C#•2w ago

✅ Cannot convert List<MyClass> to List<object>

I'm trying to design a custom generic collection editor in Winforms. I'm getting into a bit of a mess with casting the received class type onto a list of objects. I'm not very good with generics, so tried this via List<object>. See my code here: https://pastebin.com/M2DwziWj Gives me the attached error (precis: Unable to cast object of type List<SupportApplications> to type List<object> I'm sure this may be a job for List<T> or something, but I've never really got my head around generics. I know this is me getting some basic principles wrong. Can anyone help with example code?

Pastebin

public class SupportApplications: object{ public string Name { get;...

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37 Replies

jcotton42•2w ago

List is invariant, because otherwise you could do something like this

List<string> s = …;
List<object> o = s;
o.Add(1); // kaboom!

List<string> s = …;
List<object> o = s;
o.Add(1); // kaboom!

stigzlerOP•2w ago

Yes - that makes sense. How should I approach this then?

jcotton42•2w ago

You could make the form generic I guess?

stigzlerOP•2w ago

Like I say - I'm rubbish with generics. I tried this in GenericCollectionEditorForm:

public List<T> Objects { get; set; }

public List<T> Objects { get; set; }

but get the error "namespace T cannot be found" oh wait!

    public partial class GenericCollectionEditorForm<T> : FormViewBase
    {
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
        public List<T> Objects { get; set; }

    public partial class GenericCollectionEditorForm<T> : FormViewBase
    {
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
        public List<T> Objects { get; set; }

works¬ !

Angius•2w ago

Yep, you need to get the T from somewhere for it to work

stigzlerOP•2w ago

how does one add a new object of type T to a list? Tried variations on the line below in GenericCollectionEditorForm<T> : FormViewBase , but no dice:

Objects.Add(<T>);

Objects.Add(<T>);

Also, Objects.Remove(objectsBindingSource.Current); doesn't compile - "cannot convert from object to T"

Angius•2w ago

Just... add that object? If T is int, just add an int If T is Person, just add an instance of Person If T is IFoo then add an instance of any class that implements IFoo You get the idea

stigzlerOP•2w ago

but that defeats the point for the exercise - it's a generic collection editor - that is - GenericCollectionEditorForm won't know what type is being passed in - can you not achieve this via T?

Angius•2w ago

Why won't it know the type? Just instantiate GenericCollectionEditorForm with a given type You have to, to even create the instance

jcotton42•2w ago

It sounds like they want to be able to make new Ts inside the editor?

stigzlerOP•2w ago

@Angius Code as currently stands:

public partial class GenericCollectionEditorForm<T> : FormViewBase
{
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public List<T> Objects { get; set; }

    private Type type = null;

    private BindingSource objectsBindingSource = new BindingSource();

   public GenericCollectionEditorForm(List<T> objects, Type type)
    {
        //InitializeComponent();

        Objects = objects;
        this.type = type;

        objectsBindingSource.DataSource = Objects;
        MainLB.DataSource = objectsBindingSource;
    }

    private void AddBT_Click(object sender, EventArgs e)
    {
        if (type == null) return;
        Objects.Add((T)Activator.CreateInstance(typeof(T), new object()));
        RefreshEffectListDataBindings();
    }

    private void MainLB_SelectedValueChanged(object sender, EventArgs e)
    {
        MainPG.SelectedObject = MainLB.SelectedItem;
    }

    private void RefreshEffectListDataBindings()
    {
        MainLB.DataSource = null;
        MainLB.DataSource = objectsBindingSource;
    }

    private void MainPG_PropertyValueChanged(object s, PropertyValueChangedEventArgs e)
    {
        //ComponentVM.UpdateDesignItemEffectProperties(MainPG.SelectedObject as Effect);
        //ComponentVM.MainDatabase.Collections.Effects.Upsert(MainPG.SelectedObject as Effect);
    }

