C# - How to use Select LINQ method using properties from a dynamically list of properties

Question

Suppose we have a simple class called Person with properties as FirstName, LastName, Age.

Now I have a list of properties as a list of strings.

var props = new List<string> { "FirstName", "Age"}.

Now I have a List of Person objects.

var persons = new List<Person> {....}

How to use Select LINQ method to get just the Person properties that appear in the props?

var result = persons.Select(p => code here)

The result should be a list of anonymous objects, where an anonymous object should contain the properties from props list.

I'm pretty sure you need to use a `Where` instead of a `Select`. — baltermia, Nov 17 '21 at 13:48
Show us the exact input and output you expect. This question is unclear as it stands. — DavidG, Nov 17 '21 at 14:00
@DavidG, the output should be a list of anonymous objects. Every anonymous object should have the properties from `props` list. — Mihai Alexandru-Ionut, Nov 17 '21 at 14:03
No, give us an ACTUAL example of input and output. What is `Person`? What is the exact output? Note you can't create anonymous type on the fly. And even if you could, how would you expect to use them? — DavidG, Nov 17 '21 at 14:06
https://stackoverflow.com/questions/16516971/linq-dynamic-select - does it answer your question? Take a look for `Nick Butler's` answer, seems it very similar for your question — Yaroslav Bres, Nov 17 '21 at 14:10
@MihaiAlexandru-Ionut post the *actual* code. Anonymous types can't be returned from a method so whatever you read must be used in the method itself. Why use a list of property names in that case instead of an actual anonymous type expression? `.Select(x=>new {x.Name,x.Surname,....})`. If you really want a dynamic `select` you'll have to build that `Expression>` by yourself. It's doable, but better to not have to do this. If you want to query a database - why use LINQ ? — Panagiotis Kanavos, Nov 17 '21 at 14:50

baltermia · Answer 1 · 2021-11-17T13:59:37.173

0

Could you try this:

IEnumerable<string> personProps = typeof(Person).GetProperties().Select(p => p.Name);

List<string> props = new List<string> { "FirstName", "Age"};

IEnumerable<string> selectedProps = props.Where(p => personProps.Contains(p));

edited Nov 17 '21 at 13:59

answered Nov 17 '21 at 13:52

baltermia

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i have updated my question to be more clearly. – Mihai Alexandru-Ionut Nov 17 '21 at 13:57
1

@MihaiAlexandru-Ionut what kind of object do you want as end-result? If you only want to select specific properties there must be some sort of list that holds these? – baltermia Nov 17 '21 at 13:58
I can have a simple anonoymous object – Mihai Alexandru-Ionut Nov 17 '21 at 13:59
1

@MihaiAlexandru-Ionut now I can understand what you want. I will try to bring a solution to this but honestly, I don't think there's a simple answer to the problem. – baltermia Nov 17 '21 at 14:06

score 0 · Answer 2 · answered Nov 17 '21 at 14:30

If I get your question correctly you want to kind of trim your Person object. If that's correct I'd recommend to create an instance method inside your Person class that gives you the opportunity to create an unnamed object or Tuple (for example like here: C# ValueTuple of any size) directly from your Person instanciated object. You just need to pass your props List for matching the properties (like here: enter link description here) then and just use this very method inside your Select statement.

Serge · Accepted Answer · 2021-11-17T17:33:15.380

try this code. It was tested in Visual Studio and works properly

var props = new List<string> { "FirstName", "Age" };

var persons = new List<Person> { 
                  new Person { FirstName = "FirstName1", LastName = "FirstName1", Age=10},
                  new Person { FirstName = "FirstName2", LastName = "FirstName2", Age=20},
                  new Person { FirstName = "FirstName3", LastName = "FirstName3", Age=30}
    
    };

    var json = Newtonsoft.Json.JsonConvert.SerializeObject(persons);
    var jArray = JArray.Parse(json);

    List<JObject> list = new List<JObject>();

    foreach (var item in jArray)
    {
        JObject jsonObj = new JObject();
        foreach (var prop in props)
        {
            foreach (JProperty jprop in item)
            {
                if (jprop.Name == prop)
                    jsonObj.Add(jprop);
            }
        }

        list.Add(jsonObj);
    }

json = JsonConvert.SerializeObject(jsonList);

to use it you can convert to dictionary for example

var dictArray = JsonConvert.DeserializeObject<Dictionary<string, object>[]>(json);

or list of tuples or key-value pairs

List<(string,object)> values = dictArray.SelectMany(i=>i).Select(i =>(i.Key,i.Value)).ToList();

output

    FirstName   FirstName1
    Age         10
    FirstName   FirstName2
    Age         20
    FirstName   FirstName3
    Age         30

or you can convert to dynamic object

dynamic eo = dictArray.Select( e=> e.Aggregate(new ExpandoObject() as IDictionary<string, Object>,
                                (a, p) => { a.Add(p.Key, p.Value); return a; })).ToList();

test

var firstName = eo[1].FirstName; //FirstName2

class

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }
}

C# - How to use Select LINQ method using properties from a dynamically list of properties

3 Answers3