C#: Do not insert a synthetic ToString call in interpolation expressions, if the type implements IFormattable.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs

@@ -532,6 +532,12 @@ attribute.AttributeClass is INamedTypeSymbol nt &&
             return isInline;
         }
 
+        /// <summary>
+        /// Returns true if this type implements `System.IFormattable`.
+        /// </summary>
+        public static bool ImplementsIFormattable(this ITypeSymbol type) =>
+            type.AllInterfaces.Any(i => i.Name == "IFormattable" && i.ContainingNamespace.ToString() == "System");
+
         /// <summary>
         /// Holds if this type is of the form <code>System.ReadOnlySpan&lt;byte&gt;</code>.
         /// </summary>

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/InterpolatedString.cs

@@ -1,4 +1,5 @@
 using System.IO;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Semmle.Extraction.Kinds;
@@ -20,7 +21,15 @@ protected override void PopulateExpression(TextWriter trapFile)
                 {
                     case SyntaxKind.Interpolation:
                         var interpolation = (InterpolationSyntax)c;
-                        ImplicitToString.Create(Context, interpolation.Expression, this, child++);
+                        var exp = interpolation.Expression;
+                        if (Context.GetTypeInfo(exp).Type is ITypeSymbol type && !type.ImplementsIFormattable())
+                        {
+                            ImplicitToString.Create(Context, exp, this, child++);
+                        }
+                        else
+                        {
+                            Create(Context, exp, this, child++);
+                        }
                         break;
                     case SyntaxKind.InterpolatedStringText:
                         // Create a string literal