Overload resolution, conversions, and patterns

Overload resolution, conversions, and patterns

When you call a method, Roslyn runs a deterministic search: find applicable candidates, then apply the betterness rules. You do not need the spec in your head — you need the handful of rules that decide real bugs:

1. Exact type beats implicit conversion. Pick(5) prefers Pick(int) over Pick(long).
2. Non-generic beats generic when both apply without conversion; a fixed type argument beats both (Pick<object>(5) calls the generic).
3. Implicit numeric widening never happens implicitly in overload choice between int and long arguments — the int literal binds to int.
4. User-defined conversions are consulted last and never chain.

Pattern matching is compiled to type checks and value extraction:

static string Describe(object o) => o switch
{
    int n when n < 0         => "negative",
    int and > 100            => "big int",
    string { Length: 0 }     => "empty string",
    Point(_, 0)              => "on the X axis",
    null                     => "null",
    _                        => "other",
};

switch expressions compile to a decision tree — isinst/unbox for type patterns, comparisons for relational patterns. Recursive patterns nest declarations, not behavior. The design win: exhaustive-looking shapes push state decisions out of if-ladders into one declaration the compiler can check (add a case for a new enum member and the _ discards it silently — that silence is the tax).

When patterns go wrong: matching on object erases generic type info (rely on when guards, not on runtime surprises), and is type patterns on value types box the operand — one more silent allocation on hot paths.