C# -> VB: Declaration patterns don't convert

[RussellSmith2]

Input code

using System;

namespace ICSharpCode.CodeConverter.Util
{
    static partial class ObjectExtensions
    {

        public static TResult TypeSwitch<TBaseType, TDerivedType1, TResult>(this TBaseType obj, Func<TDerivedType1, TResult> matchFunc1, Func<TBaseType, TResult> defaultFunc = null) where TDerivedType1 : TBaseType
        {
            switch (obj) {
                case TDerivedType1 tObj:
                    return matchFunc1(tObj);
                case TBaseType tObj when (defaultFunc != null):
                    return defaultFunc(tObj);
                default:
                    return default(TResult);
            }
        }
	
    }
}

Erroneous output

Imports System
Imports System.Runtime.CompilerServices

Namespace ICSharpCode.CodeConverter.Util
    Friend Partial Module ObjectExtensions
        <Extension()>
        Function TypeSwitch(Of TBaseType, TDerivedType1 As TBaseType, TResult)(ByVal obj As TBaseType, ByVal matchFunc1 As Func(Of TDerivedType1, TResult), ByVal Optional defaultFunc As Func(Of TBaseType, TResult) = Nothing) As TResult
            Select Case obj
                Case
                    Return matchFunc1(tObj)
                Case
                    Return defaultFunc(tObj)
                Case Else
                    Return Nothing
            End Select
        End Function
    End Module
End Namespace

Details

Product in use: web converter Version in use: 6.2.0.0

Right now it looks like this should convert to if else statements, though it looks like this feature may get added to VB.net in the future.

[RussellSmith2]

The VB variable declaration gets generated twice when a Declaration Pattern is inside a Local Declaration Statement.

Input code

public async Task<IEnumerable<ExpressionSyntax>> ConvertArrayBounds(ArgumentListSyntax argumentListSyntax)
{
  return await argumentListSyntax.Arguments.SelectAsync(a => {
	VBSyntax.ExpressionSyntax upperBoundExpression = a is SimpleArgumentSyntax sas ? sas.Expression
		: a is RangeArgumentSyntax ras ? ras.UpperBound
		: throw new ArgumentOutOfRangeException(nameof(a), a, null);

	return IncreaseArrayUpperBoundExpression(upperBoundExpression);
  });
}

Erroneous output

Public Async Function ConvertArrayBounds(ByVal argumentListSyntax As ArgumentListSyntax) As Task(Of IEnumerable(Of ExpressionSyntax))
	Dim sas As SimpleArgumentSyntax = Nothing, ras As RangeArgumentSyntax = Nothing
	Return Await argumentListSyntax.Arguments.SelectAsync(Function(a)
									Dim sas As SimpleArgumentSyntax = Nothing, ras As RangeArgumentSyntax = Nothing
									Dim upperBoundExpression As VBSyntax.ExpressionSyntax = If(CSharpImpl.__Assign(sas, TryCast(a, SimpleArgumentSyntax)) IsNot Nothing, sas.Expression, If(CSharpImpl.__Assign(ras, TryCast(a, RangeArgumentSyntax)) IsNot Nothing, ras.UpperBound, CSharpImpl.__Throw(Of)(New ArgumentOutOfRangeException(NameOf(a), a, Nothing))))
									Return IncreaseArrayUpperBoundExpression(upperBoundExpression)
								  End Function)
End Function

Expected output

Public Async Function ConvertArrayBounds(ByVal argumentListSyntax As ArgumentListSyntax) As Task(Of IEnumerable(Of ExpressionSyntax))
	Return Await argumentListSyntax.Arguments.SelectAsync(Function(a)
									Dim sas As SimpleArgumentSyntax = Nothing, ras As RangeArgumentSyntax = Nothing
									Dim upperBoundExpression As VBSyntax.ExpressionSyntax = If(CSharpImpl.__Assign(sas, TryCast(a, SimpleArgumentSyntax)) IsNot Nothing, sas.Expression, If(CSharpImpl.__Assign(ras, TryCast(a, RangeArgumentSyntax)) IsNot Nothing, ras.UpperBound, CSharpImpl.__Throw(Of)(New ArgumentOutOfRangeException(NameOf(a), a, Nothing))))
									Return IncreaseArrayUpperBoundExpression(upperBoundExpression)
								  End Function)
End Function

Details

• Version in use: Current compiled repo

[GrahamTheCoder]

It looks like all future VB development will "focus on stability", so VB probably isn't getting the improved select case feature. Therefore I think we'll have to transform to an if-else chain as you mentioned.

So probably best to fix the double-declaration bug, then write the code to generally transform select to a switch, and activate it when a declaration pattern appears in the case expression.