    private void DeleteBT_Click(object sender, EventArgs e)
    {
        if (MainLB.SelectedItems.Count == 0) return;
        Objects.Remove(objectsBindingSource.Current);
        RefreshEffectListDataBindings();
    }

    private void UpBT_Click(object sender, EventArgs e)
    {
        int newIndex = Math.Max(MainLB.SelectedIndex - 1, 0);
        Objects.Move(MainLB.SelectedIndex, newIndex);
        RefreshEffectListDataBindings();
        MainLB.SelectedIndex = newIndex;
    }

    private void DownBT_Click(object sender, EventArgs e)
    {
        int newIndex = Math.Min(MainLB.SelectedIndex + 1, MainLB.Items.Count - 1);
        Objects.Move(MainLB.SelectedIndex, newIndex);
        RefreshEffectListDataBindings();
        MainLB.SelectedIndex = newIndex;
    }

}

public partial class GenericCollectionEditorForm<T> : FormViewBase
{
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public List<T> Objects { get; set; }

    private Type type = null;

    private BindingSource objectsBindingSource = new BindingSource();

   public GenericCollectionEditorForm(List<T> objects, Type type)
    {
        //InitializeComponent();

        Objects = objects;
        this.type = type;

        objectsBindingSource.DataSource = Objects;
        MainLB.DataSource = objectsBindingSource;
    }

    private void AddBT_Click(object sender, EventArgs e)
    {
        if (type == null) return;
        Objects.Add((T)Activator.CreateInstance(typeof(T), new object()));
        RefreshEffectListDataBindings();
    }

    private void MainLB_SelectedValueChanged(object sender, EventArgs e)
    {
        MainPG.SelectedObject = MainLB.SelectedItem;
    }

    private void RefreshEffectListDataBindings()
    {
        MainLB.DataSource = null;
        MainLB.DataSource = objectsBindingSource;
    }

    private void MainPG_PropertyValueChanged(object s, PropertyValueChangedEventArgs e)
    {
        //ComponentVM.UpdateDesignItemEffectProperties(MainPG.SelectedObject as Effect);
        //ComponentVM.MainDatabase.Collections.Effects.Upsert(MainPG.SelectedObject as Effect);
    }

    private void DeleteBT_Click(object sender, EventArgs e)
    {
        if (MainLB.SelectedItems.Count == 0) return;
        Objects.Remove(objectsBindingSource.Current);
        RefreshEffectListDataBindings();
    }

    private void UpBT_Click(object sender, EventArgs e)
    {
        int newIndex = Math.Max(MainLB.SelectedIndex - 1, 0);
        Objects.Move(MainLB.SelectedIndex, newIndex);
        RefreshEffectListDataBindings();
        MainLB.SelectedIndex = newIndex;
    }

    private void DownBT_Click(object sender, EventArgs e)
    {
        int newIndex = Math.Min(MainLB.SelectedIndex + 1, MainLB.Items.Count - 1);
        Objects.Move(MainLB.SelectedIndex, newIndex);
        RefreshEffectListDataBindings();
        MainLB.SelectedIndex = newIndex;
    }

}

Objects.Add((T)Activator.CreateInstance(typeof(T), new object())); untested at runtime as getting compile error on Objects.Remove(objectsBindingSource.Current); - "cannot convert from object to T"

jcotton42•2w ago

You can just cast.

stigzlerOP•2w ago

lol - oh ffs I'm dumb Now stuck on instantiating it. The variable type shows no errors, but cannot get the line below it right:

Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object);
GenericCollectionEditorForm<T> form = new GenericCollectionEditorForm((List<type>)value);

Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object);
GenericCollectionEditorForm<T> form = new GenericCollectionEditorForm((List<type>)value);

Angius•2w ago

List<T>, assuming the type is the same as T You can't pass an instance of type as a generic parameter

stigzlerOP•2w ago

OK. Forget type. How would I instantiate GenericCollectionEditorForm? (have removed 'type' as a parameter):

 public GenericCollectionEditorForm(List<T> objects)
  {
      Objects = objects;
      // blah...
  }

  GenericCollectionEditorForm<T> form = new GenericCollectionEditorForm((List<T>)value);

 public GenericCollectionEditorForm(List<T> objects)
  {
      Objects = objects;
      // blah...
  }

  GenericCollectionEditorForm<T> form = new GenericCollectionEditorForm((List<T>)value);

Last line not working in many places!

