Add warning for --framework option on solution builds

- Added NETSDK1234 warning string in Strings.resx
- Modified FrameworkOption to set _CommandLineDefinedTargetFramework property
- Added _CheckForSolutionLevelTargetFramework target in Microsoft.NET.Sdk.Solution.targets
- Created FrameworkOptionTests.cs with test cases for the new warning

Co-authored-by: marcpopMSFT <12663534+marcpopMSFT@users.noreply.github.com>

Author: Copilot

src/Cli/dotnet/CommonOptions.cs

@@ -175,7 +175,7 @@ public static Option<string> FrameworkOption(string description) =>
             HelpName = CliStrings.FrameworkArgumentName
         }
         .AddCompletions(CliCompletion.TargetFrameworksFromProjectFile)
-        .ForwardAsSingle(o => $"--property:TargetFramework={o}");
+        .ForwardAsMany(o => [$"--property:TargetFramework={o}", "--property:_CommandLineDefinedTargetFramework=true"]);
 
     public static Option<string> ArtifactsPathOption =
         new ForwardedOption<string>(

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="CannotHaveSolutionLevelTargetFramework" xml:space="preserve">
+    <value>NETSDK1234: The "--framework" option isn't supported when building a solution. Specifying a solution-level framework results in all projects building for that framework, which can cause referenced projects to build multiple times and can lead to inconsistent builds.</value>
+    <comment>{StrBegins="NETSDK1234: "}{Locked="--framework"}</comment>
+  </data>
+  <!-- The latest message added is CannotHaveSolutionLevelTargetFramework. Please update this value with each PR to catch parallel PRs both adding a new message -->
 </root>

src/Tasks/Microsoft.NET.Build.Extensions.Tasks/msbuildExtensions-ver/SolutionFile/ImportAfter/Microsoft.NET.Sdk.Solution.targets

@@ -37,4 +37,12 @@ Copyright (c) .NET Foundation. All rights reserved.
 
   </Target>
 
+  <Target Name="_CheckForSolutionLevelTargetFramework"
+          BeforeTargets="Build;Publish;Clean;Store;VSTest;_MTPBuild"
+          Condition="'$(_CommandLineDefinedTargetFramework)' == 'true'">
+
+    <NetSdkWarning ResourceName="CannotHaveSolutionLevelTargetFramework" />
+
+  </Target>
+
 </Project>

test/dotnet.Tests/FrameworkOptionTests.cs

@@ -0,0 +1,84 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable disable
+
+using System.Runtime.CompilerServices;
+
+namespace dotnet.Tests;
+
+public class FrameworkOptionTests : SdkTest
+{
+    public FrameworkOptionTests(ITestOutputHelper log) : base(log)
+    {
+    }
+
+    [Theory]
+    [InlineData("build", true)]
+    [InlineData("clean", true)]
+    [InlineData("publish", true)]
+    [InlineData("test", true)]
+    public void FrameworkOptionGeneratesWarningsWithSolutionFiles(string command, bool shouldWarn)
+    {
+        TestFrameworkWithSolution(command, useOption: true, shouldWarn: shouldWarn);
+    }
+
+    [Theory]
+    [InlineData("build")]
+    [InlineData("clean")]
+    [InlineData("publish")]
+    [InlineData("test")]
+    public void TargetFrameworkPropertyDoesNotGenerateWarningsWithSolutionFiles(string command)
+    {
+        TestFrameworkWithSolution(command, useOption: false, shouldWarn: false);
+    }
+
+    void TestFrameworkWithSolution(string command, bool useOption, bool shouldWarn, [CallerMemberName] string callingMethod = "")
+    {
+        var testProject = new TestProject()
+        {
+            IsExe = true,
+            TargetFrameworks = $"{ToolsetInfo.CurrentTargetFramework};net8.0"
+        };
+
+        var testAsset = _testAssetsManager.CreateTestProject(testProject, callingMethod, identifier: command);
+
+        var slnDirectory = testAsset.TestRoot;
+
+        Log.WriteLine($"Test root: {slnDirectory}");
+
+        new DotnetNewCommand(Log)
+            .WithVirtualHive()
+            .WithWorkingDirectory(slnDirectory)
+            .Execute("sln")
+            .Should().Pass();
+
+        new DotnetCommand(Log)
+            .WithWorkingDirectory(slnDirectory)
+            .Execute("sln", "add", testProject.Name)
+            .Should().Pass();
+
+        Microsoft.DotNet.Cli.Utils.CommandResult commandResult;
+        if (useOption)
+        {
+            commandResult = new DotnetCommand(Log)
+                .WithWorkingDirectory(slnDirectory)
+                .Execute(command, "--framework", "net8.0");
+        }
+        else
+        {
+            commandResult = new DotnetCommand(Log)
+                .WithWorkingDirectory(slnDirectory)
+                .Execute(command, "--property:TargetFramework=net8.0");
+        }
+        commandResult.Should().Pass();
+        if (shouldWarn)
+        {
+            commandResult.Should().HaveStdOutContaining("NETSDK1234");
+        }
+        else
+        {
+            commandResult.Should().NotHaveStdOutContaining("NETSDK1234");
+        }
+    }
+}