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configProperties in .runtimeconfig.dev.json override .runtimeconfig.json

If an app defines the same runtime configProperties key in both .runtimeconfig.json and .runtimeconfig.dev.json, the .runtimeconfig.dev.json value takes precedence.

Version introduced

.NET 11 Preview 6

Previous behavior

Previously, .runtimeconfig.json took precedence over .runtimeconfig.dev.json for duplicate configProperties keys.

For example:

// app.runtimeconfig.json
{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.Concurrent": true
    }
  }
}

// app.runtimeconfig.dev.json
{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.Concurrent": false
    }
  }
}

With this configuration, the runtime used System.GC.Concurrent: true.

New behavior

To support development-time overrides in .NET 11, .runtimeconfig.dev.json takes precedence over .runtimeconfig.json for duplicate configProperties keys.

Using the same example, the runtime now uses System.GC.Concurrent: false.

Type of breaking change

This change is a behavioral change.

Reason for change

This change enables developers to override production settings during development without modifying production configuration files.

For more information, see dotnet/runtime#126606.

For predictable production behavior, only place development-time overrides in .runtimeconfig.dev.json.

To prevent a development-time override, remove the duplicate key from .runtimeconfig.dev.json.

Affected APIs

None.

See also

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