Gracefully handle format exception from Diagnostic.GetMessage() when the...

Gracefully handle format exception from Diagnostic.GetMessage() when the analyzer reports a diagnostic with incorrect format arguments.

Fixes #4589

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -6688,6 +6688,28 @@ public void AnalyzerDiagnosticThrowsInGetMessage()
             CleanupAllGeneratedFiles(srcFile.Path);
         }
 
+        [Fact]
+        [WorkItem(4589, "https://github.com/dotnet/roslyn/issues/4589")]
+        public void AnalyzerReportsMisformattedDiagnostic()
+        {
+            var srcFile = Temp.CreateFile().WriteAllText(@"class C {}");
+            var srcDirectory = Path.GetDirectoryName(srcFile.Path);
+
+            var outWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var csc = new MockCSharpCompiler(null, _baseDirectory, new[] { "/t:library", srcFile.Path },
+               analyzer: new AnalyzerReportingMisformattedDiagnostic());
+
+            var exitCode = csc.Run(outWriter);
+            Assert.Equal(0, exitCode);
+            var output = outWriter.ToString();
+
+            // Verify that the diagnostic reported by AnalyzerReportingMisformattedDiagnostic is reported with the message format string, instead of the formatted message.
+            Assert.Contains(AnalyzerThatThrowsInGetMessage.Rule.Id, output, StringComparison.Ordinal);
+            Assert.Contains(AnalyzerThatThrowsInGetMessage.Rule.MessageFormat.ToString(CultureInfo.InvariantCulture), output, StringComparison.Ordinal);
+
+            CleanupAllGeneratedFiles(srcFile.Path);
+        }
+
         [Fact]
         public void ErrorPathsFromLineDirectives()
         {

src/Compilers/Core/Portable/Diagnostic/Diagnostic_SimpleDiagnostic.cs

@@ -99,7 +99,16 @@ public override string GetMessage(IFormatProvider formatProvider = null)
                 }
 
                 var localizedMessageFormat = _descriptor.MessageFormat.ToString(formatProvider);
-                return string.Format(formatProvider, localizedMessageFormat, _messageArgs);
+
+                try
+                {
+                    return string.Format(formatProvider, localizedMessageFormat, _messageArgs);
+                }
+                catch (Exception)
+                {
+                    // Analyzer reported diagnostic with invalid format arguments, so just return the unformatted message.
+                    return localizedMessageFormat;
+                }
             }
 
             internal override IReadOnlyList<object> Arguments

src/Compilers/VisualBasic/Test/CommandLine/CommandLineTests.vb

@@ -7231,6 +7231,28 @@ End Class
             CleanupAllGeneratedFiles(source)
         End Sub
 
+        <Fact>
+        <WorkItem(4589, "https://github.com/dotnet/roslyn/issues/4589")>
+        Public Sub AnalyzerReportsMisformattedDiagnostic()
+            Dim source As String = Temp.CreateFile().WriteAllText(<text>
+Class C
+End Class
+</text>.Value).Path
+
+            Dim vbc = New MockVisualBasicCompiler(Nothing, _baseDirectory, {"/t:library", source},
+                                                  analyzer:=New AnalyzerReportingMisformattedDiagnostic)
+            Dim outWriter = New StringWriter()
+            Dim exitCode = vbc.Run(outWriter, Nothing)
+            Assert.Equal(0, exitCode)
+            Dim output = outWriter.ToString()
+
+            ' Verify that the diagnostic reported by AnalyzerReportingMisformattedDiagnostic is reported with the message format string, instead of the formatted message.
+            Assert.Contains(AnalyzerThatThrowsInGetMessage.Rule.Id, output, StringComparison.Ordinal)
+            Assert.Contains(AnalyzerThatThrowsInGetMessage.Rule.MessageFormat.ToString(CultureInfo.InvariantCulture), output, StringComparison.Ordinal)
+
+            CleanupAllGeneratedFiles(source)
+        End Sub
+
         <Fact>
         Public Sub AdditionalFileDiagnostics()
             Dim dir = Temp.CreateDirectory()

src/Test/Utilities/Desktop/CommonDiagnosticAnalyzers.cs

@@ -358,6 +358,28 @@ protected override string GetText(IFormatProvider formatProvider)
             }
         }
 
+        [DiagnosticAnalyzer(LanguageNames.CSharp, LanguageNames.VisualBasic)]
+        public sealed class AnalyzerReportingMisformattedDiagnostic : DiagnosticAnalyzer
+        {
+            public static readonly DiagnosticDescriptor Rule = new DiagnosticDescriptor(
+                "ID1",
+                "Title1",
+                "Symbol Name: {0}, Extra argument: {1}",
+                "Category1",
+                defaultSeverity: DiagnosticSeverity.Warning,
+                isEnabledByDefault: true);
+
+            public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(Rule);
+            public override void Initialize(AnalysisContext context)
+            {
+                context.RegisterSymbolAction(symbolContext =>
+                {
+                    // Report diagnostic with incorrect number of message format arguments.
+                    symbolContext.ReportDiagnostic(Diagnostic.Create(Rule, symbolContext.Symbol.Locations[0], symbolContext.Symbol.Name));
+                }, SymbolKind.NamedType);
+            }
+        }
+
         [DiagnosticAnalyzer(LanguageNames.CSharp, LanguageNames.VisualBasic)]
         public class HiddenDiagnosticsCompilationAnalyzer : DiagnosticAnalyzer
         {