Tidy up string comparison - fixes #500

Author: GrahamTheCoder

CodeConverter/CSharp/VisualBasicEqualityComparison.cs

@@ -65,9 +65,17 @@ public RequiredType GetObjectEqualityType(VBSyntax.BinaryExpressionSyntax node,
 
         public (ExpressionSyntax lhs, ExpressionSyntax rhs) VbCoerceToString(VBSyntax.ExpressionSyntax vbLeft, ExpressionSyntax csLeft, TypeInfo lhsTypeInfo, VBSyntax.ExpressionSyntax vbRight, ExpressionSyntax csRight, TypeInfo rhsTypeInfo)
         {
+            if (IsNonEmptyStringLiteral(vbLeft) || IsNonEmptyStringLiteral(vbRight)) {
+                return (csLeft, csRight);
+            }
             return (VbCoerceToString(vbLeft, csLeft, lhsTypeInfo), VbCoerceToString(vbRight, csRight, rhsTypeInfo));
         }
 
+        private static bool IsNonEmptyStringLiteral(VBSyntax.ExpressionSyntax vbExpr)
+        {
+            return vbExpr.SkipParens().IsKind(VBSyntaxKind.StringLiteralExpression) && vbExpr is VBSyntax.LiteralExpressionSyntax literal && !IsEmptyString(literal);
+        }
+
         private ExpressionSyntax VbCoerceToString(VBSyntax.ExpressionSyntax vbNode, ExpressionSyntax csNode, TypeInfo typeInfo)
         {
             bool isStringType = typeInfo.Type.SpecialType == SpecialType.System_String;
@@ -163,12 +171,16 @@ private bool IsNothingOrEmpty(VBSyntax.ExpressionSyntax expressionSyntax)
         {
             if (expressionSyntax is VBSyntax.LiteralExpressionSyntax les &&
                 (les.IsKind(VBSyntaxKind.NothingLiteralExpression) ||
-                 (les.IsKind(VBSyntaxKind.StringLiteralExpression) &&
-                  String.IsNullOrEmpty(les.Token.ValueText)))) return true;
+                 IsEmptyString(les))) return true;
             var constantAnalysis = _semanticModel.GetConstantValue(expressionSyntax);
             return constantAnalysis.HasValue && (constantAnalysis.Value == null || constantAnalysis.Value as string == "");
         }
 
+        private static bool IsEmptyString(VBSyntax.LiteralExpressionSyntax les)
+        {
+            return les.IsKind(VBSyntaxKind.StringLiteralExpression) && string.IsNullOrEmpty(les.Token.ValueText);
+        }
+
         private static ExpressionSyntax NegateIfNeeded(VBSyntax.BinaryExpressionSyntax node, InvocationExpressionSyntax positiveExpression)
         {
             return node.IsKind(VBasic.SyntaxKind.EqualsExpression)

Tests/CSharp/ExpressionTests/ByRefTests.cs

@@ -456,9 +456,9 @@ public void DoSomething(ref string str)
     public void Main()
     {
         DoSomething(ref arr[1]);
-        Debug.Assert((arr[1] ?? """") == ""test"");
+        Debug.Assert(arr[1] == ""test"");
         DoSomething(ref arr2[2, 2]);
-        Debug.Assert((arr2[2, 2] ?? """") == ""test"");
+        Debug.Assert(arr2[2, 2] == ""test"");
     }
 }");
         }

Tests/CSharp/ExpressionTests/LinqExpressionTests.cs

@@ -189,7 +189,7 @@ Return String.Empty
         }
 
         [Fact]
-        public async Task LinqAsEnumerable()
+        public async Task LinqAsEnumerableAsync()
         {
             await TestConversionVisualBasicToCSharpAsync(@"Imports System.Data
 

Tests/CSharp/ExpressionTests/StringExpressionTests.cs

@@ -126,12 +126,12 @@ public void Foo()
             throw new Exception();
         }
 
-        if ((s1 ?? """") == ""something"")
+        if (s1 == ""something"")
         {
             throw new Exception();
         }
 
-        if (""something"" == (s1 ?? """"))
+        if (""something"" == s1)
         {
             throw new Exception();
         }
@@ -318,6 +318,29 @@ public void Foo()
     {
         string x = ""x"" + 4 + ""y"";
     }
+}");
+        }
+
+        [Fact]
+        public async Task EmptyStringCoalesceSkippedForLiteralComparisonAsync()
+        {
+            await TestConversionVisualBasicToCSharpAsync(
+@"Public Class VisualBasicClass
+
+    Sub Foo()
+        Dim x = """"
+        Dim y = x = ""something""
+    End Sub
+
+End Class",
+@"
+public partial class VisualBasicClass
+{
+    public void Foo()
+    {
+        string x = """";
+        bool y = x == ""something"";
+    }
 }");
         }
     }