SDK resolver: Emit clear message when preview SDK blocked by VS settings

src/Resolvers/Microsoft.DotNet.MSBuildSdkResolver/MSBuildSdkResolver.cs

@@ -31,6 +31,7 @@ public sealed class DotNetMSBuildSdkResolver : SdkResolver
         private readonly Func<string>? _getCurrentProcessPath;
         private readonly Func<string, string, string?> _getMsbuildRuntime;
         private readonly NETCoreSdkResolver _netCoreSdkResolver;
+        private readonly VSSettings _vsSettings;
 
         private const string DOTNET_HOST_PATH = nameof(DOTNET_HOST_PATH);
         private const string DotnetHostExperimentalKey = "DOTNET_EXPERIMENTAL_HOST_PATH";
@@ -86,6 +87,7 @@ public DotNetMSBuildSdkResolver(Func<string, string?> getEnvironmentVariable, Fu
             _getCurrentProcessPath = getCurrentProcessPath;
             _netCoreSdkResolver = new NETCoreSdkResolver(getEnvironmentVariable, vsSettings);
             _getMsbuildRuntime = getMsbuildRuntime;
+            _vsSettings = vsSettings;
 
             if (_getEnvironmentVariable(EnvironmentVariableNames.DOTNET_MSBUILD_SDK_RESOLVER_ENABLE_LOG) is string val &&
                 (string.Equals(val, "true", StringComparison.OrdinalIgnoreCase) ||
@@ -167,6 +169,30 @@ private sealed class CachedState
                 if (resolverResult.ResolvedSdkDirectory == null)
                 {
                     logger?.LogMessage($"Failed to resolve .NET SDK.  Global.json path: {resolverResult.GlobalJsonPath}");
+
+#if NETFRAMEWORK
+                    // Check if we're in Visual Studio and preview SDKs are disallowed
+                    // If so, retry allowing previews to see if a preview SDK is available
+                    if (context.IsRunningInVisualStudio && _vsSettings.DisallowPrerelease())
+                    {
+                        logger?.LogString("Retrying SDK resolution allowing preview SDKs");
+                        var retryResult = _netCoreSdkResolver.ResolveNETCoreSdkDirectory(globalJsonStartDir, context.MSBuildVersion, context.IsRunningInVisualStudio, dotnetRoot, disallowPrerelease: false);
+
+                        string? previewSdkDirectory = retryResult.ResolvedSdkDirectory;
+                        if (previewSdkDirectory != null)
+                        {
+                            string previewVersion = new DirectoryInfo(previewSdkDirectory).Name;
+                            logger?.LogMessage($"Found preview SDK: {previewVersion}");
+                            return Failure(
+                                factory,
+                                logger,
+                                context.Logger,
+                                Strings.UnableToLocateNETCoreSdkButPreviewAvailable,
+                                previewVersion);
+                        }
+                    }
+#endif
+
                     return Failure(
                         factory,
                         logger,

src/Resolvers/Microsoft.DotNet.MSBuildSdkResolver/Strings.resx

@@ -138,4 +138,7 @@
   <data name="GlobalJsonResolutionFailedSpecificVersion" xml:space="preserve">
     <value>Unable to locate the .NET SDK version '{0}' as specified by global.json, please check that the specified version is installed.</value>
   </data>
+  <data name="UnableToLocateNETCoreSdkButPreviewAvailable" xml:space="preserve">
+    <value>Unable to locate the .NET SDK. A preview SDK (version {0}) is available but is being blocked by your Visual Studio settings. To use preview SDKs, go to Tools -> Options -> Environment -> Preview Features and check "Use previews of the .NET SDK", or add "allowPrerelease": true to global.json.</value>
+  </data>
 </root>

src/Resolvers/Microsoft.DotNet.SdkResolver/NETCoreSdkResolver.cs

@@ -149,7 +149,21 @@ public Version GetMinimumMSBuildVersion(string netcoreSdkDir)
 
         public SdkResolutionResult ResolveNETCoreSdkDirectory(string? globalJsonStartDir, Version msbuildVersion, bool isRunningInVisualStudio, string? dotnetExeDir)
         {
-            var result = NETCoreSdkResolverNativeWrapper.ResolveSdk(dotnetExeDir, globalJsonStartDir, _vsSettings.DisallowPrerelease());
+            return ResolveNETCoreSdkDirectory(globalJsonStartDir, msbuildVersion, isRunningInVisualStudio, dotnetExeDir, _vsSettings.DisallowPrerelease());
+        }
+
+        /// <summary>
+        /// Resolves the .NET Core SDK directory with explicit control over prerelease handling.
+        /// </summary>
+        /// <param name="globalJsonStartDir">Starting directory for global.json search</param>
+        /// <param name="msbuildVersion">Version of MSBuild</param>
+        /// <param name="isRunningInVisualStudio">Whether running in Visual Studio</param>
+        /// <param name="dotnetExeDir">Directory containing dotnet executable</param>
+        /// <param name="disallowPrerelease">Whether to disallow prerelease SDKs</param>
+        /// <returns>SDK resolution result</returns>
+        public SdkResolutionResult ResolveNETCoreSdkDirectory(string? globalJsonStartDir, Version msbuildVersion, bool isRunningInVisualStudio, string? dotnetExeDir, bool disallowPrerelease)
+        {
+            var result = NETCoreSdkResolverNativeWrapper.ResolveSdk(dotnetExeDir, globalJsonStartDir, disallowPrerelease);
 
             string? mostCompatible = result.ResolvedSdkDirectory;
             if (result.ResolvedSdkDirectory == null