Blazor: 2-way binding to nullable struct not supported.

[alexdrl]

I have a problem with defining Blazor components.

This is my component code...

@code {
    [Parameter]
    public TimeSpan? Value { get; set; }

    [Parameter]
    public EventCallback<TimeSpan?> ValueChanged { get; set; }
...
}

The idea is that I can 2-way bind like this using a TimeSpan? field:

I can't bind it like this:

<TimeSpanTotalHoursSpinEditNullable @bind-Value="@Model.Value" />

With compiler errors:

1>------ Build started: Project: MyApp.WebApp.Client, Configuration: Debug Any CPU ------
1>C:\Users\Username\code\MyCompany\MyApp\src\MyApp.WebApp.Client\obj\Debug\net9.0\Microsoft.CodeAnalysis.Razor.Compiler\Microsoft.NET.Sdk.Razor.SourceGenerators.RazorSourceGenerator\Components_Adjustments_AdjustmentEditorFormLayout_razor.g.cs(578,421,578,573): error CS1503: Argument 2: cannot convert from 'Microsoft.AspNetCore.Components.EventCallback<System.TimeSpan>' to 'Microsoft.AspNetCore.Components.EventCallback'
1>C:\Users\Username\code\MyCompany\MyApp\src\MyApp.WebApp.Client\obj\Debug\net9.0\Microsoft.CodeAnalysis.Razor.Compiler\Microsoft.NET.Sdk.Razor.SourceGenerators.RazorSourceGenerator\Components_Adjustments_AdjustmentEditorFormLayout_razor.g.cs(559,154,559,169): error CS1662: Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type
1>Done building project "MyApp.WebApp.Client.csproj" -- FAILED.

I can workaround it like this, but seems ugly:

<TimeSpanTotalHoursSpinEditNullable Value="@Model.Value" ValueChanged=@(value => {if (value != null) { Model.Value = value.Value; }} )/>

Is this a compiler issue? Is there a official better way to support having a nullable struct as parameter and binding to it?

[jjonescz]

Do I understand correctly that you want to bind between a variable of type TimeSpan and a parameter of type TimeSpan?, i.e., one nullable struct and the other non-nullable? Those are two distinct types to the Razor compiler, so it wouldn't know how to convert between them (do you want to throw on null or silently ignore nulls, etc.). I guess a general feature here would be some converters for bindings.

[alexdrl]

Do I understand correctly that you want to bind between a variable of type TimeSpan and a parameter of type TimeSpan?, i.e., one nullable struct and the other non-nullable?

yes!! this is the source of the problem here, as I imagined this is not working because they are not the same type...

Those are two distinct types to the Razor compiler, so it wouldn't know how to convert between them (do you want to throw on null or silently ignore nulls, etc.). I guess a general feature here would be some converters for bindings.

Couldn't the component be built for TimeSpan?, be able to handle nulls, and then my client components be able to send the value as TimeSpan ? This may seem easy, but the problem when bubbling up the values right?

[jjonescz]

This may seem easy, but the problem when bubbling up the values right?

Yes, it's not clear what the binding should do when there is a null propagated from the component parameter up to the variable that's not nullable.

Also, if you just ignore nulls like you do in Value="@Model.Value" ValueChanged=@(value => {if (value != null) { Model.Value = value.Value; }}, then you can in fact get your "bound" values out of sync - it might happen that the component has Value=null while the Model.Value has some non-null value at the same time, leading to potentially inconsistent UI I can imagine.