void ValidateDimensions()
{
double width = 0;
foreach (Widget child in children)
{
width += child.Width;
if (Height < child.Height)
throw new InvalidOperationException (String.Format ("Container height too small {0} < {1}", Height, child.Height));
}
if (Width < width)
throw new InvalidOperationException (String.Format ("Container witdh too small {0} < {1}", Width, width));
}