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coding_rookie · 03-10-2024, 08:39 PM

I'm facing a bit of a problem with the LINQ GroupBy method in C#. I am trying to combine a list of cars owned by people, where each person is represented multiple times in a list, with each instance having potentially a different car. What I want to achieve is a consolidated result where I get a list of people, where each person has a list of all the cars they own. To illustrate, I have a Person class, and a list that contains multiple instances with the same PersonID but different cars.
Here is how my Person class looks:

Code:
}

And here's an example list:

Code:
},

    new Person {

        PersonID = 1, car = "BMW"

    },

    new Person {

        PersonID = 2, car = "Audi"

    }

};

I want to end up with a result in this format:

Code:
}

The part where I'm stuck is after grouping by PersonID, I'm not entirely sure how to select and construct the list of cars for each person. Here's what I've tried so far:

Code:
select new {

    PersonID = g.Key, // What do I put here?

Any help on how to continue from here would be greatly appreciated.

code_Warrior · 03-10-2024, 09:27 PM

You are on the right track with the GroupBy method, but you just need to project the results properly. Inside the select clause of your query, you should create a new Result object and construct the list of cars from each group. You can use the Select method to project the car field of each person in the group into the list. Here's the code that should work for you:

Code:
};

This query groups the persons by PersonID, then for each group, it creates a new Result instance with the PersonID set to the group's key and the cars property set to the list of cars associated with that key.
Make sure that you have `using System.Linq;` at the top of your code file since you're using LINQ.

coding_rookie · 03-10-2024, 10:06 PM

Thank you for the explanation, it makes a lot more sense now. I think I understand how to use the GroupBy followed by Select to shape my data accordingly. One last question, how would I go about converting this to method syntax?

code_Warrior · 03-10-2024, 11:07 PM

Converting that query to method syntax is straightforward. You would use the GroupBy method to initially group the data, and then you can use the Select method to shape each group into the desired format. Here is how you would rewrite your query using method syntax:

Code:
})

    .ToList();

Make sure to include both `using System;` and `using System.Linq;` for the necessary LINQ extensions.

coding_rookie · 03-10-2024, 11:35 PM

That did the trick. Your guidance has helped me to figure out how to get the results I was after. Here's the complete working code including the classes and the group by logic:

Code:
using System;

using System.Collections.Generic;

using System.Linq;

public class Person {

    internal int PersonID;

    internal string car;

}

public class Result {

    public int PersonID;

    public List < string > cars;

}

class Program {

    static void Main() {

        List < Person > persons = new List < Person > {

            new Person {

                PersonID = 1, car = "Ferrari"

            },

            new Person {

                PersonID = 1, car = "BMW"

            },

            new Person {

                PersonID = 2, car = "Audi"

            }

        };

        var results = persons.GroupBy(p => p.PersonID)

            .Select(g => new Result {

                PersonID = g.Key,

                    cars = g.Select(x => x.car).ToList()

            })

            .ToList();

        // Output the results

        foreach(var result in results) {

            Console.WriteLine($ "PersonID: {result.PersonID}, Cars: {string.Join(", ", result.cars)}");

        }

    }

}

This code should compile and run, providing the grouped results as expected.

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