Angius•2w ago

Uh GenericCollectionEditorForm((T)value)...? We're firmly entering "weird shit" territory lol

stigzlerOP•2w ago

Sorry Zzzz - I'm a hobby coder - I know you're a super-duper expert coder and everything - so my amateurship may be throwing you tried: GenericCollectionEditorForm<Type> form = new GenericCollectionEditorForm((T)value);

stigzlerOP•2w ago

LHS worked, but RHS doesn't compile:

Angius•2w ago

GenericCollectionEditorForm<T> form = new GenericCollectionEditorForm((T)value); Or just var form = new GenericCollectionEditorForm((T)value); for simplicity

stigzlerOP•2w ago

naw - whichever permutation, it doens't like GenericCollectionEditorForm((T)value) it also doesn't like GenericCollectionEditorForm<T>!

Angius•2w ago

Are we still inside of GenericCollectionEditorForm class, or somewhere else? Like, are you trying to create a new GenericCollectionEditorForm from another form?

stigzlerOP•2w ago

Yeah soz - now in GenericCollectionEditor which instantiates the form. It may be helpful for you to review my OP code - which shows the full structure.

Angius•2w ago

Right, well, the generic type has to come from somewhere So you can either define it here,

var form = new GenericCollectionEditorForm<int>(69)

var form = new GenericCollectionEditorForm<int>(69)

Or it needs to come from... somewhere Be it a generic EditValue<T> or a generic GenericCollectionEditor<T>

stigzlerOP•2w ago

Let's focus the issue:

    internal class GenericCollectionEditor : UITypeEditor
    {
        public override UITypeEditorEditStyle GetEditStyle(System.ComponentModel.ITypeDescriptorContext context)
        {
            if (context == null && context.Instance == null) return base.GetEditStyle(context);
            return UITypeEditorEditStyle.Modal;
        }

        public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
        {
            if (context == null || provider == null || context.Instance == null) return value;

            IWindowsFormsEditorService editorService = (IWindowsFormsEditorService)provider.GetService(typeof(IWindowsFormsEditorService));

            Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object);

            // What here??
            GenericCollectionEditorForm<Type> form = new GenericCollectionEditorForm<Type>((List<Type>)value);

            if (editorService.ShowDialog(form) == System.Windows.Forms.DialogResult.OK)
            {
                return form.Objects.ToList();
            }

            return base.EditValue(context, provider, value);
        }

    internal class GenericCollectionEditor : UITypeEditor
    {
        public override UITypeEditorEditStyle GetEditStyle(System.ComponentModel.ITypeDescriptorContext context)
        {
            if (context == null && context.Instance == null) return base.GetEditStyle(context);
            return UITypeEditorEditStyle.Modal;
        }

        public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
        {
            if (context == null || provider == null || context.Instance == null) return value;

            IWindowsFormsEditorService editorService = (IWindowsFormsEditorService)provider.GetService(typeof(IWindowsFormsEditorService));

            Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object);

            // What here??
            GenericCollectionEditorForm<Type> form = new GenericCollectionEditorForm<Type>((List<Type>)value);

            if (editorService.ShowDialog(form) == System.Windows.Forms.DialogResult.OK)
            {
                return form.Objects.ToList();
            }

            return base.EditValue(context, provider, value);
        }

This line compiles, but erro at runtime: enericCollectionEditorForm<Type> form = new GenericCollectionEditorForm<Type>((List<Type>)value);

Angius•2w ago

Well, you're creating an editor form for a list of Type instances Type instance is not the same as a generic

stigzlerOP•2w ago

yes I know! The question is again, how do I instantiate the form from what I recieve in the EditValue method? Are you familiar with UITypeEditor first of all?

