尝试这样的：

ViewBag.ListOfCodes = new SelectList(mymodelinstance.Codes, "Id", "Name");
ViewBag.Codes = "C";

和在您的视图：

<%= Html.DropDownList(
    "Codes", 
    (IEnumerable<SelectListItem>)ViewBag.ListOfCodes, 
    new { style = "max-width: 600px;" }
) %>

对于这个工作，你显然必须有id为C的项目您的收藏里面，像这样的：

For this to work you obviously must have an item with Id = "C" inside your collection, like this:

    ViewBag.ListOfCodes = new SelectList(new[]
    {
        new { Id = "A", Name = "Code A" },
        new { Id = "B", Name = "Code B" },
        new { Id = "C", Name = "Code C" },
    }, "Id", "Name");