Binding nullable int to input causes null reference exception

[floreseken]

Minimal repro steps

Consider this Blazor page:

<input @bind(TestInt) />

@functions
{
    public int? TestInt { get; set; }

}

Expected result

Page should render without exceptions

Actual result

A nullref exception is thrown:

SCRIPT5022: System.NullReferenceException: Object reference not set to an instance of an object.
  at Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder.AddAttribute (System.Int32 sequence, System.String name, System.Object value) <0x169c870 + 0x0005c> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Datack.Blazor.Pages.Data.Index.BuildRenderTree (Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder) <0x1977930 + 0x000dc> in <cd4fad1f0c094e678f0f1612015bbf56>:0 
  at (wrapper delegate-invoke) <Module>.invoke_void_RenderTreeBuilder(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder)
  at Microsoft.AspNetCore.Blazor.Rendering.ComponentState.RenderIntoBatch (Microsoft.AspNetCore.Blazor.Rendering.RenderBatchBuilder batchBuilder, Microsoft.AspNetCore.Blazor.RenderFragment renderFragment) <0x16964f8 + 0x0006c> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Microsoft.AspNetCore.Blazor.Rendering.Renderer.RenderInExistingBatch (Microsoft.AspNetCore.Blazor.Rendering.RenderQueueEntry renderQueueEntry) <0x16960a0 + 0x00040> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Microsoft.AspNetCore.Blazor.Rendering.Renderer.ProcessRenderQueue () <0x16931b8 + 0x00048> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Microsoft.AspNetCore.Blazor.Rendering.Renderer.AddToRenderQueue (System.Int32 componentId, Microsoft.AspNetCore.Blazor.RenderFragment renderFragment) <0x16923d0 + 0x00068> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Microsoft.AspNetCore.Blazor.Components.RenderHandle.Render (Microsoft.AspNetCore.Blazor.RenderFragment renderFragment) <0x16920b8 + 0x00036> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Microsoft.AspNetCore.Blazor.Routing.Router.Refresh () <0x17672a0 + 0x00130> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at Microsoft.AspNetCore.Blazor.Routing.Router.OnLocationChanged (System.Object sender, System.String newAbsoluteUri) <0x1974c90 + 0x0002c> in <6f55c2fa30114054ab77a67cb3b359ec>:0 
  at (wrapper delegate-invoke) System.EventHandler`1[System.String].invoke_void_object_TEventArgs(object,string)
  at (wrapper delegate-invoke) System.EventHandler`1[System.String].invoke_void_object_TEventArgs(object,string)
  at Microsoft.AspNetCore.Blazor.Browser.Services.BrowserUriHelper.NotifyLocationChanged (System.String newAbsoluteUri) <0x1974b00 + 0x00026> in <aa678c00565443a1a03daf9fa7da0de1>:0 

Further technical details

Build: 0.1
OS: Windows 10 Pro
Browser: Edge

Making the nullable int non-nullable wil fix this.
So this does work as expected:

<input @bind(TestInt) />

@functions
{
    public int TestInt { get; set; }

}

[Gyciauskas]

Helpful info!

Currently i'm trying to make working selectable list or it call maybe drop-down list (I'm back-end developer). Anyway, can't make it working with drop-down list if drop-down list items is dynamic get data from server.

This is enum drop-down list working great:

            <div class="form-group">
                    <select class="form-control" @bind(_user.UserType)>
                        @foreach (var userType in Enum.GetValues(typeof(UserType)))
                        {
                            <option>@userType</option>
                        }
                    </select>
            </div>

This one is struggling:

            <div class="form-group">
                    <select class="form-control" @bind(_user.CompanyName)> // CompanyName - string
                        @foreach (var company in _companies) // _companies is List<Company> Company object contain Id and name
                        {
                            <option value=@company.name>@company.name</option>
                        }
                    </select>
                </div>
            </div>

Also possible that i'm just doing it wrong (I guess it 90% possibility)

[Suchiman]

This issue appears for any non string binding that is null, the code that causing this is
https://github.com/aspnet/Blazor/blob/0b3b84f1db4c628bd5c37a16c9c22f236e85d3ad/src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeBuilder.cs#L138
The exception occurs because calling .ToString() obviously NRE's on null, so the fix could be as easy as adding ?.
For string, null works as there's a different overload.

The interesting part is that AttributeValue declared object which makes ToString seem redundant https://github.com/aspnet/Blazor/blob/0b3b84f1db4c628bd5c37a16c9c22f236e85d3ad/src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeFrame.cs#L90
but JS assumes it to be always string https://github.com/aspnet/Blazor/blob/0b3b84f1db4c628bd5c37a16c9c22f236e85d3ad/src/Microsoft.AspNetCore.Blazor.Browser.JS/src/Rendering/RenderTreeFrame.ts#L21
This odd design appears to be for https://github.com/aspnet/Blazor/blob/0b3b84f1db4c628bd5c37a16c9c22f236e85d3ad/src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeBuilder.cs#L140-L143

[floreseken]

Tested this on 0.2 and that throws a decent understandable exception, so closing this.

[haasey]

I feel like this is a hole. If someone uses entity framework to bind to a database, it will force them to create new entities taking care of nulls. Would love to see some configuration that allows treating nulls as "" for strings and nulls as 0 for ints. I don't want to get into a philosophical discussion on why they aren't equal. In my mind I realize that but the application has to deal with it somehow, and I would prefer to handle it globally not field by field.

If I have a nullable string in a database and I want to bind using blazor I don't like having to do some type of extension method to solve this over and over.
public static string NoNull(this string value)
{
string rc = "";
if (value != null)
{
rc = value;
}
return rc;
}

[LaughingJohn]

"I don't want to get into a philosophical discussion"
I think that's unavoidable when dealing with nulls, especially true of numbers in a financial system- zero returns is very different to no returns.
That said, I agree completely that the ability to bind to nullable fields would be helpful but I'd want my nulls preserved on any round trip, not turned into empty strings/zeros.
Something along the lines of TargetNullValue or IValueConverter would be very helpful, especially as a lot of people will be coming to this from a WPF/SL background.