Configuration binding source generator fails with CS8598 error

[martincostello]

Description

Attempting to enable the Configuration binding source generator with a struct used as a custom configuration type causes the source generator to emit code that fails to compile with a CS8598 error.

Repro\Microsoft.Extensions.Configuration.Binder.SourceGeneration\Microsoft.Extensions.Configuration.Binder.SourceGeneration.ConfigurationBindingGenerator\GeneratedConfigurationBinder.g.cs(122,17): error CS8598: The suppression operator is not allowed in this context [Repro\Repro.csproj]

The method generated is below. The failing line is element! = temp4;.

public static void BindCore(IConfiguration configuration, ref IDictionary<string, Geolocation> obj, BinderOptions? binderOptions)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj));
    }

    foreach (IConfigurationSection section in configuration.GetChildren())
    {
        if (!(obj.TryGetValue(section.Key!, out Geolocation element)))
        {
            element = InitializeGeolocation(section, binderOptions);;
        }
        var temp4 = InitializeGeolocation(configuration, binderOptions);;
        BindCore(section, ref temp4, binderOptions);
        element! = temp4; // CS8598 occurs here
        obj[section.Key!] = element;
    }
}

Reproduction Steps

1. Clone martincostello/config-binding-source-generator-cs8598-repro
2. Run dotnet build

Expected behavior

The application compiles and configuration binding works equivalently to runtime binding.

Actual behavior

The application fails to compile with a CS8598 error.

Regression?

No.

Known Workarounds

None.

Configuration

.NET SDK 8.0.100-preview.6.23330.14

Other information

No response

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Issue Details

---

Description

Attempting to enable the Configuration binding source generator with a struct used as a custom configuration type causes the source generator to emit code that fails to compile with a CS8598 error.

Repro\Microsoft.Extensions.Configuration.Binder.SourceGeneration\Microsoft.Extensions.Configuration.Binder.SourceGeneration.ConfigurationBindingGenerator\GeneratedConfigurationBinder.g.cs(122,17): error CS8598: The suppression operator is not allowed in this context [Repro\Repro.csproj]

The method generated is below. The failing line is element! = temp4;.

public static void BindCore(IConfiguration configuration, ref IDictionary<string, Geolocation> obj, BinderOptions? binderOptions)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj));
    }

    foreach (IConfigurationSection section in configuration.GetChildren())
    {
        if (!(obj.TryGetValue(section.Key!, out Geolocation element)))
        {
            element = InitializeGeolocation(section, binderOptions);;
        }
        var temp4 = InitializeGeolocation(configuration, binderOptions);;
        BindCore(section, ref temp4, binderOptions);
        element! = temp4; // CS8598 occurs here
        obj[section.Key!] = element;
    }
}

Reproduction Steps

1. Clone martincostello/config-binding-source-generator-cs8598-repro
2. Run dotnet build

Expected behavior

The application compiles and configuration binding works equivalently to runtime binding.

Actual behavior

The application fails to compile with a CS8598 error.

Regression?

No.

Known Workarounds

None.

Configuration

.NET SDK 8.0.100-preview.6.23330.14

Other information

No response

Author:	martincostello
Assignees:	-
Labels:	untriaged, area-Extensions-Configuration
Milestone:	-

[layomia]

Thanks for providing a repro. Taking a look at this & #89019.

[layomia]

Side note: I notice that you're using a TypeConverter for the Geolocation type in question. The generator doesn't probe for or generate code to support TypeConverter usage. This is to facilitate AOT / trim compat, given that's the primary goal for the feature. We have an issue tracking a new extensibility feature that we can consider post .NET 8 - #83599.

After this (#89010) bug is fixed, could you try the generator again to see if the binding behavior matches your expectations?

[martincostello]

Sure. The repro is adapted from a private repo that has a suite of tests. Once it uses a version of the SDK that should compile, it should be trivial to see if the binding behaviour works correctly.

[layomia]

Looking at the core parsing logic, it looks like the generator would not handle your inputs as expected - https://github.com/martincostello/config-binding-source-generator-cs8598-repro/blob/57998e5dcd44f6ec59429240bcc63839f6d1a70e/Repro/Geolocation.cs#L69.

So IIUC the input IConfiguration shape would be something like { Geolocation: "3, 4" } rather than { Geolocation: { Latitude: 3, Longitude: 4 } }. I believe the generator would only support the latter.

--

I say this just to point out the extensibility feature if you want to chime in there.