How to convert List<string> to Dictionary<string,string> using linq?

Question

I have a list of strings:

List<string> tList=new List<string>();
tList.add("a");
tList.add("mm");

I want to convert this list to a Dictionary so the key and the value of the dictionary is the same using linq

I have tried:

var dict = tList.ToDictionary<string,string>(m => m, c => c);

but I get the following error:

Cannot convert lambda expression to type 'IEqualityComparer' because it is not a delegate type

What have you tried to do to solve this problem, and what problems have you had with your attempted solutions? — Servy
– Servy, Commented Sep 10, 2015 at 14:51

Backs · Accepted Answer · 2015-09-10 14:57:19Z

12

Use ToDictionary method:

List<string> tList = new List<string>();
tList.add("a");
tList.add("mm");
var dict = tList.ToDictionary(k => k, v => v);

Do not forget add reference to System.Linq.

edited Sep 10, 2015 at 14:57

answered Sep 10, 2015 at 14:51

Backs

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1 Comment

SIRHAMY Over a year ago

For the life of me, could not figure out what I was doing wrong in my code. I was doing basically as above, but had specified the type parameters for ToDictionary like this: ` var dict = tList.ToDictionary<string, string>(k => k, v => v); ` The trick was to remove those type parameters as this answer shows: ` var dict = tList.ToDictionary(k => k, v => v); `

juharr · Accepted Answer · 2015-09-10 15:22:15Z

5

Here are the signatures for ToDictionary

ToDictionary<TSource, TKey>(
    IEnumerable<TSource>, 
    Func<TSource, TKey>)

ToDictionary<TSource, TKey>(
    IEnumerable<TSource>, 
    Func<TSource, TKey>, 
    IEqualityComparer<TKey>)

ToDictionary<TSource, TKey, TElement>(
    IEnumerable<TSource>, 
    Func<TSource, TKey>, 
    Func<TSource, TElement>)

ToDictionary<TSource, TKey, TElement>(
    IEnumerable<TSource>, 
    Func<TSource, TKey>, 
    Func<TSource, TElement>, 
    IEqualityComparer<TKey>)

You want the 3rd one, but since you call it and specify two generic types it is instead using the 2nd one and your second argument (actually 3rd since the first is the argument the extension method is called on) is not an IEqualityComparer<TKey>. The fix is to either specify the third type

var dict = tList.ToDictionary<string,string,string>(m => m, c => c);

Don't specify the generic types and let the compiler figure it out via type inference

var dict = tList.ToDictionary(m => m, c => c);

Or since you want the items to be the values you can just use the 1st one instead and avoid the second lambda altogether.

var dict = tList.ToDictionary(c => c);

answered Sep 10, 2015 at 15:22

juharr

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1 Comment

RuudvK Over a year ago

I appreciate the clear explanation, it made me realize I could use ToDictionary<string,string,object> to create a Dictionary<string,object>.

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