Adding extra properties to an anonymous class

Question

Is there a way to add extra properties to an anonymous class (or create a new instance with the extra properties)?

string cssClasses = "hide";
object viewData = new { @class = cssClasses };
if (suppliedId != null)
{
  // add an extra id property to the viewData somehow...
  // desired end result is { @id = suppliedId, @class = cssClasses }
}
// ... code will be passing the viewData as the additionalViewData parameter in asp.mvc's .EditorFor method

Answer 1

No, you can't do it. You can redefine it, though:

dynamic viewData = new { @class = cssClasses };
if (suppliedId != null)
{
    viewData = new { @class = viewData.@class, yourExtra = Property };
}

Note that I changed it to dynamic. This is just so you can edit the properties without casting to dynamic.

Answer 2

You can not add extra properties to anonymous types, but you can use two anonymous types.

If you don't like dynamic (and there are reasons not to), you could do:

object viewData;
if (suppliedId == null)
    viewData = new { @class = cssClasses, };
else
    viewData = new { @id = suppliedId, @class = cssClasses, };

If you want to keep separate exactly typed references to each object instance, then use:

var viewDataSimple = new { @class = cssClasses, };
var viewDataExtra = new { @id = suppliedId, @class = cssClasses, };
object viewDataToUse;
if (suppliedId == null)
    viewDataToUse = viewDataSimple;
else
    viewDataToUse = viewDataExtra;

instead.