Error CS1934 Failed to implement query pattern for source type & # 39; NavigatorCustomButtons & # 39; being found.

Friends I have a class that retrieves a value with an expression lamda of a control. This works well for me in VB .Net when it is submitted to C #. It sends me this error:
This is the code I have in a class in C #, in vB I had it in a class too, I expect your comments:

public static NavigatorCustomButton MfxDtanavGetBtn (ControlNavigator poctrlnav, string pcval)
{
// get the display button with lambda
NavigatorCustomButton aobtn = (from an object in poctrlnav.Buttons.CustomButtons, where aobject.Tag.ToString () == pcval select an object). ElementAtOrDefault (0);

return aobtn;

}