dotnet/efcore · error · NotImplementedException

Empty switch statement

Error message

Empty switch statement

What it means

When a void switch is rewritten to conditionals (RewriteSwitchToConditionals), the aggregate over an empty case list with a null default body yields null, and the translator throws NotImplementedException("Empty switch statement"). A switch with zero arms and no default is degenerate.

Source

Thrown at src/EFCore.Design/Query/Internal/LinqToCSharpSyntaxTranslator.cs:2385

            }

            default:
                throw new ArgumentOutOfRangeException();
        }

        static ConditionalExpression RewriteSwitchToConditionals(SwitchExpression node)
        {
            if (node.Type == typeof(void))
            {
                return (ConditionalExpression)(node.Cases
                        .SelectMany(c => c.TestValues, (c, tv) => new { c.Body, Label = tv })
                        .Reverse()
                        .Aggregate(
                            node.DefaultBody,
                            (expression, arm) => expression is null
                                ? Expression.IfThen(Expression.Equal(node.SwitchValue, arm.Label), arm.Body)
                                : Expression.IfThenElse(Expression.Equal(node.SwitchValue, arm.Label), arm.Body, expression))
                    ?? throw new NotImplementedException("Empty switch statement"));
            }

            Check.DebugAssert(node.DefaultBody is not null, "Switch expression with non-void return type but no default body");

            return (ConditionalExpression)node.Cases
                .SelectMany(c => c.TestValues, (c, tv) => new { c.Body, Label = tv })
                .Reverse()
                .Aggregate(
                    node.DefaultBody,
                    (expression, arm) => Expression.Condition(
                        Expression.Equal(node.SwitchValue, arm.Label),
                        arm.Body,
                        expression));
        }
    }

    /// <inheritdoc />
    protected override Expression VisitTry(TryExpression tryNode)

View on GitHub (pinned to dbf9771522)

Solutions

1. Add at least one case or a default body before the switch is translated.
2. Remove the empty switch entirely — it has no effect.
3. Validate that a switch has arms before constructing it.

Example fix

// before
var sw = Expression.Switch(value); // no cases, no default -> throws
// after
var sw = Expression.Switch(value, Expression.Empty(), new[] { case1 });

protected override Expression VisitSwitch(SwitchExpression s) {
    if (s.Cases.Count == 0 && s.DefaultBody is null) Found = true;
    return s;
}

When it happens

Trigger: A SwitchExpression of void type constructed with no cases and a null default body, e.g. Expression.Switch(value).

Common situations: Programmatically constructed degenerate switches; expression rewriting that removes all arms leaving an empty switch.

Related errors

• Switch with non-null comparison method
• Missing default arm for switch expression
• Null argument in VisitLabelTarget
• Non-void label target
• DebugInfo nodes are not supporting when translating expressi

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/62887dc851dbd2f7. Report an issue: GitHub.