Not able to make it works

[xufoo]

im new to it .
when I try it on my win7(vs2015 .. )
it just said some thing wrong ..
like : "The type "Gray","Bar" must be a non-nullable value type in order to use it as parameter in the generic type or method " It really puzzle me .. help me plz ..

[xufoo]

` class FaceData
{
public string PersonName { get; set; }
public Image<Gray, byte> FaceImage { get; set; }
public DateTime CreateDate { get; set; }

}`