c# - 如果我的 C# switch 表达式不是详尽的，会发生什么？

Question

在 C# 8 中，引入了 switch 表达式。如果 switch 表达式不是详尽的，会发生什么？换句话说，如果我不测试每个可能的值会发生什么？

    static void Main(string[] args)
    {
        int x = 1;
        int imExhaustive = x switch
        {
            3 => 4,
            _ => 0   // x = 1 matches this
        };
        int okFine = x switch
        {
            1 => 4   // x = 1 matches this
        };
        int noMatch = x switch
        {
            3 => 4   // No match
        };
    }

Answer 1

如果switch表达式并非详尽无遗，编译器会产生警告。
在运行时，如果你传递一个未被处理的值，一个 SwitchExpressionException被抛出。

这在 C# 8.0 中新模式特性的 speclet 中有记录:https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/patterns.md#switch-expression

The compiler shall produce a warning if it proves (using those techniques) that some possible input value might not match some switch_expression_arm at runtime.

At runtime, the result of the switch_expression is the value of the expression of the first switch_expression_arm for which the expression on the left-hand-side of the switch_expression matches the switch_expression_arm's pattern, and for which the case_guard of the switch_expression_arm, if present, evaluates to true.

If there is no such switch_expression_arm, the switch_expression throws an instance of the exception System.Runtime.CompilerServices.SwitchExpressionException.