Question

The Problem

It’s something I came across a while back and was able to work around it somehow. But now it came back, feeding on my curiosity – and I’d love to have a definite answer.

Basically, I have a generic dgv BaseGridView<T> : DataGridView where T : class. Constructed types based on the BaseGridView (such as InvoiceGridView : BaseGridView<Invoice>) are later used in the application to display different business objects using the shared functionality provided by BaseGridView (like virtual mode, buttons, etc.).

It now became necessary to create a user control that references those constructed types to control some of the shared functionality (eg. filtering) from BaseGridView. I was therefore hoping to create a public property on the user control that would enable me to attach it to any BaseGridView in Designer/code: public BaseGridView<T> MyGridView { get; set; }. The trouble is, it doesn’t work 🙂 When compiled, I get the following message:

The type or namespace name ‘T’ could not be found (are you missing a using directive or an assembly reference?)

Solutions?

I realise I could extract the shared functionality to an interface, mark BaseGridView as implementing that interface, and then refer to the created interface in my uesr control.

But I’m curious if there exists some arcane C# command/syntax that would help me achieve what I want – without polluting my solution with an interface I don’t really need 🙂

EDIT: For reference, I did try this innocent workaround: BaseGridView<object> MyGridView { get; set; }, and… it still isn’t the answer: Cannot implicitly convert type ‘InvoiceGridView’ to ‘BaseGridView<object>’.

Partial success (edit 2)

Ok, because covariance is only supported on interfaces, I admitted defeat and defined an interface (only showing some of it):

public interface IBaseGridView<out T> where T : class
{
    bool ScrollTo(Predicate<T> criteria);
    bool ScrollTo(T object);
}

I am now able to cast my beloved InvoiceGridView to an IBaseGridView<object> – which is awesome and I’m a happy boy again 🙂 However, the second ScrollTo is giving me trouble upon compilation:

Invalid variance: The type parameter ‘T’ must be contravariantly valid on ‘GeParts.Controls.IBaseGridView.ScrollTo(T)’. ‘T’ is covariant.

I’m now having to modify the signature to ScrollTo(object o) – which isn’t ideal but gets the job done. What suprised me was that the compiler complained about the second ScrollTo yet was happy with the first one. So it seems that one isn’t allowed to pass instances of an out T, but using the type itself (eg. in Predicate<T>) is fine? Seems rather picky…

Answer 1

Since you wrote

But I’m curious if there exists some arcane C# command/syntax that would help me achieve what I want

I’d like to add that C# 4.0 makes it possible to substitute derived types for a base type using < out T > for covariance. So you could do

public BaseGridView<Object> MyGridView { get; set; }

So you get a well known type but you can return whatever BaseGridView you want. The only catch is unfortunately that covariance is only allowed on interfaces! 🙁