[main] Fix sdk check crash when SDK has no releases.json

src/Cli/dotnet/Commands/Sdk/Check/ProductCollectionProvider.cs

@@ -30,9 +30,15 @@ public IEnumerable<ProductRelease> GetProductReleases(Deployment.DotNet.Releases
         {
             return product.GetReleasesAsync().Result;
         }
-        catch (Exception e)
+        catch (Exception ex) when (ex is HttpRequestException ||
+                                     ex is FileNotFoundException ||
+                                     ex is AggregateException aggregateEx && 
+                                     (aggregateEx.InnerException is HttpRequestException || 
+                                      aggregateEx.InnerException is FileNotFoundException))
         {
-            throw new GracefulException(string.Format(CliCommandStrings.ReleasesLibraryFailed, e.Message));
+            // Return empty collection if releases are not available (e.g., unreleased preview versions)
+            // This handles cases where the releases.json file doesn't exist yet for a version
+            return Enumerable.Empty<ProductRelease>();
         }
     }
 }

src/Cli/dotnet/Commands/Sdk/Check/SdkOutputWriter.cs

@@ -68,7 +68,13 @@ private bool NewerSdkPatchExists(NetSdkInfo bundle)
 
     private ReleaseVersion? NewestSdkPatchVersion(NetSdkInfo bundle)
     {
-        var product = _productCollection.First(product => product.ProductVersion.Equals($"{bundle.Version.Major}.{bundle.Version.Minor}"));
+        var product = _productCollection.FirstOrDefault(product => product.ProductVersion.Equals($"{bundle.Version.Major}.{bundle.Version.Minor}"));
+        if (product == null)
+        {
+            // No release information available for this SDK version
+            return null;
+        }
+
         if (product.LatestSdkVersion.SdkFeatureBand == bundle.Version.SdkFeatureBand)
         {
             // This is the latest feature band
@@ -88,11 +94,24 @@ private bool NewerSdkPatchExists(NetSdkInfo bundle)
 
     private bool NewFeatureBandAvailable()
     {
-        return NewestFeatureBandAvailable() > _sdkInfo.Select(sdk => sdk.Version).Max();
+        if (!_sdkInfo.Any())
+        {
+            return false;
+        }
+
+        var newestAvailable = NewestFeatureBandAvailable();
+        return newestAvailable != null && newestAvailable > _sdkInfo.Select(sdk => sdk.Version).Max();
     }
 
-    private ReleaseVersion NewestFeatureBandAvailable()
+    private ReleaseVersion? NewestFeatureBandAvailable()
     {
-        return _productCollection.OrderByDescending(product => product.ProductVersion).First().LatestSdkVersion;
+        var newestProduct = _productCollection.OrderByDescending(product => product.ProductVersion).FirstOrDefault();
+        if (newestProduct != null)
+        {
+            return newestProduct.LatestSdkVersion;
+        }
+
+        // Fallback to the newest installed SDK if no product collection is available
+        return _sdkInfo.Any() ? _sdkInfo.Select(sdk => sdk.Version).Max() : null;
     }
 }

test/dotnet.Tests/CommandTests/Sdk/Check/GivenDotnetSdkCheck.cs

@@ -207,6 +207,32 @@ public void ItUsesConfigFile()
             _reporter.Lines.Should().Contain(replacementString);
         }
 
+        [Fact]
+        public void WhenSdkHasNoReleasesJsonItShowsVersionCheckUnavailable()
+        {
+            var parseResult = Parser.Parse(new string[] { "dotnet", "sdk", "check" });
+            // Install SDK 99.0.100 which doesn't have releases.json in the test assets
+            var bundles = GetFakeEnvironmentInfo(new[] { "3.1.100", "5.0.100", "99.0.100" }, new[] { "3.1.0", "5.0.0" });
+
+            // This should not throw even though 99.0 doesn't have releases.json
+            new SdkCheckCommand(parseResult, new MockNETBundleProvider(bundles), new MockProductCollectionProvider(fakeReleasesPath), _reporter).Execute();
+
+            // Verify all SDKs are shown
+            foreach (var version in bundles.SdkInfo.Select(b => b.Version.ToString()))
+            {
+                string.Join(' ', _reporter.Lines)
+                    .Should()
+                    .Contain(version);
+            }
+
+            // The SDK without releases should show version check failure
+            string.Join(' ', _reporter.Lines)
+                .Should()
+                .Contain("99.0.100")
+                .And
+                .Contain(CliCommandStrings.VersionCheckFailure);
+        }
+
         private NetEnvironmentInfo GetFakeEnvironmentInfo(IEnumerable<string> sdkVersions, IEnumerable<string> runtimeVersions)
         {
             var sdks = sdkVersions.Select(version => new NetSdkInfo(version, string.Empty));