Convert List to Dictionary<string, string[]>

Question

I have a simple custom object:

class CertQuestion
{
    public string Field {get;set;}
    public string Value {get;set;}
}

Subsequently I find myself with a List in some code. I'm trying to figure out how to format a list of CertQuestions into a corresponding Dictionary with similar Field names grouped together. For instance, given the following list:

        List<CertQuestion> certQuestions = new List<CertQuestion>()
            {
                new CertQuestion("Key", "Value1"),
                new CertQuestion("Key", "Value2"),
                new CertQuestion("Key2", "Value"),
                new CertQuestion("Key2", "Value2")
            };

I would like to convert that (trying to use LINQ) into a Dictionary with two entries such as

{{"Key", "Value1, Value2"}, {"Key2", "Value, Value2"}}

So you want a Dictionary<string,List<string>>?

Matt Burland
– Matt Burland

2015-03-10 17:28:25 +00:00
Commented Mar 10, 2015 at 17:28 — Matt Burland
– Matt Burland, Commented Mar 10, 2015 at 17:28
So what problem(s) are you having creating this dictionary?

Servy
– Servy

2015-03-10 17:35:36 +00:00
Commented Mar 10, 2015 at 17:35 — Servy
– Servy, Commented Mar 10, 2015 at 17:35
certQuestions.ToLookup(x => x.Field, x => x.Value);

EZI
– EZI

2015-03-10 17:41:02 +00:00
Commented Mar 10, 2015 at 17:41 — EZI
– EZI, Commented Mar 10, 2015 at 17:41

rclement · Accepted Answer · 2015-03-10 20:10:47Z

13

Group the questions by field, then convert to dictionary by selecting key, then value. Value becomes the grouping's list.

certQuestions.GroupBy(c => c.Field)
             .ToDictionary(k => k.Key, v => v.Select(f => f.Value).ToList())

Or for an array:

certQuestions.GroupBy(c => c.Field)
             .ToDictionary(k => k.Key, v => v.Select(f => f.Value).ToArray())

Edit based on question in comment:

class CertTest 
{
    public string TestId {get;set;}
    public List<CertQuestion> Questions {get;set;}
}
var certTests = new List<CertTest>();

You would use the SelectMany extension method. It is designed to aggregate a property list object that is in each element of the original list:

certTests.SelectMany(t => t.Questions)
         .GroupBy(c => c.Field)
         .ToDictionary(k => k.Key, v => v.Select(f => f.Value).ToList())

edited Mar 10, 2015 at 20:10

answered Mar 10, 2015 at 17:30

rclement

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3 Comments

Matt Burland Over a year ago

Not quite right, that'll give you a Dictionary<string,List<CertQuestion>>. You need to Select the Value. Something like .ToDictionary(k => k.Key, v => v.Select(cq => cq.Value).ToList())

rclement Over a year ago

Yes you are right, missed the string type vs. CertQuestion type in desired result.

Nate222 Over a year ago

It always seems so simple after someone has explained how to do it. Just out of curiosity, what if I had a list of objects and each object had a List<CertQuestion>, how could I wrap up all the objects' List<CertQuesion> fields into a single Dictionary<string, string[]>?

Jamiec · Accepted Answer · 2015-03-10 17:35:53Z

3

Your requirement was for a comma-separated list of values, that can be done like this:

var dict = certQuestions.GroupBy(c => c.Field)
             .ToDictionary(k => k.Key, v => String.Join(", ", v.Select(x => x.Value)))

Live example: http://rextester.com/LXS58744

(You should consider whether what you actually want is the values to be an Array or List<string> - see other answers)

answered Mar 10, 2015 at 17:35

Jamiec

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