Add deprecation warning NETSDK1234 for RoslynCompilerType=Framework

src/Tasks/Common/Resources/Strings.resx

@@ -1010,5 +1010,9 @@ You may need to build the project on another operating system or architecture, o
     <value>NETSDK1233: Targeting .NET 10.0 or higher in Visual Studio 2022 17.14 is not supported.</value>
     <comment>{StrBegins="NETSDK1233: "}</comment>
   </data>
-  <!-- The latest message added is Net10NotCompatibleWithVS17. Please update this value with each PR to catch parallel PRs both adding a new message -->
+  <data name="RoslynCompilerTypeFrameworkIsDeprecated" xml:space="preserve">
+    <value>NETSDK1234: RoslynCompilerType 'Framework' is deprecated and will be removed in a future version. Please refer to {0} for more information.</value>
+    <comment>{StrBegins="NETSDK1234: "}</comment>
+  </data>
+  <!-- The latest message added is RoslynCompilerTypeFrameworkIsDeprecated. Please update this value with each PR to catch parallel PRs both adding a new message -->
 </root>

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RoslynCompilerTypeDeprecation.targets

@@ -0,0 +1,31 @@
+<!--
+***********************************************************************************************
+Microsoft.NET.RoslynCompilerTypeDeprecation.targets
+
+WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and have
+          created a backup copy.  Incorrect changes to this file will make it
+          impossible to load or build your projects from the command-line or the IDE.
+
+Copyright (c) .NET Foundation. All rights reserved.
+***********************************************************************************************
+-->
+<Project>
+
+  <!--
+    This target detects when RoslynCompilerType is set to 'Framework' and warns that it is deprecated.
+    
+    RoslynCompilerType='Framework' is deprecated and will be removed in .NET 11.
+    Users should migrate to using the default compiler type or explicitly set RoslynCompilerType to 'Core' or 'FrameworkPackage'.
+    
+    This check runs before PrepareForBuild to catch the issue early in the build process.
+  -->
+  <Target Name="_CheckForDeprecatedRoslynCompilerType"
+          BeforeTargets="PrepareForBuild"
+          Condition="'$(RoslynCompilerType)' == 'Framework'">
+
+    <NETSdkWarning ResourceName="RoslynCompilerTypeFrameworkIsDeprecated"
+                   FormatArguments="https://aka.ms/dotnet-compiler-platform" />
+
+  </Target>
+
+</Project>

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.BeforeCommon.targets

@@ -87,6 +87,9 @@ Copyright (c) .NET Foundation. All rights reserved.
   <!-- Checks for deprecated Aspire workload usage -->
   <Import Project="$(MSBuildThisFileDirectory)Microsoft.NET.AspireWorkloadDeprecation.targets" />
 
+  <!-- Checks for deprecated RoslynCompilerType usage -->
+  <Import Project="$(MSBuildThisFileDirectory)Microsoft.NET.RoslynCompilerTypeDeprecation.targets" />
+
   <!-- Check if the Target Framework is coreclr based -->
   <PropertyGroup Condition="'$(_IsNETCoreOrNETStandard)' == ''">
     <_IsNETCoreOrNETStandard Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">true</_IsNETCoreOrNETStandard>