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System.diagnostics entry in app.config

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For applications that have an app.config file, the <configuration><configSections> entry is no longer allowed to contain a <section name="system.diagnostics"> entry. If present, you must remove the entry.

Having a <section name="system.diagnostics"> entry throws the following run-time exception when the configuration system is first used:

ConfigurationErrorsException: Section or group name 'system.diagnostics' is already defined. Updates to this may only occur at the configuration level where it is defined.

For example, the following app.config file contains the unnecessary entry:

<configuration>
 <configSections>
   <section name="system.diagnostics"
            type="System.Diagnostics.SystemDiagnosticsSection,
            System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
 </configSections>

This break will likely only occur for apps that:

  • Were migrated from .NET Framework to .NET.
  • Had explicitly added <section name="system.diagnostics"> to the app.config file to support manual reading of the <system.diagnostics> section.
  • Were upgraded to .NET 7, which has an implicit <section name="system.diagnostics"> entry.

Previous behavior

Specifying <section name="system.diagnostics"> was allowed and necessary if there was a later <system.diagnostics> configuration section like the following:

<configuration>
    <system.diagnostics>

However, the section wasn't automatically read. That's because System.Diagnostics did not yet support the feature to add listeners and configure other diagnostic features by processing that section.

.NET Framework, however, does support processing of a <system.diagnostics> section and has a <section name="system.diagnostics"> entry in the machine.config file.

New behavior

System.Diagnostics now supports reading the <system.diagnostics> section from the config file and adds an implicit <section name="system.diagnostics"> entry. Having an explicit <section name="system.diagnostics"> entry in the app.config file causes a duplicate entry, which throws ConfigurationErrorsException.

Version introduced

.NET 7 RC 1

Type of breaking change

This change can affect binary compatibility.

Reason for change

To support a new feature where System.Diagnostics reads from the app.config file, we had to add the implicit <section name="system.diagnostics"> element.

Remove the unnecessary <section name="system.diagnostics" ... > section.

Affected APIs

N/A