Angius•2w ago

The gist of it is: if you want to use generic types, the generic must be known at some point. You can't do

var foo = new Foo<whatever_lmao_idk>();

var foo = new Foo<whatever_lmao_idk>();

nor can you do

var t = typeof(69);
var foo = new Foo<t>();

var t = typeof(69);
var foo = new Foo<t>();

UITypeEditor

Honestly, no, first time I hear of it

stigzlerOP•2w ago

ah OK - well that may be part of the communicaiton break down. One moment

Angius•2w ago

From what I can see in the docs (https://learn.microsoft.com/en-us/dotnet/api/system.drawing.design.uitypeeditor?view=windowsdesktop-9.0) it's not really generic I see a bunch of casting and typeofs So you might have to just forgo type-safety here and yolo it with typeofs, reflections, and objects Ancient .NET APIs were... not always well thought-out

stigzlerOP•2w ago

OK - so this may help you to help me (I do appreciate your efforts). Think property grid in Vis Studio. If you decorate a Property of an object with a UITypeEditor attribute, then when you edit it in the property grid, the class derived from UITypeEditor is called (you don't manually call it yourself). The value parameter of public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value) holds the value of the property you want to edit. Thus this could be antyhning - a List, a string, an int etc. Have just tested my code. the line Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object); successfully detemrines the Type of value Now the remaining question is, how do I instantiate GenericCollectionEditorForm given I know the Type of value at runtime? Specifically - what is the syntax?

Angius•2w ago

Jusr forgo the generics and pass object that is your value Pass the type intance as well if needed, sure

stigzlerOP•2w ago

no - that takes us right back to the very beginning again! See JCotton42's first comment!

Angius•2w ago

But any generic types like lists will have to be List<object> or constructed with reflections

stigzlerOP•2w ago

nevermind - just leave it - maybe someone else can help

Angius•2w ago

Well, if you have no generic type, you have no generic type The "source" only has an object instance You cannot, in any way shape or form, get a generic parameter out of it You could, maybe, cast all elements of the list to object in order to get the List<object> you need theList.Cast<object>() So that wherever it is the original error happens is happy

stigzlerOP•2w ago

Solved this for any that are in a similar boat. Found some Reflection voodoo that instantiated x from the determined Type:

Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object);

Type genericClass = typeof(GenericCollectionEditorForm<>);
Type constructedClass = genericClass.MakeGenericType(type);
dynamic form = Activator.CreateInstance(constructedClass);

switch (type)
{
    case Type t when t == typeof(SupportApplications):
        List<SupportApplications> list = new List<SupportApplications>();
        foreach (var item in (List<SupportApplications>)value) list.Add(item);
        form.Objects = list;
        break;
    default:
        form.Text = $"Edit {type.Name} Collection";
        break;
}

Type type = value.GetType().GetGenericArguments().SingleOrDefault() ?? typeof(object);

Type genericClass = typeof(GenericCollectionEditorForm<>);
Type constructedClass = genericClass.MakeGenericType(type);
dynamic form = Activator.CreateInstance(constructedClass);

switch (type)
{
    case Type t when t == typeof(SupportApplications):
        List<SupportApplications> list = new List<SupportApplications>();
        foreach (var item in (List<SupportApplications>)value) list.Add(item);
        form.Objects = list;
        break;
    default:
        form.Text = $"Edit {type.Name} Collection";
        break;
}

Full code here: https://pastebin.com/gLHd2jqX

Pastebin

internal class GenericCollectionEditor : UITypeEditor { ...

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Anton•2w ago

yes, you have to do that. or make factories, put them in a dictionary and look up by type, if you want more flexibility in how they are created idk why no one mentioned this the form might be doing type checks on the generic inside to select behavior, this might break

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✅ Cannot convert List<MyClass> to List<object>

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