ConfigurationBinding source generator ignores init-only properties without warning

[RichardD2]

Description

I am trying to migration some (working) configuration code to use the new configuration source binding generator. However, the source generator never initializes the properties of the configuration object, and no warning or error is generated to highlight the issue.

Reproduction Steps

Configuration section:

public class SqlServerSettings
{
    public int? CompatibilityLevel { get; init; }
}

Usage:

private static int? GetCompatibilityLevel(IConfiguration configuration)
{
    return configuration.GetSection("SqlServer").Get<SqlServerSettings>()?.CompatibilityLevel;
}

appSettings.json:

{
    "SqlServer": {
        "CompatibilityLevel": 130
    }
}

Expected behavior

Ideally, GetCompatibilityLevel should return 130 (probably impossible).

Alternatively, a compiler warning / error should be issued to highlight the fact that the source generator cannot set init-only properties.

Actual behavior

GetCompatibilityLevel returns 0.

The generated BindCore method does not attempt to set any properties:

public static void BindCore(IConfiguration configuration, ref SqlServerSettings instance, bool defaultValueIfNotFound, BinderOptions? binderOptions)
{
    ValidateConfigurationKeys(typeof(SqlServerSettings), s_configKeys_SqlServerSettings, configuration, binderOptions);
}

No errors or warnings are issued.

Regression?

No response

Known Workarounds

Changing the init-only property to a writeable property restores the correct behaviour.

public class SqlServerSettings
{
    public int? CompatibilityLevel { get; set; }
}

public static void BindCore(IConfiguration configuration, ref SqlServerSettings instance, bool defaultValueIfNotFound, BinderOptions? binderOptions)
{
    ValidateConfigurationKeys(typeof(SqlServerSettings), s_configKeys_SqlServerSettings, configuration, binderOptions);

    if (configuration["CompatibilityLevel"] is string value2)
    {
        instance.CompatibilityLevel = ParseInt(value2, () => configuration.GetSection("CompatibilityLevel").Path);
    }
}

Configuration

.NET SDK:
Version: 8.0.400
Commit: 36fe6dda56
Workload version: 8.0.400-manifests.6c274a57
MSBuild version: 17.11.3+0c8610977

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22631
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\8.0.400\

Visual Studio 2022 v17.11.3

Other information

Possibly related to #92638?

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[ericstj]

Similar issue to #95006

[KalleOlaviNiemitalo]

If the type does not have a parameterless constructor, and the source generator uses a constructor with parameters, then it correctly initialises the init-only properties. I posted an example of that in #110909 (comment). I think the source generator should be changed to use ObjectInstantiationStrategy.ParameterizedConstructor whenever there are any init-only properties, even if there is a parameterless constructor.

runtime/src/libraries/Microsoft.Extensions.Configuration.Binder/gen/ConfigurationBindingGenerator.Parser.cs

Line 699 in 9d5a6a9

initializationStrategy = ctor.Parameters.Length is 0 ? ObjectInstantiationStrategy.ParameterlessConstructor : ObjectInstantiationStrategy.ParameterizedConstructor;