Switch expressions and is patterns

A JS switch is a statement: it falls through, needs break, and produces nothing. A C# switch expression is an expression: it evaluates to a value, has no fall-through, and each arm is pattern => result. You’ll use it to turn a value into another value, and it reads top to bottom like a lookup table.

// This is the shape Fusion endpoints use: match on the ERROR TYPE, not a string.
IResult response = result.Errors[0] switch
{
    EntityNotFound e => Results.NotFound(e.Message),
    DecisionNotFound e => Results.NotFound(e.Message),
    DecisionAlreadyExists e => Results.Conflict(e.Message),
    _ => Results.BadRequest(result.Errors),
};

Notice you’re matching on the type of the error (EntityNotFound e), binding it to e in one move. That’s the is pattern: x is EntityNotFound e both tests the type and gives you a typed variable, replacing the TS if (x instanceof EntityNotFound) plus a cast.

The _ arm is the discard: the catch-all. In a mapping like the HTTP one above, a sensible default is fine. But when you’re switching over a closed set of domain values, a discard that silently returns a default is dangerous, because adding a new member later maps it to the wrong thing with no warning. Fusion anti-pattern #8 is explicit: in that case, the discard should throw.

// Wrong: a new enum member silently becomes 0.
var factor = reason switch { Weather => 1.0m, Mechanical => 0.5m, _ => 0m };

// Right: reaching the discard is genuinely unexpected, so shout.
var factor = reason switch
{
    Weather => 1.0m,
    Mechanical => 0.5m,
    _ => throw new ArgumentOutOfRangeException(nameof(reason), reason, null),
};