Concatening objets with LINQ, under a condition

Question

I'm new to LINQ, and I have trouble with a query I'm trying to write to teach myself LINQ.

I have a list of MyError objects. A MyError object has a errorcode (an enum) and an arraylist of MyEntryError objects. Some MyError may have the same errorcode.

I would like to create a LINQ query that take as input my list of MyError and outputs a new list of MyError so that, for each errorcode, there is only one MyError that will contain all the MyEntryError that failed with the errorcode of the MyError.

What I mean is let's say I have a list of MyError objects such as this one :

• MyError1 : errorcode = ErrorType1 Arraylist : A, C
• MyError2 : errorcode = ErrorType2 ArrayList : B, D, E
• MyError3 : errorcode = ErrorType2 ArrayList : F
• MyError4 : errorcode = ErrorType1 ArrayList : G
• MyError5 : errorcode = ErrorType3 ArrayList : Z

My desired output is a list of MyError objects with the following objects :

• MyError1 : errorcode = ErrorType1 ArrayList : A, C, G
• MyError2 : errorcode = ErrorType2 ArrayList : B, D, E, F
• MyError3 : errorcode = ErrorType3 ArrayList : Z

Is it possible with LINQ ? If so, How ?

I'm guessing that using an ArrayList instead of a List in the MyError class may be a problem so if it can't be done with an ArrayList, i would like to see a query that would works with a modified MyError class that uses a List

Thanks in advance

Answer 1

something like, where errorList is your list of MyError1 t/m MyError5, and your arraylist is called errors.

from err in errorList
group err by err.errorcode into g
select new MyError { errorcode = g.Key, errors = new ArrayList(g.SelectMany(a => a.errors.Cast<YourType>()).ToArray()) };

Answer 2

    public List<MyError> GroupErrorsByErrorCode(List<MyError> errors)
    {
        var result = errors.GroupBy(e => e.ErrorCode).Select(group => new MyError
            {
                ErrorCode = group.Key,
                // EDIT: was 
                // Errors = group.SelectMany(g => g.Errors)
                Errors = new ArrayList(group
                           .SelectMany(g => g.Errors.Cast<MyEntryError>())
                           .ToList())
            });
        return result;
    }

EDIT: As ArrayList has a constructor that takes an ICollection, you can use that overload to instantiate the Errors property.

Using a strongly typed list instead of ArrayList is always a good idea...