What happens if my C# switch expression is non-exhaustive?

Question

In C# 8, switch expressions were introduced. What happens if a switch expression is not exhaustive? In other words, what happens if I don't test for every possible value?

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

If the switch expression is not exhaustive, the compiler produces a warning. At runtime, if you pass a value that is not handled, a SwitchExpressionException is thrown.

This is documented in the speclet for new patterns feature in C# 8.0: 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.