Operator '==' cannot be applied to operands of type 'KeyValuePair' and 'string'?

Question

This question might appear to be a duplicate and/or too boring, but I want to do this using this specific method.

When a user enters a string into a textbox I want to get this string key. So to do this I've created a dictionary which has n (n <= 15000) unique values. I want to get the key from this dictionary by value. The method below works well:

Dictionary<int, string> artikullar = new Dictionary<int, string>();
int key = (from elem in artikullar where elem.Value == txt_artikul.Text select elem).First().Key;

Before that I tried to use the First() method to get the key:

int key = artikullar.AsParallel().First(new Func<KeyValuePair<int, string>, bool>(val => val == txt_artikul.Text)).Key;

But it throws this error:

Operator '==' cannot be applied to operands of type 'KeyValuePair' and 'string'

I haven't used this method before.

Any helpful comment or answer would be appreciated.

Why are you holding a Dictionary<int, string>, shoulden't it be a Dictionary<string, int> if you are doing the lookup by the string? Also, if you have the same value for multiple keys your function may return a different key each time you run the function, is that acceptable? — Scott Chamberlain
– Scott Chamberlain, Commented Feb 18, 2016 at 17:03
@ScottChamberlain thanks to quick response. Well, the values should be stored as type of string and keys as type of int. So, this syntax is wrong ? I have wrote that, dictionary has n (n <= 15000) unique values. — Mirjalal
– Mirjalal, Commented Feb 18, 2016 at 17:08

René Vogt · Accepted Answer · 2016-02-18 17:21:17Z

7

Ehsan Sajjad is correct. But I'd like to add another point:

You said that the strings in your Dictionary are unique. I don't know when you created the Dictionary and if the data is rather static or dynamically changing all the time.

If it's rather static and the strings are unique, you may consider to create a reversed Dictionary like this:

Dictionary<string, int> reversedDict = artikullar.ToDictionary(
    kvp => kvp.Value,
    kvp => kvp.Key);

and then use this for your lookup:

int key = reversedDict[txt_artikul.Text];

This may be faster than querying the original dictionary.

edited Feb 18, 2016 at 17:21

answered Feb 18, 2016 at 17:13

René Vogt

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Comments

Ehsan Sajjad · Accepted Answer · 2016-02-18 17:03:23Z

3

You have to change :

val => val == txt_artikul.Text

to:

val = > val.Value == txt_artikul.Text

you have instance of KeyValuePair, you have to specify Value in lambda expression to be compared.

answered Feb 18, 2016 at 17:03

Ehsan Sajjad

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8 Comments

Scott Chamberlain Over a year ago

While this does solve the specific problem the OP asked about his design choice is a very poor one. He will not get any of the O(1) benefits of a dictionary doing this and his lookup could be very slow.

Ehsan Sajjad Over a year ago

you may be right, but question is about the problem he is facing with lambda expression

Scott Chamberlain Over a year ago

Sometimes when someone asks you if they should use a old shoe or glass bottle to pound in a tail you need to tell them to stop what they are doing and go buy a hammer.

Scott Chamberlain Over a year ago

@MirjalalTalishinski use two dictionaries, one for lookups by int, one for lookups by string. Keep them in sync whenever you add or remove a value, whatver you do to one, do to the other.

Scott Chamberlain Over a year ago

See René Vogt's answer for a example.

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