Add switch for trim out ConfigurationManager support

src/libraries/System.Configuration.ConfigurationManager/src/ILLink/ILLink.Substitutions.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<linker>
+  <assembly fullname="System.Configuration.ConfigurationManager">
+    <type fullname="System.Configuration.ConfigurationManager">
+      <method signature="System.Boolean get_DisableConfigurationManager()" body="stub" value="true" feature="Switch.System.Configuration.ConfigurationManager.DisableConfigurationManager" featurevalue="true" />
+    </type>
+  </assembly>
+</linker>

src/libraries/System.Configuration.ConfigurationManager/src/Resources/Strings.resx

@@ -694,4 +694,7 @@
   <data name="TL_InitializeData_NotSpecified" xml:space="preserve">
     <value>initializeData needs to be valid for this TraceListener.</value>
   </data>
+  <data name="ConfigurationManagerDisabled" xml:space="preserve">
+    <value>Working with ConfigurationManager is disabled.</value>
+  </data>
 </root>

src/libraries/System.Configuration.ConfigurationManager/src/System.Configuration.ConfigurationManager.csproj

@@ -15,6 +15,10 @@
     <IsPartialFacadeAssembly Condition="$([MSBuild]::GetTargetFrameworkIdentifier('$(TargetFramework)')) == '.NETFramework'">true</IsPartialFacadeAssembly>
   </PropertyGroup>
 
+  <ItemGroup>
+    <ILLinkSubstitutionsXmls Include="$(ILLinkDirectory)ILLink.Substitutions.xml" />
+  </ItemGroup>
+
   <ItemGroup Condition="'$(IsPartialFacadeAssembly)' != 'true'">
     <Compile Include="System\Configuration\DictionarySectionHandler.cs" />
     <Compile Include="System\Configuration\DpapiProtectedConfigurationProvider.cs" />

src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/ConfigurationManager.cs

@@ -145,6 +145,9 @@ private static void PrepareConfigSystem()
 
         public static object GetSection(string sectionName)
         {
+            if (DisableConfigurationManager)
+                return null;
+
             // Avoid unintended AV's by ensuring sectionName is not empty.
             // For compatibility, we cannot throw an InvalidArgumentException.
             if (string.IsNullOrEmpty(sectionName)) return null;
@@ -157,6 +160,9 @@ public static object GetSection(string sectionName)
 
         public static void RefreshSection(string sectionName)
         {
+            if (DisableConfigurationManager)
+                return;
+
             // Avoid unintended AV's by ensuring sectionName is not empty.
             // For consistency with GetSection, we should not throw an InvalidArgumentException.
             if (string.IsNullOrEmpty(sectionName)) return;
@@ -201,6 +207,9 @@ public static Configuration OpenMappedExeConfiguration(ExeConfigurationFileMap f
         private static Configuration OpenExeConfigurationImpl(ConfigurationFileMap fileMap, bool isMachine,
             ConfigurationUserLevel userLevel, string exePath, bool preLoad = false)
         {
+            if (DisableConfigurationManager)
+                throw new ArgumentException(SR.ConfigurationManagerDisabled);
+
             // exePath must be specified if not running inside ClientConfigurationSystem
             if (!isMachine &&
                 (((fileMap == null) && (exePath == null)) ||
@@ -243,6 +252,15 @@ private static void PreloadConfigurationSectionGroup(ConfigurationSectionGroup s
                 PreloadConfigurationSectionGroup(childSectionGroup);
         }
 
+        private static bool s_disableConfigurationManager;
+        private static bool DisableConfigurationManager
+        {
+            get
+            {
+                return AppContext.TryGetSwitch(@"System.Configuration.ConfigurationManager.DisableConfigurationManager", out s_disableConfigurationManager) ? s_disableConfigurationManager : false;
+            }
+        }
+
         private enum InitState
         {
             // Initialization has not yet started.