为什么将带有子对象列表的模型传递给控制器​​时，子对象列表没有通过？

Question

I have 2 Model classes:

public class ModelWithList 
{
    int Id { get; set; }
    string Name { get; set; }
    List<SetOfGroups> GroupSets { get; set; }
}

public class SetOfGroups
{
    List<Groups> Groups { get; set; }
}

When I submit my form all the fields are pass through from ModelWithList except for the GroupSets.

Normally I would use a @Html.HiddenFor(model => model.GroupSets) for properties that aren't passed through but this can't be done with lists of custom models.

Answer 1

Suppose you have this model setup:

public class ModelWithList {
    int Id { get; set; }
    string Name { get; set; }
    List<SetOfGroups> GroupSets { get; set; }
}

public class SetOfGroups
{
    public int GroupId { get; set; }
    public string GroupName { get; set; }
}

Then you can use a for loop to iterate GroupSets and assign its index for every numeric type/ string / DateTime (including Nullable<T> ) properties it has:

@model ModelWithList

@* other code *@

@for (int i = 0; i < Model.GroupSets.Count; i++)
{
    @Html.HiddenFor(model => model.GroupSets[i].GroupId)

    @Html.HiddenFor(model => model.GroupSets[i].GroupName)
}

Next, assume that you're moving SetOfGroups properties and create a new list:

public class SetOfGroups
{
    List<Group> Groups { get; set; }
}

public class Group
{
    public int GroupId { get; set; }
    public string GroupName { get; set; }
}

Then you should add another for loop to bind them:

@model ModelWithList

@* other code *@

@for (int i = 0; i < Model.GroupSets.Count; i++)
{
    @for (int j = 0; j < Model.GroupSets[i].Groups.Count; j++)
    {
        @Html.HiddenFor(model => model.GroupSets[i].Groups[j].GroupId)

        @Html.HiddenFor(model => model.GroupSets[i].Groups[j].GroupName)
    }
}

Although it may possible to add nested lists into view like above sample, the loops to render HTML helpers will become more complex and should be avoided.

Important Note:

You should not assign a HiddenFor to a List<T> like this:

@Html.HiddenFor(model => model.GroupSets)

because Razor implicitly calls ToString() method to that List<T> object, resulting in fully-qualified name of the list inserted to the value attribute and the binding will ignore it because GroupSets is not a string property:

<input id="GroupSets" name="GroupSets" type="hidden" value="System.Collections.Generic.List`1[ProjectNamespace.Models.SetOfGroups]">

Related issue:

List item inside model always null on post - Asp.net MVC