The resource expression is always needed regardless of the setting.

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -406,7 +406,8 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		protected internal override Statement VisitUsingInstruction(UsingInstruction inst)
 		{
-			var transformed = TransformToForeach(inst, out var resource);
+			var resource = exprBuilder.Translate(inst.ResourceExpression).Expression;
+			var transformed = TransformToForeach(inst, resource);
 			if (transformed != null)
 				return transformed;
 			AstNode usingInit = resource;
@@ -420,7 +421,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					AssignVariableNames.GenerateVariableName(currentFunction, disposeType)
 				);
 				return new BlockStatement {
-					new ExpressionStatement(new AssignmentExpression(exprBuilder.ConvertVariable(var).Expression, resource?.Detach())),
+					new ExpressionStatement(new AssignmentExpression(exprBuilder.ConvertVariable(var).Expression, resource.Detach())),
 					new TryCatchStatement {
 						TryBlock = ConvertAsBlock(inst.Body),
 						FinallyBlock = new BlockStatement() {
@@ -446,14 +447,12 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		Statement TransformToForeach(UsingInstruction inst, out Expression resource)
+		Statement TransformToForeach(UsingInstruction inst, Expression resource)
 		{
 			if (!settings.ForEachStatement) {
-				resource = null;
 				return null;
 			}
 			// Check if the using resource matches the GetEnumerator pattern.
-			resource = exprBuilder.Translate(inst.ResourceExpression);
 			var m = getEnumeratorPattern.Match(resource);
 			// The using body must be a BlockContainer.
 			if (!(inst.Body is BlockContainer container) || !m.Success)