C# - Accessing Contained Class Properties In Gridviews Boundfield?

I wonder why asp.net wont allow accessing property of contained class on Gridview's Boundfields while it work in ItemTemplates..

Class User
{
Diagnosis diagnosis { get; set; } // Contained class
}
Class Diagnosis
{
string DiagnosisCode { get; set; }
}
gridview.datasource =
new List<User>() {
new User() {
diagnosis = new Diagnosis() { DiagnosisCode = "MALARIA" }} }
<boundfield datafield='<#% User.diagnosis.DiagnosisCode %>' />