In order to work around this, I attempted to change the type to Dictionary<string, int> which doesn't crash, but instead it binds the entire form to the Dictionary?

I am using the example from https://docs.microsoft.com/en-us/aspnet/core/mvc/models/model-binding?view=aspnetcore-5.0 (scroll down to the Dictionaries section), and it seems like that example also crashes if selectedCourses is missing from the POST.

Model binding will fallback to an empty prefix if it does not find any values that matches the parameter's prefix - https://docs.microsoft.com/en-us/aspnet/core/mvc/models/model-binding?view=aspnetcore-5.0#prefix--parameter-name. In your case, since no keys with the prefix sortOrder were present, model binding falls back to the empty prefix at which point every field is a candidate to be bound.

You can limit what prefixes are allowed to be bound by specifying it on the parameter - https://docs.microsoft.com/en-us/aspnet/core/mvc/models/model-binding?view=aspnetcore-5.0#custom-prefix.

Also just to be certain, I would expect binding to add the format exception as a a validation error to the ModelStateDictionary in and not throw an exception in this case. Is that not the case?

It doesn't even enter the controller action but crashes before.

Ooh that's a good find. It's weird that format exceptions from dictionaries are rethrown:

An unhandled exception has occurred while executing the request.
      System.FormatException: Input string was not in a correct format.
         at System.Number.ThrowOverflowOrFormatException(ParsingStatus status, TypeCode type)
         at System.Number.ParseInt32(ReadOnlySpan`1 value, NumberStyles styles, NumberFormatInfo info)
         at System.Int32.Parse(String s, NumberStyles style, IFormatProvider provider)
         at System.ComponentModel.Int32Converter.FromString(String value, NumberFormatInfo formatInfo)
         at System.ComponentModel.BaseNumberConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Mvc.ModelBinding.ModelBindingHelper.ConvertSimpleType(Object value, Type destinationType, CultureInfo culture)
         at Microsoft.AspNetCore.Mvc.ModelBinding.ModelBindingHelper.UnwrapPossibleArrayType(Object value, Type destinationType, CultureInfo culture)
         at Microsoft.AspNetCore.Mvc.ModelBinding.ModelBindingHelper.ConvertTo(Object value, Type type, CultureInfo culture)
         at Microsoft.AspNetCore.Mvc.ModelBinding.Binders.DictionaryModelBinder`2.BindModelAsync(ModelBindingContext bindingContext)
         at Microsoft.AspNetCore.Mvc.ModelBinding.ParameterBinder.BindModelAsync(ActionContext actionContext, IModelBinder modelBinder, IValueProvider valueProvider, ParameterDescriptor parameter, ModelMetadata metadata, Object value, Object container)
         at Microsoft.AspNetCore.Mvc.Controllers.ControllerBinderDelegateProvider.<>c__DisplayClass0_0.<<CreateBinderDelegate>g__Bind|0>d.MoveNext()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
         at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)

FYI @dougbu in case this stacktrace rings a bell.

Doesn't ring a bell but we have lots of try / catch blocks in other model binders around xyz.ConvertTo(...) calls. We probably aren't testing Dictionary<{numeric type}> enough.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

Same problem here (.net core 6.0 razor page), I have this dictionary
[BindProperty] public Dictionary<Int32, String> Relations { get; set; }

When user POST and there are no "Relations" sent the page throw exception Invalid Input format string.

So, I decide to add a hidden field and works
<input type="hidden" name="Relations[0]" value="0" />

and in the Post method I check that dictionary key is not 0 in the iteration.

but definitely is a bug, dictionary should be null when no data with that name is present in the POST

I have the same issue. I've tried [CanBeNull] and it didn't help. Also, it took significant time to find the issue, since error is somewhat cryptic.

My workaround is this:

@if (Model.MyArray.Count == 0)
{
    <input type="hidden" asp-for="@Model.MyArray" value="null" />
}

With combination of [CanBeNull] (maybe not necessary).

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