ConfigurationBinder source generator generates code that doesn't compile when using StringValues

[eerhardt]

Description

When an Options object uses the StringValues type, the ConfigurationBinder Source Generator is generating code that doesn't compile.

Reproduction Steps

Build the following app:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <EnableConfigurationBindingGenerator>true</EnableConfigurationBindingGenerator>
    <PublishAot>true</PublishAot>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="9.0.0-dev" />
    <PackageReference Include="Microsoft.Extensions.Configuration" Version="9.0.0-dev" />
  </ItemGroup>

</Project>

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Primitives;

IConfigurationSection c = new ConfigurationBuilder().Build().GetSection("Options");
c.Get<MyOptions>();

public class MyOptions
{
    public IDictionary<string, StringValues> DefaultValues { get; set; } = new Dictionary<string, StringValues>(StringComparer.OrdinalIgnoreCase);
}

Expected behavior

The project should build successfully.

Actual behavior

The build fails with:

Severity	Code	Description	Project	File	Line	Suppression State	Details
Error (active)	CS8518	An expression of type 'StringValues' can never match the provided pattern.	ConsoleApp101	C:\Users\eerhardt\source\repos\ConsoleApp101\ConsoleApp101\Microsoft.Extensions.Configuration.Binder.SourceGeneration\Microsoft.Extensions.Configuration.Binder.SourceGeneration.ConfigurationBindingGenerator\BindingExtensions.g.cs	68		

The generated code that fails looks like:

        public static void BindCore(IConfiguration configuration, ref global::Microsoft.Extensions.Primitives.StringValues instance, bool defaultValueIfNotFound, BinderOptions? binderOptions)
        {
            if (instance is not ICollection<string> temp) // this line has the build error
            {
                return;
            }

Regression?

No

Known Workarounds

No response

Configuration

No response

Other information

No response

Tagging subscribers to this area: @dotnet/area-extensions-configuration
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Issue Details

---

Description

When an Options object uses the StringValues type, the ConfigurationBinder Source Generator is generating code that doesn't compile.

Reproduction Steps

Build the following app:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <EnableConfigurationBindingGenerator>true</EnableConfigurationBindingGenerator>
    <PublishAot>true</PublishAot>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="9.0.0-dev" />
    <PackageReference Include="Microsoft.Extensions.Configuration" Version="9.0.0-dev" />
  </ItemGroup>

</Project>

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Primitives;

IConfigurationSection c = new ConfigurationBuilder().Build().GetSection("Options");
c.Get<MyOptions>();

public class MyOptions
{
    public IDictionary<string, StringValues> DefaultValues { get; set; } = new Dictionary<string, StringValues>(StringComparer.OrdinalIgnoreCase);
}

Expected behavior

The project should build successfully.

Actual behavior

The build fails with:

Severity	Code	Description	Project	File	Line	Suppression State	Details
Error (active)	CS8518	An expression of type 'StringValues' can never match the provided pattern.	ConsoleApp101	C:\Users\eerhardt\source\repos\ConsoleApp101\ConsoleApp101\Microsoft.Extensions.Configuration.Binder.SourceGeneration\Microsoft.Extensions.Configuration.Binder.SourceGeneration.ConfigurationBindingGenerator\BindingExtensions.g.cs	68		

The generated code that fails looks like:

        public static void BindCore(IConfiguration configuration, ref global::Microsoft.Extensions.Primitives.StringValues instance, bool defaultValueIfNotFound, BinderOptions? binderOptions)
        {
            if (instance is not ICollection<string> temp) // this line has the build error
            {
                return;
            }

Regression?

No

Known Workarounds

No response

Configuration

No response

Other information

No response

Author:	eerhardt
Assignees:	-
Labels:	area-Extensions-Configuration
Milestone:	-

[ericstj]

Definitely the generator shouldn't emit code that doesn't compile. It can do that check at compile time since the struct must be non null and implements ICollection.

I wonder if we have an example of someone successfully using a StringValues type with configuration? Actually binding data to it. Maybe it requires collection like data to bind correctly?

[eerhardt]

This scenario was found in https://github.com/dotnet/extensions/blob/33abf0c4b36ac8ce5964938b42f06ca31f48d9f6/src/Libraries/Microsoft.AspNetCore.HeaderParsing/HeaderParsingOptions.cs#L16-L24. I tried different combinations of what the config could look like, but I wasn't able to get the reflection based config binder to bind it correctly. I could get a key added to the dictionary, but the StringValues value would always be empty.

I looked back in source history, and I don't see any indication that this property was supposed to actually be bound to configuration. All the places that touch it manually add it through code. Tagging some of the people on the original PR to see if they remember if HeaderParsingOptions.DefaultValues is supposed to be able to be bound to IConfiguration - @geeknoid @tekian @rafal-mz

[geeknoid]

This was not intended to be configured from JSON. This kind of configuration option is pretty essential to the behavior of the code, so it's not particularly compelling as a config options controlled from JSON.