Document EXTOBS obsolete diagnostics for extensions

docs/fundamentals/syslib-diagnostics/extobs0001.md

@@ -0,0 +1,97 @@
+---
+title: EXTOBS0001 warning
+description: Learn about the obsoletions that generate compile-time warning EXTOBS0001.
+ms.date: 11/12/2025
+f1_keywords:
+  - extobs0001
+ai-usage: ai-assisted
+---
+# EXTOBS0001: IResourceMonitor is obsolete
+
+The <xref:Microsoft.Extensions.Diagnostics.ResourceMonitoring.IResourceMonitor> interface and related APIs have been marked as obsolete starting in .NET 9. These APIs will be removed in a future version. The resource monitoring functionality has been replaced with a more efficient metrics-based approach using observable instruments.
+
+The following APIs are marked obsolete. Use of these APIs generates warning `EXTOBS0001` at compile time.
+
+- <xref:Microsoft.Extensions.Diagnostics.ResourceMonitoring.IResourceMonitor?displayProperty=nameWithType>
+- <xref:Microsoft.Extensions.Diagnostics.ResourceMonitoring.IResourceMonitor.GetUtilization(System.TimeSpan)?displayProperty=nameWithType>
+
+## Workarounds
+
+Instead of using `IResourceMonitor`, switch to using resource monitoring metrics with observable instruments. The metrics-based approach provides the same resource utilization information (CPU, memory) but integrates better with modern observability systems like OpenTelemetry.
+
+### Migration example
+
+Old approach using `IResourceMonitor`:
+
+```csharp
+services.AddResourceMonitoring();
+
+// Inject and use IResourceMonitor
+public class MyService
+{
+    private readonly IResourceMonitor _resourceMonitor;
+
+    public MyService(IResourceMonitor resourceMonitor)
+    {
+        _resourceMonitor = resourceMonitor;
+    }
+
+    public void CheckResources()
+    {
+        var utilization = _resourceMonitor.GetUtilization(TimeSpan.FromSeconds(1));
+        Console.WriteLine($"CPU: {utilization.CpuUsedPercentage}%");
+        Console.WriteLine($"Memory: {utilization.MemoryUsedPercentage}%");
+    }
+}
+```
+
+New approach using metrics:
+
+```csharp
+services.AddResourceMonitoring();
+
+// Configure metrics collection.
+services.AddOpenTelemetry()
+    .WithMetrics(builder =>
+    {
+        builder.AddMeter("Microsoft.Extensions.Diagnostics.ResourceMonitoring");
+        builder.AddConsoleExporter(); // Or use any other exporter.
+    });
+```
+
+The resource monitoring metrics are automatically published and can be consumed by any OpenTelemetry-compatible metrics pipeline. For more information, see [Resource Monitoring observable instruments](../../core/diagnostics/built-in-metrics-diagnostics.md#microsoftextensionsdiagnosticsresourcemonitoring).
+
+## Suppress a warning
+
+If you must use the obsolete APIs, you can suppress the warning in code or in your project file.
+
+To suppress only a single violation, add preprocessor directives to your source file to disable and then re-enable the warning.
+
+```csharp
+// Disable the warning.
+#pragma warning disable EXTOBS0001
+
+// Code that uses obsolete API.
+// ...
+
+// Re-enable the warning.
+#pragma warning restore EXTOBS0001
+```
+
+To suppress all the `EXTOBS0001` warnings in your project, add a `<NoWarn>` property to your project file.
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+   ...
+   <NoWarn>$(NoWarn);EXTOBS0001</NoWarn>
+  </PropertyGroup>
+</Project>
+```
+
+For more information, see [Suppress warnings](obsoletions-overview.md#suppress-warnings).
+
+## See also
+
+- [Diagnostic resource monitoring](../../../core/diagnostics/diagnostic-resource-monitoring.md)
+- [Built-in metrics - Resource Monitoring](../../../core/diagnostics/built-in-metrics-diagnostics.md#microsoftextensionsdiagnosticsresourcemonitoring)

docs/fundamentals/syslib-diagnostics/obsoletions-overview.md

@@ -1,23 +1,26 @@
 ---
 title: Obsolete features in .NET 5+
 description: Learn about APIs that are marked as obsolete in .NET 5 and later versions that produce SYSLIB compiler warnings.
-ms.date: 01/14/2025
+ms.date: 11/12/2025
 ai-usage: ai-assisted
 ---
 
 # Obsolete features in .NET 5+
 
 Starting in .NET 5, some APIs that are newly marked as obsolete make use of two new properties on <xref:System.ObsoleteAttribute>.
 
-- The <xref:System.ObsoleteAttribute.DiagnosticId?displayProperty=nameWithType> property tells the compiler to generate build warnings using a custom diagnostic ID. The custom ID allows for obsoletion warning to be suppressed specifically and separately from one another. In the case of the .NET 5+ obsoletions, the format for the custom diagnostic ID is `SYSLIB0XXX`.
-
+- The <xref:System.ObsoleteAttribute.DiagnosticId?displayProperty=nameWithType> property tells the compiler to generate build warnings using a custom diagnostic ID. The custom ID allows for obsoletion warning to be suppressed specifically and separately from one another. In the case of the `System*` namespace obsoletions, the format for the custom diagnostic ID is `SYSLIB0XXX`. In the case of the `Microsoft.Extensions` obsoletions, the format for the custom diagnostic ID is `EXTOBS0XXX`.
 - The <xref:System.ObsoleteAttribute.UrlFormat?displayProperty=nameWithType> property tells the compiler to include a URL link to learn more about the obsoletion.
 
