Question

In my method, I have this query:

var query =
    _db.STEWARDSHIP
        .OrderBy(r => r.SITE.SITE_NAME)
        .Where(r => SiteId == null || r.SITE_ID == iSiteId)
        .Where(r => SiteTypeId == null || r.SITE.SITE_TYPE_VAL.SITE_TYPE_ID == iSiteTypeId)
        .Where(r => EcoregionId == null || r.SITE.ECOREGION_VAL.ECOREGION_ID == iEcoregionId)
        .Where(r => ConservationAreaId == null || r.SITE.CONSERVATION_AREA_VAL.CONSERVATION_AREA_ID == iConservationAreaId)
        .Where(r => VisitTypeId == null || r.VISIT_TYPE_VAL.VISIT_TYPE_ID == iVisitTypeId)
        .Where(r => Staff == null || r.STAFF.Contains(sStaff))
        .Where(r => Comment == null || r.VISIT_COMMENTS.Contains(sComment))
        .Select(r => new SiteVisitDetails
        {
            Id = r.STEWARDSHIP_ID,
            Name = r.SITE.SITE_NAME,
            VisitType = r.VISIT_TYPE_VAL.VISIT_TYPE_DESC,
            VisitDate = r.VISIT_DATE
        });

This fails with the error:

SiteVisitDetails does not contain a constructor that takes 0 arguments

Here the class:

public class SiteVisitDetails : ISiteVisitDetails
{

    private int _id;
    private string _name;
    private DateTime _visitDate;
    private string _visitType;

    public SiteVisitDetails(int Id, string Name, DateTime VisitDate, 
                            string VisitType)
    {
        _id = Id;
        _name = Name;
        _visitDate = VisitDate;
        _visitType = VisitType;
    }

    public int Id
    {
        get { return _id; }
        set { _id = value; }
    }

    public string Name
    {
        get { return _name; }
        set { _name = value; }
    }

    public DateTime VisitDate
    {
        get { return _visitDate; }
        set { _visitDate = value; }
    }

    public string VisitType
    {
        get { return _visitType; }
        set { _visitType = value; }
    }

    public string getShortVisitDate()
    { 
        return _visitDate.ToShortDateString();
    }
}

The Interface:

interface ISiteVisitDetails
{
    int Id { get; set; }
    string Name { get; set; }
    DateTime VisitDate { get; set; }
    string VisitType { get; set; }
    string getShortVisitDate;
}

I don’t understand why I would need a constructor that takes 0 arguments. Can somebody help me understand?

Thanks.

Answer 1

don’t understand why I would need a constructor that takes 0
arguments.

The expression:

new SiteVisitDetails
{
    Id = r.STEWARDSHIP_ID,
    Name = r.SITE.SITE_NAME,
    VisitType = r.VISIT_TYPE_VAL.VISIT_TYPE_DESC,             
    VisitDate = r.VISIT_DATE
}

is really shorthand for:

new SiteVisitDetails() // Note the parantheses
{
    Id = r.STEWARDSHIP_ID,
    Name = r.SITE.SITE_NAME,
    VisitType = r.VISIT_TYPE_VAL.VISIT_TYPE_DESC,
    VisitDate = r.VISIT_DATE
}

From the language specification:

7.6.10.1 Object creation expressions

[…] An object creation expression can omit the constructor argument list
and enclosing parentheses provided it includes an object initializer
or collection initializer. Omitting the constructor argument list and
enclosing parentheses is equivalent to specifying an empty argument
list.

In other words, you really are trying to call the parameterless constructor of the SiteVisitDetails type, which doesn’t exist of course.