How to update value of dictionary with specific key in C#

Question

1.00/5 (1 vote)

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C#5.0

dictionary

Is there any way to add new value into existing dictionary, using it's key parameter for check up?

Posted 9-Apr-15 3:17am

Nemanja M

Updated 9-Apr-15 5:44am

Sergey Alexandrovich Kryukov

v2

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3 solutions

Solution 1

Of course:

C#

// this will work regardless if "someKey" was previously present in the dictionary or not:
yourDictionary[someKey] = newValue;


// if you want to ensure that "someKey" actually is already present:
if (yourDictionary.Contains(someKey))
   yourDictionary[someKey] = newValue;
else
   // throw an Exception or something


// if you want to ensure that "someKey" is NOT already present:
yourDictionary.Add(someKey, newValue);
// (will throw an Exception otherwise)

Edit:

You could create an extension method for the "second case" above:

C#

using System;
using System.Collections.Generic;

namespace MyExtensionClasses
{
  public static class MyDictionaryExtension
  {
    // will add or update
    // returns true if the key-value-pair was added or updated
    // return false if the new value was equal to the already existing value
    public static bool AddOrUpdate<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue value)
    {
        bool addedOrUpdated = false;
        TValue oldValue;
        if (!dict.TryGetValue(key, out oldValue) || !value.Equals(oldValue))
        {
            dict[key] = value;
            addedOrUpdated = true;
        }
        return addedOrUpdated;
    }

    // will only update a value if the key already exists
    // and throw an exception otherwise
    public static void UpdateExisting<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue value)
    {
        if (dict.ContainsKey(key))
            dict[key] = value;
        else
            throw new InvalidOperationException("Can't update because key not found in dictionary: " + key.ToString());
    }
  }
}

You can just copy the code above into a new code file, insert a using MyExtensionClasses; at the top of your code files where you want to use it and then use the methods as if they were regular methods of the Dictionary-class.

Posted 9-Apr-15 3:32am

Sascha Lefèvre

Updated 9-Apr-15 7:16am

v5

Comments

Sergey Alexandrovich Kryukov 9-Apr-15 11:50am

Up-voted, but... not quite clear. It would help if you wrote just a functions with parameters, which would be self-explained.

More importantly, "throw an exception" is wrong approach. Write approaches are: 1) do nothing; 2) add to the dictionary unconditionally and let the dictionary itself throw the exception. Why creating superfluous thunk between your and dictionary code?

—SA

Sascha Lefèvre 9-Apr-15 12:08pm

Thank you, Sergey!

But in the case "I want to update a value in the dictionary and ensure that there actually already is a value with the same key" (that's how I interpreted the question) the Dictionary doesn't offer a method that would fail if there is no such key present yet. That's why constructed that middle part in the code-block.

By your first suggestion do you mean the following?:

void UpdateExistingValueOrFail<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue value)
{
   if (dict.Contains(key))
      dict[key] = value;
   else
      throw new InvalidOperationException("can't update because key not found");
}

Sergey Alexandrovich Kryukov 9-Apr-15 12:21pm

Ah, you are right, I missed it.

Anyway, if you show the function ModifyKey(string key, string newValue); it would be clear. By they way, you could simply return false to indicate absence of the key (or, better, "no changes have been done", then you would need to do nothing also when old value is the same as new value). Depending on intent of the use, throwing exception would also be a choice.

—SA

Sascha Lefèvre 9-Apr-15 12:52pm

Alright, I updated my solution. I like the idea of returning "no changes have been done" - but that would require to restrict TValue to reference types.
/Sascha

Sergey Alexandrovich Kryukov 9-Apr-15 13:07pm

No, it won't. Everything remains the same. You can always compare two values using Object.Equals and act accordingly. It will be semantic comparison based on overridden method Equals in each value type, which always can be called in type-agnostic call of a generic class/method.
—SA

Sascha Lefèvre 9-Apr-15 13:09pm

Oh, right. That's something that I learned just recently and didn't think of here. Updating again.. ;)

Sergey Alexandrovich Kryukov 9-Apr-15 13:10pm

:-)

Sascha Lefèvre 9-Apr-15 13:19pm

Soo.... can you approve of v5? ;)

Sergey Alexandrovich Kryukov 9-Apr-15 14:18pm

Not quite... that's not all yet. 4 is a good mark. :-)
Please see my Solution 3. Sorry, I found that I can spend less time if I put all together myself. I explained what else could be addressed.

Thank you.
—SA

Sergey Alexandrovich Kryukov 9-Apr-15 13:09pm

I already commented on what it takes. It will work on structures, string, int, anything. All those types, on boxed form, will do semantic comparison.
—SA

Maciej Los 9-Apr-15 16:30pm

+5!

