Convert Select x = into let x = within Linq - fixes #717

Author: GrahamTheCoder

CHANGELOG.md

@@ -10,7 +10,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ### VB -> C#
 
-* Prevent overrides and overloads appearing on the same property (https://github.com/icsharpcode/CodeConverter/issues/681)
+* Prevent overrides and overloads appearing on the same property [#681](https://github.com/icsharpcode/CodeConverter/issues/681)
+* Convert `Select x = ` into `let x = ` within Linq [#717](https://github.com/icsharpcode/CodeConverter/issues/717)
 
 ### C# -> VB
 

CodeConverter/CSharp/ExpressionNodeVisitor.cs

@@ -652,6 +652,9 @@ await node.Name.AcceptAsync<ExpressionSyntax>(TriviaConvertingExpressionVisitor)
             );
         }
 
+        public override async Task<CSharpSyntaxNode> VisitVariableNameEquals(VBSyntax.VariableNameEqualsSyntax node) =>
+            SyntaxFactory.NameEquals(SyntaxFactory.IdentifierName(ConvertIdentifier(node.Identifier.Identifier)));
+
         public override async Task<CSharpSyntaxNode> VisitObjectCollectionInitializer(VBasic.Syntax.ObjectCollectionInitializerSyntax node)
         {
             return await node.Initializer.AcceptAsync<CSharpSyntaxNode>(TriviaConvertingExpressionVisitor); //Dictionary initializer comes through here despite the FROM keyword not being in the source code

CodeConverter/CSharp/QueryConverter.cs

@@ -93,7 +93,7 @@ public async Task<CSharpSyntaxNode> ConvertClausesAsync(SyntaxList<VBSyntax.Quer
                     new Queue<(SyntaxList<CSSyntax.QueryClauseSyntax>, VBSyntax.QueryClauseSyntax)>();
                 while (vbBodyClauses.Any() && !RequiresMethodInvocation(vbBodyClauses.Peek())) {
                     var convertedClauses = new List<CSSyntax.QueryClauseSyntax>();
-                    while (vbBodyClauses.Any() && !RequiredContinuation(vbBodyClauses.Peek())) {
+                    while (vbBodyClauses.Any() && !RequiredContinuation(vbBodyClauses.Peek(), vbBodyClauses.Count - 1)) {
                         convertedClauses.Add(await ConvertQueryBodyClauseAsync(vbBodyClauses.Dequeue()));
                     }
 
@@ -233,11 +233,11 @@ private static bool RequiresMethodInvocation(VBSyntax.QueryClauseSyntax queryCla
                    || queryClauseSyntax is VBSyntax.DistinctClauseSyntax;
         }
 
-        private static bool RequiredContinuation(VBSyntax.QueryClauseSyntax queryClauseSyntax)
-        {
-            return queryClauseSyntax is VBSyntax.GroupByClauseSyntax
-                   || queryClauseSyntax is VBSyntax.SelectClauseSyntax;
-        }
+        /// <summary>
+        /// In VB, multiple selects work like Let clauses, but the last one needs to become the actual select (its name is discarded)
+        /// </summary>
+        private static bool RequiredContinuation(VBSyntax.QueryClauseSyntax queryClauseSyntax, int clausesAfter) => queryClauseSyntax is VBSyntax.GroupByClauseSyntax
+                || queryClauseSyntax is VBSyntax.SelectClauseSyntax sc && (sc.Variables.Any(v => v.NameEquals is null) || clausesAfter == 0);
 
         private async Task<CSSyntax.FromClauseSyntax> ConvertFromClauseSyntaxAsync(VBSyntax.FromClauseSyntax vbFromClause)
         {
@@ -284,20 +284,19 @@ private static CSSyntax.SelectClauseSyntax CreateDefaultSelectClause(SyntaxToken
             return SyntaxFactory.SelectClause(SyntaxFactory.IdentifierName(reusableCsFromId));
         }
 
