The type 'System.Windows.Forms.GroupBox' cannot be used as type parameter 'T' in the generic type or method

my Class:

public static class Global
{
    public static void TextBoxEmpty<T>(T ContainerControl) where T : ContainerControl
    {
        foreach (var t in ContainerControl.Controls.OfType<TextBox>())
        {
            t.Text = string.Empty; 
        }
    }
}

use :

private void btnCancel_Click(object sender, EventArgs e)
{
    Global.TextBoxEmpty<GroupBox>(this.grpInfoBook);
}

error :

The type 'System.Windows.Forms.GroupBox'开发者_如何学运维 cannot be used as type parameter 'T' in the generic type or method 'Global.TextBoxEmpty(T)'. There is no implicit reference conversion from 'System.Windows.Forms.GroupBox' to 'System.Windows.Forms.ContainerControl'.

What is the correct code?

You don't really need the where restriction at all since in the code you're using OfType to filter the list anyways. However, if you want to keep the restriction, change it to reference System.Windows.Controls.Control:

public static class Global
{
    public static void TextBoxEmpty<T>(T ContainerControl) where T : Control
    {
        foreach (var t in ContainerControl.Controls.OfType<TextBox>())
        {
            t.Text = string.Empty; 
        }
    }
}

Take a look at the docs for GroupBox and you'll see it does not inherit from ContainerControl:

System.Object
  System.Windows.Threading.DispatcherObject
    System.Windows.DependencyObject
      System.Windows.Media.Visual
        System.Windows.UIElement
          System.Windows.FrameworkElement
            System.Windows.Controls.Control
              System.Windows.Controls.ContentControl
                System.Windows.Controls.HeaderedContentControl
                  System.Windows.Controls.GroupBox

http://msdn.microsoft.com/en-us/library/system.windows.controls.groupbox.aspx

The inheritance hierarchy of GroupBox is:

  System.Object 
    System.MarshalByRefObject
      System.ComponentModel.Component
        System.Windows.Forms.Control
          System.Windows.Forms.GroupBox

The type ContainerControl is not in this inheritance tree, hence the reason for the error message.

The generic constraint, you have defined restricts the usage to ContainerControl type. But GroupBox is not a containercontrol. It derives from Control class.