将LINQ Group By Data传递给新的对象列表

Question

I am trying to create a LINQ query in C# which takes a list of objects, performs a group by and count on the list, and then passes the results into a new list of objects.

This is my LINQ query at the moment which performs the group by and count, but not transferring the data into the list I need, instead a list of anonymous types.

var dataPoints = from e in evaluations.Take(20)
                 group e.EvaluationID by e.description into g
                 select new { Profession = g.Key, Count = g.Count() };

The dataPoints variable returns a list of anonymous types and the data looks something like:

 [0] Profession = "Doctor", Count = 12 [1] Profession = "Nurse", Count = 6 [2] Profession = "Health Visitor", Count = 2 

Rather than return the data into a list of anonymous types, I'd like to return a list of DataPoint objects. Below is my DataPoint object.

public class DataPoint
{
    public DataPoint(double y, string label)
    {
        this.Y = y;
        this.Label = label;
    }

    [DataMember(Name = "y")]
    public Nullable<double> Y = null;

    [DataMember(Name = "label")]
    public string Label;
}

I've tried amending my query to something like this, but I get an error saying DataPoint does not contain a constructor taking 0 arguments.

var dataPoints = from e in evaluations.Take(20)
                  group e.EvaluationID by e.description into g
                  select new DataPoint { Count = g.Count(), Profession = g.Key};

Any suggestions much appreciated.

Answer 1

For the property initializer to work you need a parameterless constructor and your fields should be properties to be accessible:

public class DataPoint
{
    [DataMember(Name = "y")]
    public Nullable<double> Y  { get; set; };

    [DataMember(Name = "label")]
    public string Label { get; set; }

    public DataPoint()
    {
    }

    public DataPoint(double y, string label)
    {
       Y = y;
       Label = label;
    }
}

and now your code should compile, alternatively use the constructor with parameters as someone suggested in comments.