Range syntax in combination with ?[] does not work as expected

[mostevercxz]

Input code

using System;
using System.IO;

namespace switchTest
{
    static class TestSwitch
    {
        public static string Create(string url)
        {
            string ext = url.Contains('.') ? Path.GetExtension(url)?[1..] : url;
            // The following code works correctly
            //string ext = url;
            string ret;
            switch (ext)
            {
                case "dog": ret = "1"; break;
                case "fish":
                case "cat": ret = "2"; break;
                default: throw new Exception($"unsupported:{url}");
            }
            return ret;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var ani = TestSwitch.Create(args[0]);
            Console.WriteLine(ani);
        }
    }
}

Erroneous output

Error decompiling @06000001 switchTest.TestSwitch.Create
in assembly "D:\bin\Release\netcoreapp3.1\switchTest.dll"
 ---> System.InvalidCastException: Cast from Int64 to Object not supported.
   at ICSharpCode.Decompiler.Util.CSharpPrimitiveCast.CSharpPrimitiveCastUnchecked(TypeCode targetType, Object input) in CSharpPrimitiveCast.cs:line 769
   at ICSharpCode.Decompiler.Util.CSharpPrimitiveCast.Cast(TypeCode targetType, Object input, Boolean checkForOverflow) in CSharpPrimitiveCast.cs:line 44
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.<CreateTypedCaseLabel>d__17.MoveNext() in StatementBuilder.cs:line 176
   at System.Linq.Enumerable.<SelectManyIterator>d__17`2.MoveNext() in offset 138
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source) in offset 114
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source) in offset 20
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.TranslateSwitch(BlockContainer switchContainer, SwitchInstruction inst) in StatementBuilder.cs:line 227
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.VisitBlockContainer(BlockContainer container) in StatementBuilder.cs:line 1139
   at ICSharpCode.Decompiler.IL.BlockContainer.AcceptVisitor[T](ILVisitor`1 visitor) in Instructions.cs:line 851
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.Convert(ILInstruction inst) in StatementBuilder.cs:line 78
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertBlockContainer(BlockStatement blockStatement, BlockContainer container, IEnumerable`1 blocks, Boolean isLoop) in StatementBuilder.cs:line 1317
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertBlockContainer(BlockContainer container, Boolean isLoop) in StatementBuilder.cs:line 1257
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.VisitBlockContainer(BlockContainer container) in StatementBuilder.cs:line 1143
   at ICSharpCode.Decompiler.IL.BlockContainer.AcceptVisitor[T](ILVisitor`1 visitor) in Instructions.cs:line 851
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.Convert(ILInstruction inst) in StatementBuilder.cs:line 78
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertAsBlock(ILInstruction inst) in StatementBuilder.cs:line 83
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in CSharpDecompiler.cs:line 1467
-- continuing with outer exception (ICSharpCode.Decompiler.DecompilerException) --
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in CSharpDecompiler.cs:line 1483
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DoDecompile(IMethod method, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in CSharpDecompiler.cs:line 1374
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DoDecompile(ITypeDefinition typeDef, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in CSharpDecompiler.cs:line 1212
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.Decompile(IEnumerable`1 definitions) in CSharpDecompiler.cs:line 938
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.Decompile(EntityHandle[] definitions) in CSharpDecompiler.cs:line 907
   at ICSharpCode.ILSpy.CSharpLanguage.DecompileType(ITypeDefinition type, ITextOutput output, DecompilationOptions options) in CSharpLanguage.cs:line 322
   at ICSharpCode.ILSpy.TreeNodes.TypeTreeNode.Decompile(Language language, ITextOutput output, DecompilationOptions options) in TypeTreeNode.cs:line 123
   at ICSharpCode.ILSpy.TextView.DecompilerTextView.DecompileNodes(DecompilationContext context, ITextOutput textOutput) in DecompilerTextView.cs:line 881
   at ICSharpCode.ILSpy.TextView.DecompilerTextView.<>c__DisplayClass48_0.<DecompileAsync>b__0() in DecompilerTextView.cs:line 851

Details

• Product in use: The lastest ILSpy github source code (2020-09-11)
• Version in use: vs2019, F5 run ILSpy

Try to fix

ILSpy/ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

Line 151 in 41edef7

else if (type.IsKnownType(KnownTypeCode.String) && map != null)

ILSpy will decompile without exception if you change line 151 to the following line:

} else if ((type.IsKnownType(KnownTypeCode.String) || type.IsKnownType(KnownTypeCode.Object)) && map != null) {

Besides, if you uncomment the following line in Input Code, ILSpy will work correctly:

// The following code works correctly
//string ext = url;

[siegfriedpammer]

Current decompiler output:

// switchTest.TestSwitch
using System;
using System.IO;

public static string Create(string url)
{
	object obj;
	if (!url.Contains('.'))
	{
		obj = url;
	}
	else
	{
		string? extension = Path.GetExtension(url);
		obj = extension?[new Range(end: extension!.Length, start: 1)];
	}
	switch ((string)obj)
	{
	case "dog":
		return "1";
	case "fish":
	case "cat":
		return "2";
	default:
		throw new Exception("unsupported:" + url);
	}
}

Two/three things that should be improved:

1. obj should be typed either string or string?
2. range syntax detection should work
3. possibly use ?: instead of an if-statement

[dgrunwald]

obj should be typed either string or string?

This is a bit tricky. For single-assignment stack slots we can use the C# type of the expression being assigned, but that approach won't work for multiple assignments.
Maybe add a ILAst-level type inference step?

range syntax detection should work

This is really the root cause of the issue.
If we can detect extension?[1..] then the second use of extension disappears.
That in turn allows us to inline extension, which makes the if simple enough so that ? : can be used, which then in turn allows us to inline obj.