如何转换List <string> 到字典 <string,string> 使用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

Answer 1

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 .

Answer 2

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);