How to use generics

debugcn Published at Dev

Krowi

So, I have a class:

internal class GridBox<T> : BoxBase where T : new()
{
    public GridBox(Grid grid, GridBoxView view, MessageBoxIcon icon, string caption, ObservableCollection<T> dataSource, MessageBoxButton button)
        : base(grid, icon, caption, button)
    {
        View = view;
        DataSource = dataSource;
    }

    public GridBoxView View { get; set; }
    public ObservableCollection<T> DataSource { get; set; }
}

I use this GridBox Class here:

public static T Show<T>(DependencyObject sender, MessageBoxIcon icon, string caption, ObservableCollection<T> dataSource, MessageBoxButton button) where T : IComparable<T>
    {
        Window window = Window.GetWindow(sender);
        Grid grid = Extensions.FindChild<Grid>(window);
        GridBoxView gridBox = new GridBoxView();

        return gridBox.Show<T>(new GridBox<T>(grid, gridBox, icon, caption, dataSource, button));
    }

I get an error here tho at the new GridBox<T>:

'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method

So, how can I use new GridBox<T> if the T is coming from public static T Show<T>?

Chris Sinclair

GridBox<T> has a constraint on T that requires the type to have a public parameterless constructor. This is what the where T : new() specifies. (See the MSDN article on the new constraint)

As such, when you attempt to use it in your Show method, the T there must still satisfy this constraint. If you update your Show method to:

public static T Show<T>(DependencyObject sender, MessageBoxIcon icon, string caption, ObservableCollection<T> dataSource, MessageBoxButton button)
    where T : IComparable<T>, new()

By adding the new() constraint to Show, you will satisfy the constraints for GridBox as well.

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