小开

下面的代码只在包含唯一值数据时有效:

public string getKey(string Value)
{
if (dictionary.ContainsValue(Value))
{
var ListValueData = new List<string>();
var ListKeyData   = new List<string>();


var Values = dictionary.Values;
var Keys = dictionary.Keys;


foreach (var item in Values)
{
ListValueData.Add(item);
}


var ValueIndex = ListValueData.IndexOf(Value);
foreach (var item in Keys)
{
ListKeyData.Add(item);
}


return  ListKeyData[ValueIndex];
}
return string.Empty;
}
小开

也许是这样的:

foreach (var keyvaluepair in dict)
{
if(Object.ReferenceEquals(keyvaluepair.Value, searchedObject))
{
//dict.Remove(keyvaluepair.Key);
break;
}
}
小开

我遇到的情况是LINQ绑定不可用，必须显式展开lambda。它的结果是一个简单的函数:

public static T KeyByValue<T, W>(this Dictionary<T, W> dict, W val)
{
T key = default;
foreach (KeyValuePair<T, W> pair in dict)
{
if (EqualityComparer<W>.Default.Equals(pair.Value, val))
{
key = pair.Key;
break;
}
}
return key;
}

这样称呼它:

public static void Main()
{
Dictionary<string, string> dict = new Dictionary<string, string>()
{
{"1", "one"},
{"2", "two"},
{"3", "three"}
};


string key = KeyByValue(dict, "two");
Console.WriteLine("Key: " + key);
}

它适用于。net 2.0和其他有限的环境。

小开

我创建了一个双重查找类:

/// <summary>
/// dictionary with double key lookup
/// </summary>
/// <typeparam name="T1">primary key</typeparam>
/// <typeparam name="T2">secondary key</typeparam>
/// <typeparam name="TValue">value type</typeparam>
public class cDoubleKeyDictionary<T1, T2, TValue> {
private struct Key2ValuePair {
internal T2 key2;
internal TValue value;
}
private Dictionary<T1, Key2ValuePair> d1 = new Dictionary<T1, Key2ValuePair>();
private Dictionary<T2, T1> d2 = new Dictionary<T2, T1>();


/// <summary>
/// add item
/// not exacly like add, mote like Dictionary[] = overwriting existing values
/// </summary>
/// <param name="key1"></param>
/// <param name="key2"></param>
public void Add(T1 key1, T2 key2, TValue value) {
lock (d1) {
d1[key1] = new Key2ValuePair {
key2 = key2,
value = value,
};
d2[key2] = key1;
}
}


/// <summary>
/// get key2 by key1
/// </summary>
/// <param name="key1"></param>
/// <param name="key2"></param>
/// <returns></returns>
public bool TryGetValue(T1 key1, out TValue value) {
if (d1.TryGetValue(key1, out Key2ValuePair kvp)) {
value = kvp.value;
return true;
} else {
value = default;
return false;
}
}


/// <summary>
/// get key1 by key2
/// </summary>
/// <param name="key2"></param>
/// <param name="key1"></param>
/// <remarks>
/// 2x O(1) operation
/// </remarks>
/// <returns></returns>
public bool TryGetValue2(T2 key2, out TValue value) {
if (d2.TryGetValue(key2, out T1 key1)) {
return TryGetValue(key1, out value);
} else {
value = default;
return false;
}
}


/// <summary>
/// get key1 by key2
/// </summary>
/// <param name="key2"></param>
/// <param name="key1"></param>
/// <remarks>
/// 2x O(1) operation
/// </remarks>
/// <returns></returns>
public bool TryGetKey1(T2 key2, out T1 key1) {
return d2.TryGetValue(key2, out key1);
}


/// <summary>
/// get key1 by key2
/// </summary>
/// <param name="key2"></param>
/// <param name="key1"></param>
/// <remarks>
/// 2x O(1) operation
/// </remarks>
/// <returns></returns>
public bool TryGetKey2(T1 key1, out T2 key2) {
if (d1.TryGetValue(key1, out Key2ValuePair kvp1)) {
key2 = kvp1.key2;
return true;
} else {
key2 = default;
return false;
}
}


/// <summary>
/// remove item by key 1
/// </summary>
/// <param name="key1"></param>
public void Remove(T1 key1) {
lock (d1) {
if (d1.TryGetValue(key1, out Key2ValuePair kvp)) {
d1.Remove(key1);
d2.Remove(kvp.key2);
}
}
}


/// <summary>
/// remove item by key 2
/// </summary>
/// <param name="key2"></param>
public void Remove2(T2 key2) {
lock (d1) {
if (d2.TryGetValue(key2, out T1 key1)) {
d1.Remove(key1);
d2.Remove(key2);
}
}
}


/// <summary>
/// clear all items
/// </summary>
public void Clear() {
lock (d1) {
d1.Clear();
d2.Clear();
}
}


/// <summary>
/// enumerator on key1, so we can replace Dictionary by cDoubleKeyDictionary
/// </summary>
/// <param name="key1"></param>
/// <returns></returns>
public TValue this[T1 key1] {
get => d1[key1].value;
}


/// <summary>
/// enumerator on key1, so we can replace Dictionary by cDoubleKeyDictionary
/// </summary>
/// <param name="key1"></param>
/// <returns></returns>
public TValue this[T1 key1, T2 key2] {
set {
lock (d1) {
d1[key1] = new Key2ValuePair {
key2 = key2,
value = value,
};
d2[key2] = key1;
}
}
}
小开 
public static string GetKeyFromValue(string valueVar)
{
foreach (string keyVar in dictionaryVar.Keys)
{
if (dictionaryVar[keyVar] == valueVar)
{
return keyVar;
}
}
return null;
}

其他人可能有更有效的答案，但我个人觉得这个更直观，对我来说很管用。

小开

Dictionary< K, V>可以扩展。我已经用了很长时间了::

public static bool TryGetKey<K, V>(this IDictionary<K, V> instance, V value, out K key)
{
foreach (var entry in instance)
{
if (!entry.Value.Equals(value))
{
continue;
}
key = entry.Key;
return true;
}
key = default(K);
return false;
}

并使用as:

public static void Main()
{
Dictionary<string, string> dict = new Dictionary<string, string>()
{
{"1", "one"},
{"2", "two"},
{"3", "three"}
};
     

string value="two";
if (dict.TryGetKey(value, out var returnedKey))
Console.WriteLine($"Found Key {returnedKey}");
else
Console.WriteLine($"No key found for value {value}");
}
小开

keys中的键的顺序是未指定的，但它与values中的相关值相同(来自c#文档)。

因此，(在某些情况下)对值的集合执行此操作的有效方法如下:

    /// <summary>
/// Gets the 1st key matching each value
/// </summary>
public static IEnumerable<TKey> GetKeys<TKey,TValue>(this Dictionary<TKey, TValue> dic, IEnumerable<TValue> values) where TKey : notnull
{
//The order of the keys in Keys is unspecified, but it is the same as the associated values in Values
var dicKeys = dic.Keys.ToList();
var dicValues = dic.Values.ToList();
foreach (var value in values)
{
var i = dicValues.IndexOf(value); //Will return the index of the 1st found value, even when multiple values are present
//we could test if i==-1 there.
yield return dicKeys[i];
}
}

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