-If you encounter build warnings or errors due to usage of an obsolete API, follow the specific guidance provided for the diagnostic ID listed in the [Reference](#reference) section. Warnings or errors for these obsoletions *can't* be suppressed using the [standard diagnostic ID (CS0618)](../../csharp/language-reference/compiler-messages/cs0618.md) for obsolete types or members; use the custom `SYSLIB0XXX` diagnostic ID values instead. For more information, see [Suppress warnings](#suppress-warnings).
+If you encounter build warnings or errors due to usage of an obsolete API, follow the specific guidance provided for the diagnostic ID listed in the [Reference](#reference) section. Warnings or errors for these obsoletions *can't* be suppressed using the [standard diagnostic ID (CS0618)](../../csharp/language-reference/compiler-messages/cs0618.md) for obsolete types or members; use the custom `SYSLIB0XXX` or `EXTOBS0XXX` diagnostic ID values instead. For more information, see [Suppress warnings](#suppress-warnings).
 
 ## Reference
 
-The following table provides an index to the `SYSLIB0XXX` obsoletions in .NET 5+.
+The following tables provide an index to obsolete APIs with custom diagnostic IDs in .NET 5 and later versions.
+
+### SYSLIB obsoletions
+
+The following table lists the `SYSLIB0XXX` obsoletions in .NET 5+.
 
 | Diagnostic ID | Warning or error | Description |
 |---------------|------------------|-------------|
@@ -84,9 +87,17 @@ The following table provides an index to the `SYSLIB0XXX` obsoletions in .NET 5+
 | [SYSLIB0061](syslib0061.md) | Warning | The `Queryable` <xref:System.Linq.Queryable.MaxBy``2(System.Linq.IQueryable{``0},System.Linq.Expressions.Expression{System.Func{``0,``1}},System.Collections.Generic.IComparer{``0})?displayProperty=nameWithType> and <xref:System.Linq.Queryable.MinBy``2(System.Linq.IQueryable{``0},System.Linq.Expressions.Expression{System.Func{``0,``1}},System.Collections.Generic.IComparer{``0})?displayProperty=nameWithType> taking an `IComparer<TSource>` are obsolete. Use the new ones that take an `IComparer<TKey>`. |
 | [SYSLIB0062](syslib0062.md) | Warning | <xref:System.Xml.Xsl.XsltSettings.EnableScript?displayProperty=nameWithType> is obsolete. |
 
+### EXTOBS obsoletions
+
+The following table lists the `EXTOBS0XXX` obsoletions from the `Microsoft.Extensions` libraries.
+
+| Diagnostic ID | Warning or error | Description |
+|---------------|------------------|-------------|
+| [EXTOBS0001](extobs0001.md) | Warning | <xref:Microsoft.Extensions.Diagnostics.ResourceMonitoring.IResourceMonitor> is obsolete and will be removed in a future version. Consider using [Resource Monitoring observable instruments](../../core/diagnostics/built-in-metrics-diagnostics.md#microsoftextensionsdiagnosticsresourcemonitoring). |
+
 ## Suppress warnings
 
-It's recommended that you use an available workaround whenever possible. However, if you cannot change your code, you can suppress warnings through a `#pragma` directive or a `<NoWarn>` project setting. If you must use the obsolete APIs and the `SYSLIB0XXX` diagnostic does not surface as an error, you can suppress the warning in code or in your project file.
+It's recommended that you use an available workaround whenever possible. However, if you cannot change your code, you can suppress warnings through a `#pragma` directive or a `<NoWarn>` project setting. If you must use the obsolete APIs and the `SYSLIB0XXX` or `EXTOBS0XXX` diagnostic does not surface as an error, you can suppress the warning in code or in your project file.
 
 To suppress the warnings in code:
 
@@ -123,7 +134,6 @@ To suppress the warnings in a project file:
 
 ## See also
 
-- [API obsoletions with non-default diagnostic IDs (.NET 5)](../../core/compatibility/core-libraries/5.0/obsolete-apis-with-custom-diagnostics.md)
-- [API obsoletions with non-default diagnostic IDs (.NET 6)](../../core/compatibility/core-libraries/6.0/obsolete-apis-with-custom-diagnostics.md)
-- [API obsoletions with non-default diagnostic IDs (.NET 7)](../../core/compatibility/core-libraries/7.0/obsolete-apis-with-custom-diagnostics.md)
 - [API obsoletions with non-default diagnostic IDs (.NET 8)](../../core/compatibility/core-libraries/8.0/obsolete-apis-with-custom-diagnostics.md)
+- [API obsoletions with non-default diagnostic IDs (.NET 9)](../../core/compatibility/core-libraries/9.0/obsolete-apis-with-custom-diagnostics.md)
+- [API obsoletions with non-default diagnostic IDs (.NET 10)](../../core/compatibility/core-libraries/10.0/obsolete-apis-with-custom-diagnostics.md)

docs/navigate/tools-diagnostics/toc.yml

@@ -3936,7 +3936,7 @@ items:
       - name: Obsoletions
         items:
           - name: Overview
-            displayName: syslib, obsolete, obsoletion
+            displayName: syslib, extobs, obsolete, obsoletion
             href: ../../fundamentals/syslib-diagnostics/obsoletions-overview.md
           - name: SYSLIB0001
             href: ../../fundamentals/syslib-diagnostics/syslib0001.md
@@ -4170,6 +4170,10 @@ items:
           - name: SYSLIB1230
             href: ../../fundamentals/syslib-diagnostics/syslib1230.md
             displayProperty: syslib1230, syslib1231, syslib1232, syslib1233, syslib1234, syslib1235, syslib1236, syslib1237, syslib1238, syslib1239
+  - name: EXTOBS diagnostics
+    items:
+      - name: EXTOBS0001
+        href: ../../fundamentals/syslib-diagnostics/extobs0001.md
   - name: API compatibility
     items:
       - name: Overview