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Check key / value pair in List

I have list declared as
List<KeyValuePair<string, Int64>> KPList = new List<KeyValuePair<string, long>>();
I need to check if a combination of key and value already exists. I can't use dictionary since I need to have unique combination of key and value where in case a key will have multiple values but as a single pair
if(!KPList.Any(p=> p.Key ==keyp && p.Value == valuep))
What is wrong in this?

You can also use HashSet, which doesn't have values, but works as you need.
HashSet< Tuple<string, long> > KPSet = new HashSet< Tuple<string, long> >();
...
if(KPSet.Contains(p))
{
...
}

For easy-use and best performance I would like to suggest use combination of Dictionary and HashSet :
var KPDict = new Dictionary<string, HashSet<long>>();
Then it will provides you O(1)+O(1) lookup complexity and easy check of value:
if (KPDict.ContainsKey(keyp) && KPDict[keyp].Contains(valuep)) {
//do some actions
}
else{
//some logic in case keyp, valuep pair not found in KPDict
}

Beginner python, about keys and array [duplicate]

I only want the Keys and not the Values of a Dictionary.
I haven't been able to get any code to do this yet. Using another array proved to be too much work as I use remove also.
How do I get a List of the Keys in a Dictionary?

List<string> keyList = new List<string>(this.yourDictionary.Keys);

You should be able to just look at .Keys:
Dictionary<string, int> data = new Dictionary<string, int>();
data.Add("abc", 123);
data.Add("def", 456);
foreach (string key in data.Keys)
{
Console.WriteLine(key);
}

Update for .NET 3.5+
To get list of all keys:
using System.Linq;
List<String> myKeys = myDict.Keys.ToList();
If you face any issues using System.Linq, see the following:
Visual Studio Does not recognize System.Linq
System.Linq Namespace

Marc Gravell's answer should work for you. myDictionary.Keys returns an object that implements ICollection<TKey>, IEnumerable<TKey> and their non-generic counterparts.
I just wanted to add that if you plan on accessing the value as well, you could loop through the dictionary like this (modified example):
Dictionary<string, int> data = new Dictionary<string, int>();
data.Add("abc", 123);
data.Add("def", 456);
foreach (KeyValuePair<string, int> item in data)
{
Console.WriteLine(item.Key + ": " + item.Value);
}

I can't believe all these convoluted answers. Assuming the key is of type: string (or use 'var' if you're a lazy developer): -
List<string> listOfKeys = theCollection.Keys.ToList();

The question is a little tricky to understand but I'm guessing that the problem is that you're trying to remove elements from the Dictionary while you iterate over the keys. I think in that case you have no choice but to use a second array.
ArrayList lList = new ArrayList(lDict.Keys);
foreach (object lKey in lList)
{
if (<your condition here>)
{
lDict.Remove(lKey);
}
}
If you can use generic lists and dictionaries instead of an ArrayList then I would, however the above should just work.

Or like this:
List< KeyValuePair< string, int > > theList =
new List< KeyValuePair< string,int > >(this.yourDictionary);
for ( int i = 0; i < theList.Count; i++)
{
// the key
Console.WriteLine(theList[i].Key);
}

For a hybrid dictionary, I use this:
List<string> keys = new List<string>(dictionary.Count);
keys.AddRange(dictionary.Keys.Cast<string>());

I often used this to get the key and value inside a dictionary: (VB.Net)
For Each kv As KeyValuePair(Of String, Integer) In layerList
Next
(layerList is of type Dictionary(Of String, Integer))

How to add duplicate keys into the Dictionary

I have some lines from text files that i want to add into the Dictionary.I am using Dictionary for the first time.While adding up starting lines it was Ok but suddenly i got error:
An item with the same key has already been added
Here in my code there are duplicate keys which i can not change.Here is my code in c#
Dictionary<string, string> previousLines = new Dictionary<string, string> { };
previousLines.Add(dialedno, line);
Here dialedno is the key and line is the textfile line.
Here is the code from which i am retrieving the given line based on key.
string tansferOrginExt = previousLines[dialedno];
So my concern is how to allow to add duplicate keys in Dictionary if possible and if not how can i get similar functionality.

how to allow to add duplicate keys in Dictionary
It is not possible. All keys should be unique. As Dictionary<TKey, TValue> implemented:
Every key in aDictionary<TKey, TValue> must be unique according to
the dictionary's equality comparer.
Possible solutions - you can keep collection of strings as value (i.e. use Dictionary<string, List<string>>), or (better) you can use Lookup<TKey, TValue> instead of dictionary.
how to check for duplicate keys and delete previous value from
Dictionary?
You can check if the key exists with previousLines.ContainsKey(dialedno) but if you always want to hold the last line, then just replace whatever dictionary had for the key, or add the new key if it is not in the dictionary:
previousLines[dialedno] = line;

We can Use a List of Key Value Pair
List<KeyValuePair<string, string>> myduplicateLovingDictionary= new List<KeyValuePair<string, string>>();
KeyValuePair<string,string> myItem = new KeyValuePair<string,string>(dialedno, line);
myduplicateLovingDictionary.Add(myItem);

Its not possible to add duplicate items to a Dictionary - an alternative is to use the Lookup class.
Enumerable.ToLookup Method
Creates a generic Lookup from an IEnumerable.

