HashSet(Of T) - how can I get elements by position number ?

[Sascha Lefevre]

Sure, here you go:

class ChildClass
{ }

class MyClass
{
public ICollection<ChildClass> Children { get; set; }

public MyClass()
{
    Children = new HashSet<ChildClass>() { new ChildClass() };
}

}

T CopyEntity(T entity)
where T : class, new()
{
T copy = new T();
// actual copying omitted here
// for testing only the return type matters
return copy;
}

void Test()
{
MyClass myClass = new MyClass();
dynamic children = typeof(MyClass).GetProperty("Children").GetValue(myClass);
dynamic child = Enumerable.ElementAt(children, 0);
dynamic copy = CopyEntity(child);
// copy is of type ChildClass
}

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

[dilkonika]

Thank you ! But your code in C# when is converted in vb.net ( with code converters ) , is :

Class ChildClass
End Class

Class [MyClass]
Public Property Children() As ICollection(Of ChildClass)
Get
Return m_Children
End Get
Set
m_Children = Value
End Set
End Property
Private m_Children As ICollection(Of ChildClass)

Public Sub New()
	Children = New HashSet(Of ChildClass)() From { \_
		New ChildClass() \_
	}
End Sub

End Class

Private Function CopyEntity(Of T As {Class, New})(entity As T) As T
Dim copy As New T()
' actual copying omitted here
' for testing only the return type matters
Return copy
End Function

Private Sub Test()
Dim [myClass] As New [MyClass]()
Dim children As dynamic = GetType([MyClass]).GetProperty("Children").GetValue([myClass])
Dim child As dynamic = Enumerable.ElementAt(children, 0)
Dim copy As dynamic = CopyEntity(child)
' copy is of type ChildClass
End Sub

and this code is not using CTypeDynamic. So what you have tested in vb.net that doesn't work ? and on my vb.net code , the error came up on the line :

Dim en = ctx.Entry(clone)

and you don't have a such line in your code. ............................. Also I try to implement your version with reflection , but I can't make it work on vb.net ? can you provide some more help ? .............................. Thank you !

[Sascha Lefevre]

dilkonika wrote:

and this code is not using CTypeDynamic.

Because it works without it. For completeness, I included the analog into the VB-version:

Public Class ChildClass
End Class

Public Class SomeClass
    Public Property Children As ICollection(Of ChildClass) = New HashSet(Of ChildClass)
End Class

Public Function CopyEntity(Of T As {Class, New})(entity As T) As T
    Dim clone As New T()
    Return clone
End Function

Sub Main()
    Dim someClass = New SomeClass
    someClass.Children.Add(New ChildClass)

    Dim el = Enumerable.ElementAt(CallByName(someClass, "Children", CallType.Get), 0)

    Dim elcopy = CopyEntity(el)   REM without CTypeDynamic. doesn't work. (elcopy is Object)

    Dim el1 = CType(el, ChildClass)
    Dim el1copy = CopyEntity(el1) REM works (el1copy is ChildClass)

    Dim el2 = CTypeDynamic(Of ChildClass)(el)
    Dim el2copy = CopyEntity(el2) REM works (el2copy is ChildClass)

    Dim el3 = CTypeDynamic(el, GetType(ChildClass))
    Dim el3copy = CopyEntity(el3) REM doesn't work (el3copy is Object)
End Sub

Quote:

and on my vb.net code , the error came up on the line : Dim en = ctx.Entry(clone) and you don't have a such line in your code.

Because the reason why it not works (when it not works) is because clone is of type Object and not of type ChildClass. So I only have to check for the type of clone to know if your line ctx.Entry(clone) would work.

dilkonika wrote:

Also I try to implement your version with reflection , but I can't make it work on vb.net ? can you provide some more help ?

Please post what you have so far.

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

[dilkonika]

Thank you ! But for the Reflection version , I just converted the code to vb.net but I don't know how to integreate with my code. If you can just post your full version for C# with reflection so I can understand more ( I don't have so much knowledge about reflection , but at this point I really need a solution for my problem , so I need to use it )

[Sascha Lefevre]

class Program
{
class ChildClass
{ }

class SomeClass
{
    public ICollection Children { get; set; }

    public SomeClass()
    {
        Children = new HashSet() { new ChildClass() };
    }
}

public static T CopyEntity(T entity)
    where T : class, new()
{
    T copy = new T();
    return copy;
}

static void Main(string\[\] args)
{
    SomeClass someClass = new SomeClass();

    dynamic children = typeof(SomeClass).GetProperty("Children").GetValue(someClass);
    object child = Enumerable.ElementAt(children, 0);

    // if CopyEntity would be not a static method, BindingFlags.Instance
    // would have to be used, instead of BindingFlags.Static 
    MethodInfo mi = typeof(Program).GetMethod("CopyEntity", BindingFlags.Static | BindingFlags.Public);
    mi = mi.MakeGenericMethod(new Type\[\] { /\* +typeof(YourContext) \*/ child.GetType() /\* +typeof(bool) \*/ });

    // if CopyEntity would be not a static method, the first argument
    // to Invoke would have to be the class-instance on which to invoke
    // CopyEntity, instead of null
    object clone = mi.Invoke(null, new object\[\] { /\* +ctx \*/ child /\* +copyKeys \*/ });
    // -> clone is of type ChildClass
}

}

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

[dilkonika]

Thank you ! Your code is working. Just for curiosity , the other variant is working too. I found this solution in vb.net , to make a change on CopyEntityFunction like this : instead of

Dim clone as New(T)

I use this :

Dim clone = Activator.CreateInstance(entity.GetType())

Now it's working. Thank you for your help.

[Sascha Lefevre]

dilkonika wrote:

Dim clone = Activator.CreateInstance(entity.GetType())

Yes, that's a simpler solution if you don't need CopyEntity to be generic (that is, if you don't need the static type T for anything else). If you haven't already, you can remove that generic stuff from the declaration of CopyEntity then.

dilkonika wrote:

Thank you for your help.

You're welcome.

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson