jurabek

public class ChildAttribute : Attribute
{ }
public class Parent
{
    public string AnotherProperty { get; set; }
    [Child]
    public ChildClass Child { get; set; }
}
public class ChildClass
{
    public string Name { get; set; }
}

 static void Main(string[] args)
 {
     var parent = new Parent();
     foreach (PropertyInfo propertyInfo in parent.GetType().GetProperties())
     {
          var childAttribute = propertyInfo.GetCustomAttribute<ChildAttribute>();
          if (childAttribute != null)
          {
              //Get value of child property
              var childPropertyValue = propertyInfo.GetValue(parent);
               propertyInfo.SetValue(parent, new ChildClass { Name = "Child class created from reflection" });
          }
     }
     bool result = parent.Child.Name == "Child class created from reflection";
}