Example:
class Program
{
private static List<KeyValuePair<string, int>> d = new List<KeyValuePair<string, int>>();
static void Main(string[] args)
{
d.Add(new KeyValuePair<string, int>("joe", 100));
d.Add(new KeyValuePair<string, int>("joe", 200));
d.Add(new KeyValuePair<string, int>("jim", 100));
var result = d.Where(x => x.Key == "joe");
foreach(var q in result)
Console.WriteLine(q.Value );
Console.ReadLine();
}
}

List< KeyValuePair < string, string>> listKeyValPair= new List< KeyValuePair< string, string>>();
KeyValuePair< string, string> keyValue= new KeyValuePair< string, string>("KEY1", "VALUE1");
listKeyValPair.Add(keyValue);

If your question is if you can add the same key twice, the answer is No.
However if you want to just iterate through the item and then increase the count of the value for the particular Key, you can achieve that by using "TryAdd" method.
var dict = new Dictionary<int, int>();
foreach (var item in array)
{
dict.TryAdd(item, 0);
dict[item]++;
}
The same thing we are trying to achieve with if else, can be achieved with this method.``
https://learn.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentdictionary-2.tryadd?view=netframework-4.7.2

Finding already existing value in Key Value pair

I am storing a string and int value in Key value pair.
var list = new List<KeyValuePair<string, int>>();
While adding i need to check if string(Key) already exists in list, if exists i need to add it to Value instead of adding new key.
How to check and add?

Instead of List you can use Dictionary and check if it contains key then add the new value to the existing key
int newValue = 10;
Dictionary<string, int> dictionary = new Dictionary<string, int>();
if (dictionary.ContainsKey("key"))
dictionary["key"] = dictionary["key"] + newValue;

use dictonary. Dictionary in C# and I suggest you to read this post Dictonary in .net
Dictionary<string, int> dictionary =
new Dictionary<string, int>();
dictionary.Add("cat", 2);
dictionary.Add("dog", 1);
dictionary.Add("llama", 0);
dictionary.Add("iguana", -1);
to check. use ContainsKey ContainsKey
if (dictionary.ContainsKey("key"))
    dictionary["key"] = dictionary["key"] + yourValue;

If you need use the list,you must foreach the list,and look for the keys.
Simplely,you can use hashtable.

For anyone who has to use a List (which was the case for me, since it does things Dictionary doesn't), you can just use a lambda expression to see if the List contains the Key:
list.Any(l => l.Key == checkForKey);

Your needs exactly describe the design of Dictionarys?
Dictionary<string, string> openWith =
new Dictionary<string, string>();
// Add some elements to the dictionary. There are no
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";

For sure, dictionary is preferable in your case. You can not modify the Value of KeyValue<string,int> class as it is Immutable.
But even if you still want to use List<KeyValuePair<string, int>>();. You can use IEqualityComparer<KeyValuePair<string, int>>. Code will be like.
public class KeyComparer : IEqualityComparer<KeyValuePair<string, int>>
{
public bool Equals(KeyValuePair<string, int> x, KeyValuePair<string, int> y)
{
return x.Key.Equals(y.Key);
}
public int GetHashCode(KeyValuePair<string, int> obj)
{
return obj.Key.GetHashCode();
}
}
And use it in Contains like
var list = new List<KeyValuePair<string, int>>();
string checkKey = "my string";
if (list.Contains(new KeyValuePair<string, int>(checkKey, int.MinValue), new KeyComparer()))
{
KeyValuePair<string, int> item = list.Find((lItem) => lItem.Key.Equals(checkKey));
list.Remove(item);
list.Add(new KeyValuePair<string, int>("checkKey", int.MinValue));// add new value
}
which does not sounds good way.
hope this info helps..

How to insert an item into a key/value pair object?

I just need to be able to insert a key/value pair into an object at a specific position. I'm currently working with a Hashtable which, of course, doesn't allow for this functionality. What would be the best approach?
UPDATE: Also, I do need the ability to lookup by the key.
For example (oversimplified and pseudocoded but should convey the point):
// existing Hashtable
myHashtable.Add("somekey1", "somevalue1");
myHashtable.Add("somekey2", "somevalue2");
myHashtable.Add("somekey3", "somevalue3");
// Some other object that will allow me to insert a new key/value pair.
// Assume that this object has been populated with the above key/value pairs.
oSomeObject.Insert("newfirstkey","newfirstvalue");

List<KeyValuePair<string, string>> kvpList = new List<KeyValuePair<string, string>>()
{
new KeyValuePair<string, string>("Key1", "Value1"),
new KeyValuePair<string, string>("Key2", "Value2"),
new KeyValuePair<string, string>("Key3", "Value3"),
};
kvpList.Insert(0, new KeyValuePair<string, string>("New Key 1", "New Value 1"));
Using this code:
foreach (KeyValuePair<string, string> kvp in kvpList)
{
Console.WriteLine(string.Format("Key: {0} Value: {1}", kvp.Key, kvp.Value);
}
the expected output should be:
Key: New Key 1 Value: New Value 1
Key: Key 1 Value: Value 1
Key: Key 2 Value: Value 2
Key: Key 3 Value: Value 3
The same will work with a KeyValuePair or whatever other type you want to use..
Edit -
To lookup by the key, you can do the following:
var result = stringList.Where(s => s == "Lookup");
You could do this with a KeyValuePair by doing the following:
var result = kvpList.Where (kvp => kvp.Value == "Lookup");

Maybe the OrderedDictonary will help you out.

Do you need to look up objects by the key? If not, consider using List<Tuple<string, string>> or List<KeyValuePair<string, string>> if you're not using .NET 4.

I would use the Dictionary<TKey, TValue> (so long as each key is unique).
EDIT: Sorry, realised you wanted to add it to a specific position. My bad. You could use a SortedDictionary but this still won't let you insert.

You could use an OrderedDictionary, but I would question why you would want to do that.

Use a linked list. It was designed for this exact situation.
If you still need the dictionary O(1) lookups, use both a dictionary and a linked list.

Hashtables are not inherently sorted, your best bet is to use another structure such as a SortedList or an ArrayList