issue with collection vs generic list.

[tshad]

This is similar to my previous post where I am trying to resolve the issue of setting up a class then a collection of the class. But I am not calling it a List<class> but as a collection that inherits from that class.

It seems to work except when it actually assigns the collection.

I get this error:

{"Unable to cast object of type 'System.Collections.Generic.List`1[BusinessLayer.DotNetType]' to type 'BusinessLayer.DotNetTypeCollection'."}

The classes are:

    public class DotNetType
    {
        public string DatabaseDataType { get; set; }
        public string DotNetDataType { get; set; }

    }

    public class DotNetTypeCollection : List<DotNetType>
    {
        public DotNetTypeCollection()
        {
        }

        public DotNetTypeCollection GetTypes()
        {
            DbObject dbo = new DbObject();
            DataSet ds = null;
            ds = dbo.GetDataSet("dbGen.GetDotNetDataTypes");

            return (DotNetTypeCollection)ds.Tables[0].AsEnumerable().Select(row => new DotNetType
                            {
                                DotNetDataType = row.Field<string>("DotNetDataType"),
                                DatabaseDataType = row.Field<string>("DatabaseDataType")
                            }).ToList();
        }
    }

Then I call it like this from my code:

public DotNetTypeCollection dntcol { get; set; }
public DotNetTypeCollection dntcol2 { get; set; }

...
dntcol = new DotNetTypeCollection();
dntcol2 = dntcol.GetTypes();

the assignment to dntcol2 causes the error.

Thanks,

Tom

 

[tshad]

I also tried this where I assign the results to a variable that inherits from a List<> which I assume would act the same as if the type was either the collection or the list.

        public DotNetTypeCollection GetTypes()
        {
            DotNetTypeCollection dntc2;

            DbObject dbo = new DbObject();
            DataSet ds = null;
            ds = dbo.GetDataSet("dbGen.GetDotNetDataTypes");

            dntc2 = (DotNetTypeCollection)ds.Tables[0].AsEnumerable().Select(row => new DotNetType
                            {
                                DotNetDataType = row.Field<string>("DotNetDataType"),
                                DatabaseDataType = row.Field<string>("DatabaseDataType")
                            }).ToList();
            return dntc2;
        }

Thanks,

Tom

 

[Moregelen]

You can't cast a base type into a subtype - that doesn't work. To give a good example from stack overflow, you can't cast a mammal (your base type) into a dog, because it might be a cat.

I suggest you read up on inheritance:

https://msdn.microsoft.com/en-us/library/ms173149.aspx

 

[Moregelen]

If you'd like to see it in action, compile and run this. You'll see that going from mammal to cat fails, but going from cat to mammal works just fine.

[COLOR=#0000FF]using[/color] System;
[COLOR=#0000FF]using[/color] System.Collections.Generic;
[COLOR=#0000FF]using[/color] System.Data;
[COLOR=#0000FF]using[/color] System.Linq;
[COLOR=#0000FF]using[/color] System.Text;
[COLOR=#0000FF]using[/color] System.Threading.Tasks;
 
[COLOR=#0000FF]namespace[/color] ConsoleApplication1
{
    [COLOR=#0000FF]class[/color] [COLOR=#2B91AF]Program[/color]
    {
        [COLOR=#0000FF]static[/color] [COLOR=#0000FF]void[/color] Main([COLOR=#0000FF]string[/color][] args)
        {
 
            [COLOR=#0000FF]try[/color]
            {
                [COLOR=#2B91AF]Mammal[/color] m = [COLOR=#0000FF]new[/color] [COLOR=#2B91AF]Mammal[/color]();
                [COLOR=#2B91AF]Cat[/color] c = ([COLOR=#2B91AF]Cat[/color])m;
                [COLOR=#2B91AF]Console[/color].WriteLine([COLOR=#A31515]"Cast from mammal to cat good"[/color]);
            }
            [COLOR=#0000FF]catch[/color] {
                [COLOR=#2B91AF]Console[/color].WriteLine([COLOR=#A31515]"Cast from mammal to cat failed"[/color]);
            }
 
            [COLOR=#0000FF]try[/color]
            {
                [COLOR=#2B91AF]Cat[/color] c = [COLOR=#0000FF]new[/color] [COLOR=#2B91AF]Cat[/color]();
                [COLOR=#2B91AF]Mammal[/color] m = ([COLOR=#2B91AF]Mammal[/color])c; [COLOR=#008000]//cast is actually unnecessary here because it is already a mammal object[/color]
                [COLOR=#2B91AF]Console[/color].WriteLine([COLOR=#A31515]"You can go from a sub-class to the base-class"[/color]);
            }
            [COLOR=#0000FF]catch[/color] {
                [COLOR=#2B91AF]Console[/color].WriteLine([COLOR=#A31515]"You can't go from a sub-class to the base-class"[/color]);
            }
 
            [COLOR=#2B91AF]Console[/color].ReadKey();
        }
    }
 
 
    [COLOR=#0000FF]public[/color] [COLOR=#0000FF]class[/color] [COLOR=#2B91AF]Mammal[/color]
    {
        [COLOR=#0000FF]public[/color] [COLOR=#0000FF]string[/color] name { [COLOR=#0000FF]get[/color]; [COLOR=#0000FF]set[/color]; }
    }
    [COLOR=#0000FF]public[/color] [COLOR=#0000FF]class[/color] [COLOR=#2B91AF]Cat[/color] : [COLOR=#2B91AF]Mammal[/color]
    {
        [COLOR=#0000FF]public[/color] [COLOR=#0000FF]int[/color] age { [COLOR=#0000FF]get[/color]; [COLOR=#0000FF]set[/color]; }
    }
    [COLOR=#0000FF]public[/color] [COLOR=#0000FF]class[/color] [COLOR=#2B91AF]Dog[/color] : [COLOR=#2B91AF]Mammal[/color]
    {
        [COLOR=#0000FF]public[/color] [COLOR=#0000FF]int[/color] dogyears { [COLOR=#0000FF]get[/color]; [COLOR=#0000FF]set[/color]; }
    }
}