-        private async Task<CSSyntax.QueryClauseSyntax> ConvertQueryBodyClauseAsync(VBSyntax.QueryClauseSyntax node)
+        private Task<CSSyntax.QueryClauseSyntax> ConvertQueryBodyClauseAsync(VBSyntax.QueryClauseSyntax node)
         {
-            return await node
-                .TypeSwitch<VBSyntax.QueryClauseSyntax, VBSyntax.FromClauseSyntax, VBSyntax.JoinClauseSyntax,
-                    VBSyntax.LetClauseSyntax, VBSyntax.OrderByClauseSyntax, VBSyntax.WhereClauseSyntax,
-                    Task<CSSyntax.QueryClauseSyntax>>(
-                    //(VBSyntax.AggregateClauseSyntax ags) => null,
-                    async syntax => (CSSyntax.QueryClauseSyntax) await ConvertFromClauseSyntaxAsync(syntax),
-                    ConvertJoinClauseAsync,
-                    ConvertLetClauseAsync,
-                    ConvertOrderByClauseAsync,
-                    ConvertWhereClauseAsync,
-                    _ => throw new NotImplementedException(
-                        $"Conversion for query clause with kind '{node.Kind()}' not implemented"));
+            return node switch {
+                VBSyntax.FromClauseSyntax x => ConvertFromQueryClauseSyntaxAsync(x),
+                VBSyntax.JoinClauseSyntax x => ConvertJoinClauseAsync(x),
+                VBSyntax.SelectClauseSyntax x => ConvertSelectClauseAsync(x),
+                VBSyntax.LetClauseSyntax x => ConvertLetClauseAsync(x),
+                VBSyntax.OrderByClauseSyntax x => ConvertOrderByClauseAsync(x),
+                VBSyntax.WhereClauseSyntax x => ConvertWhereClauseAsync(x),
+                _ => throw new NotImplementedException($"Conversion for query clause with kind '{node.Kind()}' not implemented")
+            };
+
+            async Task<CSSyntax.QueryClauseSyntax> ConvertFromQueryClauseSyntaxAsync(VBSyntax.FromClauseSyntax x) => await ConvertFromClauseSyntaxAsync(x);
         }
 
         private async Task<CSSyntax.ExpressionSyntax> GetGroupExpressionAsync(VBSyntax.GroupByClauseSyntax gs)
@@ -337,6 +336,13 @@ private IEnumerable<string> GetGroupKeyIdentifiers(VBSyntax.GroupByClauseSyntax
             return SyntaxFactory.WhereClause(await ws.Condition.AcceptAsync<CSSyntax.ExpressionSyntax>(_triviaConvertingVisitor));
         }
 
+        private async Task<CSSyntax.QueryClauseSyntax> ConvertSelectClauseAsync(VBSyntax.SelectClauseSyntax sc)
+        {
+            var singleVariable = sc.Variables.Single();
+            var identifier = CommonConversions.ConvertIdentifier(singleVariable.NameEquals.Identifier.Identifier);
+            return SyntaxFactory.LetClause(identifier, await singleVariable.Expression.AcceptAsync<CSSyntax.ExpressionSyntax>(_triviaConvertingVisitor));
+        }
+
         private async Task<CSSyntax.QueryClauseSyntax> ConvertLetClauseAsync(VBSyntax.LetClauseSyntax ls)
         {
             var singleVariable = ls.Variables.Single();

Tests/CSharp/ExpressionTests/LinqExpressionTests.cs

@@ -332,6 +332,47 @@ public void Foo()
             // Current characterization is slightly wrong, I think it still needs this on the end "into g select new { Length = g.Key.Length, Count = g.Key.Count, Group = g.AsEnumerable() }"
         }
 
+        [Fact()]
+        public async Task LinqSelectVariableDeclarationAsync()
+        {
+            await TestConversionVisualBasicToCSharpAsync(@"Imports System
+Imports System.Linq
+
+Public Class Class717
+        Sub Main()
+        Dim arr(1) as Integer
+        arr(0) = 0
+        arr(1) = 1
+
+        Dim r = From e In arr
+                Select p = $""value: {e}""
+                Select l = p.Substring(1)
+                Select x = l
+
+        For each m In r
+            Console.WriteLine(m)
+        Next
+    End Sub
+End Class", @"using System;
+using System.Linq;
+
+public partial class Class717
+{
+    public void Main()
+    {
+        var arr = new int[2];
+        arr[0] = 0;
+        arr[1] = 1;
+        var r = from e in arr
+                let p = $""value: {e}""
+                let l = p.Substring(1)
+                select l;
+        foreach (var m in r)
+            Console.WriteLine(m);
+    }
+}");
+        }
+
         [Fact]
         public async Task LinqGroupByAnonymousAsync()
         {