"Null values preserved in configuration": source generator and in-memory provider statements don't match .NET 9.0.22 / 10.0.14 behavior

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阅读链接的兼容性页面，尤其是“Non-nullable value types”部分，并将该陈述与 issue 中的复现示例和生成代码进行比较。修正该陈述，说明 reflection binder 和 source generator 在何种情况下会为 null 或空的配置值保留初始值。页面准确说明所报告的 .NET 9 和 .NET 10 行为后，即为完成。

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描述

Page: https://learn.microsoft.com/dotnet/core/compatibility/extensions/10.0/configuration-null-values-preserved (read 2026-11-08)

Statement on the page (section "Non-nullable value types"):

Providers that store a real null value, such as the in-memory provider, already bound null to non-nullable value-type properties by setting the default value, even before .NET 10, and the configuration source generator also matches this behavior.

What we observe: neither part holds when the property has an initial value.

Repro (Microsoft.NET.Sdk.Web, TargetFrameworks net9.0;net10.0, built once with EnableConfigurationBindingGenerator false and once with true):

using Microsoft.Extensions.Configuration;

var config = new ConfigurationBuilder()
    .AddInMemoryCollection(new Dictionary<string, string?>
    {
        ["Settings:Name"] = "x",
        ["Settings:TimeoutSeconds"] = null,
    })
    .Build();

Settings settings = config.GetSection("Settings").Get<Settings>()!;
Console.WriteLine($"{Environment.Version} TimeoutSeconds = {settings.TimeoutSeconds}");

public sealed class Settings
{
    public string? Name { get; set; }
    public int TimeoutSeconds { get; set; } = 30;
}

Output (SDK 10.0.403, Windows):

Binder .NET 9.0.22 .NET 10.0.14
Reflection 30 0
Source generator 30 30
  • With the in-memory provider, the reflection binder on .NET 9 kept the initial value (30); it did not set default(int).
  • The source generator keeps the initial value on both versions. The generated code assigns a non-nullable value-type property only when the value is neither null nor empty, for example (.NET 10.0.14 generator output):
if (TryGetConfigurationValue(configuration, key: "TimeoutSeconds", out string? value6))
{
    if (!string.IsNullOrEmpty(value6))
    {
        instance.TimeoutSeconds = ParseInt(value6, configuration.GetSection("TimeoutSeconds").Path);
    }
}

The same holds for a JSON null (AddJsonFile): reflection binder 0 on .NET 10, source generator 30 on .NET 9 and .NET 10. So on .NET 10 the same configuration produces different values depending on whether the generator is enabled, which matters for apps where it is on by default (AOT web apps, PublishTrimmed).

Suggested fix: remove or correct the sentence, and mention that the source generator does not bind null (or "") to non-nullable value-type properties, so the initial value is kept.

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