Why model binding with a struct doesn't work? - asp.net-core

Question

I'm trying to understand why this binding doesn't work.
The binding didn't work until I changed the type from struct to class.
Is this by design or am I missing something?
I'm using asp.net core 2.2 MVC
View Models
Not working
public class SettingsUpdateModel
{
public DeviceSettingsStruct DeviceSettings { get; set; }
}
Working
public class SettingsUpdateModel
{
public DeviceSettingsClass DeviceSettings { get; set; }
}
public class DeviceSettingsClass
{
public bool OutOfScheduleAlert { get; set; }
// other fields removed for brevity
}
public struct DeviceSettingsStruct
{
public bool OutOfScheduleAlert { get; set; }
// other fields removed for brevity
}
Controller
[HttpPost]
public IActionResult Update(SettingsUpdateModel newSettings)
{
// newSettings.DeviceSettings.OutOfScheduleAlert always false on struct but correct on class
return Index(null);
}
View
<input class="form-check-input" type="checkbox" id="out_of_schedule_checkbox" asp-for="DeviceSettings.OutOfScheduleAlert">
Expected: DeviceSettings.OutOfScheduleAlert to bind to a struct the same as class
Actual: only the class parameter was binded

Answer 1

It is by design in complex type model bindings. A struct type is a value type that is typically used to encapsulate small groups of related variables, such as the coordinates of a rectangle or the characteristics of an item in an inventory.
In ComplexTypeModelBinder.cs , the CanUpdateReadOnlyProperty method will mark the properties of value-type model as readonly due to value types have copy-by-value semantics, which prevents us from updating
internal static bool CanUpdatePropertyInternal(ModelMetadata propertyMetadata)
{
return !propertyMetadata.IsReadOnly || CanUpdateReadOnlyProperty(propertyMetadata.ModelType);
}
private static bool CanUpdateReadOnlyProperty(Type propertyType)
{
// Value types have copy-by-value semantics, which prevents us from updating
// properties that are marked readonly.
if (propertyType.GetTypeInfo().IsValueType)
{
return false;
}
// Arrays are strange beasts since their contents are mutable but their sizes aren't.
// Therefore we shouldn't even try to update these. Further reading:
// http://blogs.msdn.com/ericlippert/archive/2008/09/22/arrays-considered-somewhat-harmful.aspx
if (propertyType.IsArray)
{
return false;
}
// Special-case known immutable reference types
if (propertyType == typeof(string))
{
return false;
}
return true;
}
Reference here for more details .
BTY ,if you want to bind struct type model , you could try to send the json data by using ajax request from the view.