Question

I am struggling with converting a LINQ statement which returns an anonymous type to an ObservableCollection with a custom class, I’m happy with the LINQ statement, and the class definition, the problem (I think) is to do with how I am implementing the IQueryable interface between my anonymous type and the class itself.

public class CatSummary : INotifyPropertyChanged
{
    private string _catName;
    public string CatName
    {
        get { return _catName; }
        set { if (_catName != value) { _catName = value; NotifyPropertyChanged("CatName"); } }
    }

    private string _catAmount;
    public string CatAmount
    {
        get { return _catAmount; }
        set { if (_catAmount != value) { _catAmount = value; NotifyPropertyChanged("CatAmount"); } }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    // Used to notify Silverlight that a property has changed.
    private void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

            //MessageBox.Show("NotifyPropertyChanged: " + propertyName);

        }
    }

}

private void GetCategoryAmounts()
{
    var myOC = new ObservableCollection<CatSummary>();


    var myQuery = BoughtItemDB.BoughtItems
                        .GroupBy(item => item.ItemCategory)
                        .Select(g => new 
                        { 
                            _catName = g.Key, 
                            _catAmount = g.Sum(x => x.ItemAmount)
                        });

    foreach (var item in myQuery) myOC.Add(item);
}

The error I am getting is on the last line and is
"Argument 1: cannot convert from 'AnonymousType#1' to 'CatSummary'"

I’m relatively new to c# and need pointing in the right direction – if anyone has any tutorials on this sort of thing that would help as well.

Answer 1

This is because the anonymous object you are creating has no type relationship with CatSummary. If you want to add those items into your ObservableCollection, you will need to construct a CatSummary like so:

BoughtItemDB.BoughtItems.GroupBy(item => item.Category)
       .Select(x => new CatSummary
       {
           CatName = x.Key,
           CatAmount = x.Sum(amt => amt.ItemAmount)
       });

This way, your query creates an IEnumerable<CatSummary> instead of IEnumerable<a'>. Unlike other languages and their duck typing, just because your newly created anonymous object has a CatName and CatAmount property does not mean it can stand in for the actual type.