Use string.IsNullOrEmpty when comparing string to string.Empty

CHANGELOG.md

@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 * Convert immediately executed lambdas without causing a compiler error [#869](https://github.com/icsharpcode/CodeConverter/issues/869)
 * Fix binary expressions for nullable types in VB->C# conversion [#840](https://github.com/icsharpcode/CodeConverter/issues/840)
+* Use string.IsNullOrEmpty when comparing string to string.Empty [#874] (https://github.com/icsharpcode/CodeConverter/issues/874)
 
 ### C# -> VB
 

CodeConverter/CSharp/VisualBasicEqualityComparison.cs

@@ -84,7 +84,8 @@ public static RequiredType GetObjectEqualityType(params TypeInfo[] typeInfos)
 
     private static bool IsNonEmptyStringLiteral(VBSyntax.ExpressionSyntax vbExpr)
     {
-        return vbExpr.SkipIntoParens().IsKind(VBSyntaxKind.StringLiteralExpression) && vbExpr is VBSyntax.LiteralExpressionSyntax literal && !IsEmptyString(literal);
+        vbExpr = vbExpr.SkipIntoParens();
+        return vbExpr.IsKind(VBSyntaxKind.StringLiteralExpression) && vbExpr is VBSyntax.LiteralExpressionSyntax literal && !IsEmptyString(literal);
     }
 
     public ExpressionSyntax VbCoerceToNonNullString(VBSyntax.ExpressionSyntax vbNode, ExpressionSyntax csNode, TypeInfo typeInfo, bool knownNotNull = false)
@@ -97,16 +98,16 @@ public ExpressionSyntax VbCoerceToNonNullString(VBSyntax.ExpressionSyntax vbNode
             return csNode;
         }
 
-        csNode = typeInfo.Type.SpecialType == SpecialType.System_String
-            ? SyntaxFactory.ParenthesizedExpression(Coalesce(csNode, EmptyStringExpression()))
+        csNode = typeInfo.Type?.SpecialType == SpecialType.System_String
+            ? Coalesce(csNode, EmptyStringExpression()).AddParens()
             : Coalesce(csNode, EmptyCharArrayExpression());
 
         return VbCoerceToString(csNode, typeInfo);
     }
 
     public static ExpressionSyntax VbCoerceToString(ExpressionSyntax csNode, TypeInfo typeInfo)
     {
-        return typeInfo.Type.SpecialType switch
+        return typeInfo.Type?.SpecialType switch
         {
             SpecialType.System_String => csNode,
             SpecialType.System_Char => SyntaxFactory.InvocationExpression(ValidSyntaxFactory.MemberAccess(csNode, nameof(ToString))),
@@ -188,11 +189,26 @@ public bool TryConvertToNullOrEmptyCheck(VBSyntax.BinaryExpressionSyntax node, E
 
     private bool IsNothingOrEmpty(VBSyntax.ExpressionSyntax expressionSyntax)
     {
+        expressionSyntax = expressionSyntax.SkipIntoParens();
+
         if (expressionSyntax is VBSyntax.LiteralExpressionSyntax les &&
-            (les.IsKind(VBSyntaxKind.NothingLiteralExpression) ||
-             IsEmptyString(les))) return true;
+            (les.IsKind(VBSyntaxKind.NothingLiteralExpression) || IsEmptyString(les))) {
+            return true;
+        }
+
+        bool IsStringEmptyExpression() =>
+            expressionSyntax is VBSyntax.MemberAccessExpressionSyntax memberAccess && memberAccess.IsKind(VBSyntaxKind.SimpleMemberAccessExpression) &&
+            memberAccess.Expression is VBSyntax.PredefinedTypeSyntax predefinedType && predefinedType.Keyword.IsKind(VBSyntaxKind.StringKeyword) &&
+            memberAccess.Name.Identifier.ValueText.Equals("Empty", StringComparison.OrdinalIgnoreCase);
+
+        if (IsStringEmptyExpression()) {
+            return true;
+        }
+
         var constantAnalysis = _semanticModel.GetConstantValue(expressionSyntax);
         return constantAnalysis.HasValue && (constantAnalysis.Value == null || constantAnalysis.Value as string == "");
+
+
     }
 
     private static bool IsEmptyString(VBSyntax.LiteralExpressionSyntax les)

Tests/CSharp/ExpressionTests/ExpressionTests.cs

@@ -6,6 +6,43 @@ namespace ICSharpCode.CodeConverter.Tests.CSharp.ExpressionTests;
 
 public class ExpressionTests : ConverterTestBase
 {
+    [Fact]
+    public async Task ComparingStringsUsesCoerceToNonNullOnlyWhenNeededAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(@"Class TestClass
+    Private Sub TestMethod(a as String)
+        Dim result = a = ("""")
+        result = """" = a
+        result = a = (String.Empty)
+        result = String.Empty = a
+        result = a = (Nothing)
+        result = Nothing = a
+        result = a Is Nothing
+        result = a IsNot Nothing
+        result = a = a
+        result = a = (""test"")
+        result = ""test"" = a
+    End Sub
+End Class", @"
+internal partial class TestClass
+{
+    private void TestMethod(string a)
+    {
+        bool result = string.IsNullOrEmpty(a);
+        result = string.IsNullOrEmpty(a);
+        result = string.IsNullOrEmpty(a);
+        result = string.IsNullOrEmpty(a);
+        result = string.IsNullOrEmpty(a);
+        result = string.IsNullOrEmpty(a);
+        result = a is null;
+        result = a is not null;
+        result = (a ?? """") == (a ?? """");
+        result = a == ""test"";
+        result = ""test"" == a;
+    }
+}");
+    }
+
     [Fact]
     public async Task ConversionOfNotUsesParensIfNeededAsync()
     {