Blazor WASM NullReferenceException

[jameswoodley]

1. Description

When attempting to run a simple query on a collection in Blazor, getting a NullReferenceException. The exact same query, on the exact same dataset, works perfectly in the Unit Tests, however when running in the browser you get the error

2. Exception

Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
Unhandled exception rendering component: Arg_NullReferenceException
System.NullReferenceException: Arg_NullReferenceException
at System.Linq.Dynamic.Core.Parser.KeywordsHelper..ctor(ParsingConfig config)
at System.Linq.Dynamic.Core.Parser.ExpressionParser..ctor(ParameterExpression[] parameters, String expression, Object[] values, ParsingConfig parsingConfig)
at System.Linq.Dynamic.Core.DynamicExpressionParser.ParseLambda(Type delegateType, ParsingConfig parsingConfig, Boolean createParameterCtor, ParameterExpression[] parameters, Type resultType, String expression, Object[] values)
at System.Linq.Dynamic.Core.DynamicExpressionParser.ParseLambda(ParsingConfig parsingConfig, Boolean createParameterCtor, ParameterExpression[] parameters, Type resultType, String expression, Object[] values)
at System.Linq.Dynamic.Core.DynamicExpressionParser.ParseLambda(ParsingConfig parsingConfig, Boolean createParameterCtor, Type itType, Type resultType, String expression, Object[] values)
at System.Linq.Dynamic.Core.DynamicQueryableExtensions.Any(IQueryable source, ParsingConfig config, String predicate, Object[] args)
at System.Linq.Dynamic.Core.DynamicQueryableExtensions.Any(IQueryable source, String predicate, Object[] args)

See attachments for data sets, and try the following code:

[Fact]
public async Task Should_Work_With_Real_World_Values()
{
    // Arrange
    var formData = await File.ReadAllTextAsync("FormElement/form_data.json");
    var formElementData = await File.ReadAllTextAsync("FormElement/form_elements.json");
    var dataContext = JsonConvert.DeserializeObject<Dictionary<string, FormValue<string>>>(formData);
    var formElements = JsonConvert.DeserializeObject<FormElementReadModel[]>(formElementData);
    
    // Act
    var groupedElements = formElements
        .Where(element => element.Page == 2)
        .Where(element => dataContext.Values.AsQueryable()
            .Any(string.IsNullOrEmpty(element.InclusionRule) ? "true == true" : element.InclusionRule))
        .GroupBy(element => element.Group)
        .Select(group => new GroupedFormElements
        {
            Legend = group.Key ?? string.Empty,
            Elements = group.ToArray()
        }).ToArray();
    
    // Assert
    groupedElements.Any().ShouldBeTrue();
}

I've even tried the following code to take any of the dynamic elements out of the equation:

_groupedFormElements = _form.Elements
        .Where(element => element.Page == CurrentPage)
        .Where(element => _dataContext.Values.AsQueryable().Any("true == true"))
        .GroupBy(element => element.Group)
        .Select(group => new GroupedFormElements
        {
            Legend = group.Key ?? string.Empty,
            Elements = group.ToArray()
        }).ToArray();

but the same error persists.

As I say, only in Blazor. I have followed the documentation and do have

in the .csproj file

Any help would be gratefully appreciated

form_elements.zip

[StefH]

@jameswoodley

Can you please fork this project, and add your failing to"src-blazor/BlazorWASMExample" application I just created? And then make a PR so that I can analyse your code.

[jameswoodley]

Think have resolved with some data analysis and null being returned from another query so lets close this for now. I don't want to waste your time.

Thanks for replying however!

[jomalbert1594]

@jameswoodley i'm experiencing this problem, how did you fix this?

[plachor]

Hi @StefH I believe we have similar issue. I also observer NullReferenceException in KeywaordsHelper ctor. I think this line of code is behind:

https://github.com/zzzprojects/System.Linq.Dynamic.Core/blob/master/src/System.Linq.Dynamic.Core/CustomTypeProviders/AbstractDynamicLinqCustomTypeProvider.cs#L98

Have not spend much on analysis but this reflectionTypeLoadException.Types in XML comment state that they might return null : https://learn.microsoft.com/en-us/dotnet/api/System.Reflection.ReflectionTypeLoadException.Types?view=net-6.0#property-value

Additionally this is odd that for non-exception case you apply filter and for one that was retruned in excption you do not.

Can you check this ?

[plachor]

Also how this KeywordsHelper might be affected by this none-deterministic 3 paths:

1. happy path - out-filtered set of types from assemblies
2. partially loaded types - with possibly all types from assembly?
3. some other exception yielding no results

[plachor]

@StefH could you check those leads?

[jameswoodley]

@jameswoodley i'm experiencing this problem, how did you fix this?

Apologies for delay, what fixed it for us was that some of the elements within the collection were null. Filtering them out 1st resolved the issue.

Doesn't explain why in Blazor it fails, and not outside of Blazor, but got us through the issue we were facing at the time

[StefH]

@plachor
I've some difficulties following your leads. Can you please provide a full working Blazor WASM example which reproduces this error?

[plachor]

I have observed this exception in regular net6 app deployed in kubernetes. Have you checked this line of code ? Documentation states that result list can contain null which is the case I believe. Also can you elaborate how this 3 different cases can affect this KeywordsHelper?

[StefH]

Hi @StefH I believe we have similar issue. I also observer NullReferenceException in KeywaordsHelper ctor. I think this line of code is behind:

https://github.com/zzzprojects/System.Linq.Dynamic.Core/blob/master/src/System.Linq.Dynamic.Core/CustomTypeProviders/AbstractDynamicLinqCustomTypeProvider.cs#L98

Have not spend much on analysis but this reflectionTypeLoadException.Types in XML comment state that they might return null : https://learn.microsoft.com/en-us/dotnet/api/System.Reflection.ReflectionTypeLoadException.Types?view=net-6.0#property-value

Additionally this is odd that for non-exception case you apply filter and for one that was retruned in excption you do not.

Can you check this ?

Indeed, the Types property can have some results which are null, so I've added an extra check.
See PR
#665

I did not look into the KeywordsHelper yet, do you expect also an error there?

[plachor]

Well the outcome of this code is used there and that was the reason of null reference exception i believe.

But i'm also worring about not deterministic outcome. It seems this code take what ever is returned on reflectionTypeLoadException. Any other error yields empty result. And filtered types are returned only for happy path.

Like I listed here: #504 (comment)