很奇怪的问题 错误提示 ：C#扩展方法必须在非泛型静态类中定

我只是在代码中加入了以下代码，运行时就出现这个错误，而且错误显示是出在Form1.
// DLL调用注册
[DllImport("user32.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.Winapi)]
private static extern IntPtr GetFocus();
/// <summary>
/// 当前拥有焦点的控件
/// </summary>
/// <param name="formControl"></param>
/// <returns></returns>
public static Control GetFocusedControl(this Control formControl)
{
Control focusedControl = null;
try
{
IntPtr focusedHandle = GetFocus();
if (focusedHandle != IntPtr.Zero)
{
focusedControl = Control.FromChildHandle(focusedHandle);
}
}
catch { }
return focusedControl;
}