如何将linq表达式转换为字典<string,string>

Question

can you help me, how to convert linq expression to dictionary? The following code is throwing me ArgumentException: An item with the same key has already been added.

        IDictionary<string, string> listAllCoursesWithAreaAsDictionary = new Dictionary<string, string>();

        var dictionary =
            (from b in bookListRecord
             select new { b.CourseCode, b.Area }).Distinct();

        listAllCoursesWithAreaAsDictionary = dictionary.AsEnumerable().ToDictionary(x => x.CourseCode, x => x.Area); 

        return listAllCoursesWithAreaAsDictionary;

When I try this:

        listAllCoursesWithAreaAsDictionary = dictionary.AsEnumerable().ToDictionary(x => x.CourseCode); 

I get error: Cannot implicitly convert type 'System.Collections.Generic.Dictionary' to 'System.Collections.Generic.IDictionary'. An explicit conversion exists (are you missing a cast?)

Answer 1

The problem you are having is that the

dictionary.AsEnumerable().ToDictionary(x => x.CourseCode); 

line executes your query.

In your database, where you retrieve your data from, you are receiving the same CourseCode twice, hence you get the error message:

"An item with the same key has already been added."

Your Distinct() doesn't behave as might you expect, you will have to check that. You wanted distinct by key, but you are calling distinct on an anonymous Key/Value.

Answer 2

Agreed with Aharon, you're looking for grouping operator:

  var dictionary = bookListRecord
    .GroupBy(g => g.CourseCode)
    .Select(g => new { g.Key, 
      AreaList = g.Select(c => c.Area).ToList(), 
      Area = g.Select(c => c.Area).FirstOrDefault() })
    .ToDictionary(g => g.Key);