如何使用泛型

Question

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> ?

Answer 1

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.

Answer 2

The generic argument of GridBox applies a constraint that you don't, which means that, if this were allowed to compile, you'd be able to pass in a type that doesn't meet its constraints. The fix is of course simple, add the constraint to your generic argument that the type needs to have a parameterless constructor:

public static T Show<T>(...) where T : IComparable<T>, new()

Answer 3

您需要添加new约束：

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

Answer 4

您还需要将new（）约束添加到静态函数中：

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