Do not remove .ToString() from base

src/EditorFeatures/CSharpTest/SimplifyInterpolation/SimplifyInterpolationTests.cs

@@ -750,6 +750,55 @@ public static void PrintRightAligned ( String[] strings )
             Console.WriteLine ($""{i}.{str[||].PadRight(maxLength, ' ')}"");
         }
     }
+}");
+        }
+
+        [Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")]
+        public async Task MissingOnBaseToString()
+        {
+            await TestMissingAsync(
+@"class C
+{
+    public override string ToString() => $""Test: {base[||].ToString()}"";
+}");
+        }
+
+        [Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")]
+        public async Task MissingOnBaseToStringEvenWhenNotOverridden()
+        {
+            await TestMissingAsync(
+@"class C
+{
+    string M() => $""Test: {base[||].ToString()}"";
+}");
+        }
+
+        [Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")]
+        public async Task MissingOnBaseToStringWithArgument()
+        {
+            await TestMissingAsync(
+@"class Base
+{
+    public string ToString(string format) => format;
+}
+
+class Derived : Base
+{
+    public override string ToString() => $""Test: {base[||].ToString(""a"")}"";
+}");
+        }
+
+        [Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")]
+        public async Task PadLeftSimplificationIsStillOfferedOnBaseToString()
+        {
+            await TestInRegularAndScriptAsync(
+@"class C
+{
+    public override string ToString() => $""Test: {base.ToString()[||].PadLeft(10)}"";
+}",
+@"class C
+{
+    public override string ToString() => $""Test: {base.ToString(),10}"";
 }");
         }
     }

src/EditorFeatures/VisualBasicTest/SimplifyInterpolation/SimplifyInterpolationTests.vb

@@ -510,6 +510,59 @@ Class C
     Sub M(someValue As String)
         Dim v = $""prefix {[||]PadLeft(3)} suffix""
     End Sub
+End Class")
+        End Function
+
+        <Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")>
+        Public Async Function MissingOnBaseToString() As Task
+            Await TestMissingAsync(
+"Class C
+    Public Overrides Function ToString() As String
+        Return $""Test: {MyBase[||].ToString()}""
+    End Function
+End Class")
+        End Function
+
+        <Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")>
+        Public Async Function MissingOnBaseToStringEvenWhenNotOverridden() As Task
+            Await TestMissingAsync(
+"Class C
+    Function M() As String
+        Return $""Test: {MyBase[||].ToString()}""
+    End Function
+End Class")
+        End Function
+
+        <Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")>
+        Public Async Function MissingOnBaseToStringWithArgument() As Task
+            Await TestMissingAsync(
+"Class Base
+    Public Function ToString(format As String) As String
+        Return format
+    End Function
+End Class
+
+Class Derived
+    Inherits Base
+
+    Public Overrides Function ToString() As String
+        Return $""Test: {MyBase[||].ToString(""a"")}""
+    End Function
+End Class")
+        End Function
+
+        <Fact, WorkItem(42669, "https://github.com/dotnet/roslyn/issues/42669")>
+        Public Async Function PadLeftSimplificationIsStillOfferedOnBaseToString() As Task
+            Await TestInRegularAndScriptAsync(
+"Class C
+    Public Overrides Function ToString() As String
+        Return $""Test: {MyBase.ToString()[||].PadLeft(10)}""
+    End Function
+End Class",
+"Class C
+    Public Overrides Function ToString() As String
+        Return $""Test: {MyBase.ToString(),10}""
+    End Function
 End Class")
         End Function
     End Class

src/Features/CSharp/Portable/SimplifyInterpolation/CSharpSimplifyInterpolationDiagnosticAnalyzer.cs

@@ -3,9 +3,11 @@
 // See the LICENSE file in the project root for more information.
 
 using Microsoft.CodeAnalysis.CSharp.EmbeddedLanguages.VirtualChars;
+using Microsoft.CodeAnalysis.CSharp.LanguageServices;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.EmbeddedLanguages.VirtualChars;
+using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.SimplifyInterpolation;
 
 namespace Microsoft.CodeAnalysis.CSharp.SimplifyInterpolation
@@ -16,5 +18,8 @@ internal class CSharpSimplifyInterpolationDiagnosticAnalyzer : AbstractSimplifyI
     {
         protected override IVirtualCharService GetVirtualCharService()
             => CSharpVirtualCharService.Instance;
+
+        protected override ISyntaxFacts GetSyntaxFacts()
+            => CSharpSyntaxFacts.Instance;
     }
 }

src/Features/Core/Portable/SimplifyInterpolation/AbstractSimplifyInterpolationCodeFixProvider.cs

@@ -14,6 +14,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Editing;
 using Microsoft.CodeAnalysis.EmbeddedLanguages.VirtualChars;
+using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Operations;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 
@@ -63,7 +64,8 @@ public override Task RegisterCodeFixesAsync(CodeFixContext context)
                     interpolationSyntax.Parent is TInterpolatedStringExpressionSyntax interpolatedString)
                 {
                     Helpers.UnwrapInterpolation<TInterpolationSyntax, TExpressionSyntax>(
-                        document.GetRequiredLanguageService<IVirtualCharService>(), interpolation, out var unwrapped,
+                        document.GetRequiredLanguageService<IVirtualCharService>(),
+                        document.GetRequiredLanguageService<ISyntaxFactsService>(), interpolation, out var unwrapped,
                         out var alignment, out var negate, out var formatString, out _);
 
                     if (unwrapped == null)

src/Features/Core/Portable/SimplifyInterpolation/AbstractSimplifyInterpolationDiagnosticAnalyzer.cs