Sascha Lefèvre 9-Apr-15 16:31pm

Thank you, Maciej! :)

Solution 2

XML

Dictionary<string, string> dictionary = new Dictionary<string, string>();
            
//Check if "Key" is already present in dictionary
if (!dictionary.ContainsKey("Key"))
            {
                dictionary["Key"] = "Value" ;

            }

Posted 9-Apr-15 4:09am

ConnectingKamlesh

Updated 14-Apr-15 23:54pm

v2

Comments

Sergey Alexandrovich Kryukov 9-Apr-15 11:45am

Will throw exception or do nothing. Useless because "Key" and "Value" are hard-coded.
Always test your solutions before posting. Please...
—SA

ConnectingKamlesh 10-Apr-15 2:45am

SA, it's not useless, "Key" here is the key which needs to be matched and needs to be replaced by the user, it seems you just copy and paste the answers and don't read or try to understand it. This is the simplest way to add new value into existing dictionary<string, string="">, using it's key parameter for check up.

Sergey Alexandrovich Kryukov 10-Apr-15 11:32am

I did not copy and paste anything. This is just too bad style answering the question. Naive readers may think that you suggest not to replace hard-coded key and value, but to hard-code then. And you cannot add a key-value pair on already contained key. This is you who need to copy and paste your own "solution" to real code and check up if it works.
—SA

ConnectingKamlesh 13-Apr-15 3:22am

This code is from a working program, you just cannot say anyone's solution/answers as "useless" because you think you are genious. Please check on your language before replying, It should not hurt.

Sergey Alexandrovich Kryukov 13-Apr-15 10:01am

I don't even understand what you are talking about.
I correctly explained to you why your code is wrong. Please keep to the facts and logic not move to personal issues and accusations; in this community, this is not welcome, as well as rude behavior.
—SA

ConnectingKamlesh 13-Apr-15 14:29pm

I felt bad when you said my code is useless, it is good to be good but it is not good to make others feel bad, there are so many ways to say if things doesn't work, I felt that the logic behind my code could help Nemanja solve his problem, so I posted my answer here. Being professional doesn't allow you to hurt anyone.

Sergey Alexandrovich Kryukov 13-Apr-15 14:43pm

You should not get hurt by criticism. Your answer has negative value, does not help; and I explained why; but you continue to argue. You should not do such things. You should argue only if you have some arguments, not just words and feelings.
Even for adding a key (which wasn't a question), it would work only if you wrote !dictionary.ContainsKey. Can you see the difference.
—SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2015-04-09T08:15:00

Please see our constructive discussion with Sascha Lefévre in comments to Solution 1. It is not yet quite comprehensive, as not all variants of modification by key are accessible to the user. Also, exception information is not informative. It is important to remember that key and value types may be not well presentable as strings.

As I was the one who suggested most of improvements, but it wasn't enough, let me summarize it on more general solution:

C#

using System.Collections.Generic;

public class DictionaryWrapper<KEY, VALUE> {

    public enum ValueModificationResult {
        Success,
        KeyNotFound,
        NoValueChangeNeeded,
    }

    Dictionary<KEY, VALUE> dictionary = new Dictionary<KEY, VALUE>();

    public ValueModificationResult ModifyExceptionless(KEY key, VALUE value) {
        VALUE oldValue;
        if (dictionary.TryGetValue(key, out oldValue)) {
            if (value.Equals(oldValue))
                return ValueModificationResult.NoValueChangeNeeded;
            dictionary[key] = value;
            return ValueModificationResult.Success;
        } else
            return ValueModificationResult.KeyNotFound;
    } //ModifyExceptionless

    // variant:
    public bool ModifyCombined(KEY key, VALUE value) {
        VALUE oldValue;
        if (dictionary.TryGetValue(key, out oldValue)) {
            bool result = !value.Equals(oldValue);
            if (result)
                dictionary[key] = value;
            return result;
        } else
            throw new ValueModificationException<KEY, VALUE>(key, value);
    } //ModifyCombined

    public ValueModificationResult AddOrModify(KEY key, VALUE value) {
        var result = ModifyExceptionless(key, value);
        if (result == ValueModificationResult.KeyNotFound) {
            dictionary.Add(key, value);
            return ValueModificationResult.Success;
        } //if
        return result;
    } //AddOrModif

    // this implicitely exposes the rest of Dictionary functionality:
    public static implicit operator Dictionary<KEY, VALUE>(DictionaryWrapper<KEY, VALUE> wrapper)
       { return wrapper.dictionary; }

    // alternatively or additionally, wrap some Dictionary functionality
    // ...

} //class DictionaryWrapper

C#

public class ValueModificationException<KEY, VALUE> : System.ApplicationException {
    internal ValueModificationException(KEY key, VALUE value) {
        this.Key = key; this.Value = value;
    } //ValueModificationException
    public KEY Key { get; private set; }
    public VALUE Value { get; private set; }
} //class ValueModificationException

Usage:

C#

class UsageSample {

    static void Test() {
        DictionaryWrapper<string, int> wrapper = new DictionaryWrapper<string,int>();
        Dictionary<string, int> dictionary = wrapper;
        dictionary.Add("1", 1);
        var result = wrapper.AddOrModify("1", 2); //Success
        ///...
    } //Test

} //class UsageSample

Note the use of private setters of exception properties (read-only from outside class) and internal constructor (try not to give more access then required). Also note some benefits of the ModifyExceptionless variant of the Modify method: it is more suitable for use in AddOrModify.

—SA