Hidden input for Dictionary<string, string> in ASP.NET MVC 3
public class ViewModel{
 ...
 public ViewPart ViewPart { get; set; }
}

public class ViewPart {
 ...
 public Dictionary<string, string> Flags { get; set; }
}

Dictionary<string, string> flags = new Dictionary<string, string>();
flags.Add("kind", "Edit");
flags.Add("Command", "Save");
ViewModel model = new ViewModel(){ Flags  = flags };
return View(model);

@foreach(var item in Model.ViewPart.Flags) {
 <input type="hidden" id="ViewPart_Flags_@(item.Key)" value="@item.Value" name="ViewPart.Flags[@(item.Key)]" />
}

@foreach(var item in Model.ViewPart.Flags) {
  @Html.HiddenFor(modelItem => item)
}

[HttpPost]
public ActionResult MyPostAction(ViewModel model){
  //model.ViewPart.Flags is null
}

var index = 0;
foreach (var item in Model.ViewPart.Flags)
{

    <input type="hidden" value="@item.Key"
                         name="ViewPart.Flags[@(index)].Key"/>
    <input type="hidden" value="@item.Value"
                         name="ViewPart.Flags[@(index)].Value"/>

    index++;
}
    <input type="submit" value="Save"/>

foreach (var item in Model.ViewPart.Flags)
{

    <input type="hidden" value="@item.Key"
                         name="ViewPart.Flags.Index"/>
    <input type="hidden" value="@item.Key"
                         name="ViewPart.Flags[@(item.Key)].Key" />
    <input type="hidden" value="@item.Value"
                         name="ViewPart.Flags[@(item.Key)].Value" />
}
    <input type="submit" value="Save"/>
}

[HttpPost]
public ActionResult MyPostAction(ViewModel model, ViewPart viewPart)
{
    model.ViewPart.Flags = viewPart.Flags;
}

@foreach(var item in Model.ViewPart.Flags) {
  @Html.HiddenFor(modelItem => item)
}