@@ -7,6 +7,7 @@
 using Microsoft.CodeAnalysis.CodeStyle;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.EmbeddedLanguages.VirtualChars;
+using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Operations;
 
 namespace Microsoft.CodeAnalysis.SimplifyInterpolation
@@ -27,6 +28,8 @@ protected AbstractSimplifyInterpolationDiagnosticAnalyzer()
 
         protected abstract IVirtualCharService GetVirtualCharService();
 
+        protected abstract ISyntaxFacts GetSyntaxFacts();
+
         public override DiagnosticAnalyzerCategory GetAnalyzerCategory()
             => DiagnosticAnalyzerCategory.SemanticSpanAnalysis;
 
@@ -56,8 +59,8 @@ private void AnalyzeInterpolation(OperationAnalysisContext context)
             }
 
             Helpers.UnwrapInterpolation<TInterpolationSyntax, TExpressionSyntax>(
-                GetVirtualCharService(), interpolation, out _, out var alignment, out _, out var formatString,
-                out var unnecessaryLocations);
+                GetVirtualCharService(), GetSyntaxFacts(), interpolation, out _, out var alignment, out _,
+                out var formatString, out var unnecessaryLocations);
 
             if (alignment == null && formatString == null)
             {

src/Features/Core/Portable/SimplifyInterpolation/Helpers.cs

@@ -8,6 +8,7 @@
 using System.Collections.Immutable;
 using System.Linq;
 using Microsoft.CodeAnalysis.EmbeddedLanguages.VirtualChars;
+using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Operations;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
@@ -18,7 +19,7 @@ namespace Microsoft.CodeAnalysis.SimplifyInterpolation
     internal static class Helpers
     {
         public static void UnwrapInterpolation<TInterpolationSyntax, TExpressionSyntax>(
-            IVirtualCharService virtualCharService, IInterpolationOperation interpolation,
+            IVirtualCharService virtualCharService, ISyntaxFacts syntaxFacts, IInterpolationOperation interpolation,
             out TExpressionSyntax? unwrapped, out TExpressionSyntax? alignment, out bool negate,
             out string? formatString, out ImmutableArray<Location> unnecessaryLocations)
                 where TInterpolationSyntax : SyntaxNode
@@ -38,7 +39,7 @@ internal static class Helpers
 
             if (interpolation.FormatString == null)
             {
-                UnwrapFormatString(virtualCharService, expression, out expression, out formatString, unnecessarySpans);
+                UnwrapFormatString(virtualCharService, syntaxFacts, expression, out expression, out formatString, unnecessarySpans);
             }
 
             unwrapped = expression.Syntax as TExpressionSyntax;
@@ -67,11 +68,12 @@ private static IOperation Unwrap(IOperation expression)
         }
 
         private static void UnwrapFormatString(
-            IVirtualCharService virtualCharService, IOperation expression, out IOperation unwrapped,
+            IVirtualCharService virtualCharService, ISyntaxFacts syntaxFacts, IOperation expression, out IOperation unwrapped,
             out string? formatString, List<TextSpan> unnecessarySpans)
         {
             if (expression is IInvocationOperation { TargetMethod: { Name: nameof(ToString) } } invocation &&
-                HasNonImplicitInstance(invocation))
+                HasNonImplicitInstance(invocation) &&
+                !syntaxFacts.IsBaseExpression(invocation.Instance.Syntax))
             {
                 if (invocation.Arguments.Length == 1 &&
                     invocation.Arguments[0].Value is ILiteralOperation { ConstantValue: { HasValue: true, Value: string value } } literal)

src/Features/VisualBasic/Portable/SimplifyInterpolation/VisualBasicSimplifyInterpolationDiagnosticAnalyzer.vb

@@ -4,8 +4,10 @@
 
 Imports Microsoft.CodeAnalysis.Diagnostics
 Imports Microsoft.CodeAnalysis.EmbeddedLanguages.VirtualChars
+Imports Microsoft.CodeAnalysis.LanguageServices
 Imports Microsoft.CodeAnalysis.SimplifyInterpolation
 Imports Microsoft.CodeAnalysis.VisualBasic.EmbeddedLanguages.VirtualChars
+Imports Microsoft.CodeAnalysis.VisualBasic.LanguageServices
 Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.SimplifyInterpolation
@@ -16,5 +18,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.SimplifyInterpolation
         Protected Overrides Function GetVirtualCharService() As IVirtualCharService
             Return VisualBasicVirtualCharService.Instance
         End Function
+
+        Protected Overrides Function GetSyntaxFacts() As ISyntaxFacts
+            Return VisualBasicSyntaxFacts.Instance
+        End Function
     End Class
 End Namespace