Getting warning when forgiving null through Nulllable.Value property on value type-CodePudding

I've tried to reproduce a null-forgiving code applied to a value type, but it does not generate any warning: dotnetfiddle.net. In actual code, I'm getting a warning. I'm actually compiling with .NET Core 6.0.

I'm getting multiple compile-time warnings on a code base, some of them related to null-forgiving code. I'm trying to use either SomeNullableValue.Value or SomeNullableValue!, but the compiler complains I'm possibly passing null argument to a function in both ways.

The case I'm dealing with now is an enum that is initially null, but is assigned later in the same control flow; in this case, it is Ast.ContStmtTarget? t, where Ast.ContStmtTarget is an enum:

NextToken();
Ast.ContStmtTarget? t = null;
if (CurrentToken.Type == Token.Keyword) {
    switch (CurrentToken.StringValue) {
        case "do": t = Ast.ContStmtTarget.Do; break;
        case "for": t = Ast.ContStmtTarget.For; break;
        case "loop": t = Ast.ContStmtTarget.Loop; break;
        case "while": t = Ast.ContStmtTarget.While; break;
    }
}
if (!t.HasValue) {
    FatalError(0x80A0400, GetTokenSpan());
}
r = new Ast.ContStmt(t.Value);

The compiler is outputting warning in the last line, r = ...;, with the column pointing to t.Value. Also, the Ast.ContStmt constructor takes a non-nullable value (the enum).

...\Parser.cs(1410,38): warning CS8629: Nullable value type may be null.

I've put compilable project on GitHub (dotnet build):

CodePudding user response：

compiler is warning you that you can reach that line with t == null. Does FatalError return? I assume it throws but the compiler doesnt know that

if (!t.HasValue) {
    FatalError(0x80A0400, GetTokenSpan());
} else{
   r = new Ast.ContStmt(t.Value);
}

CodePudding user response：

You can mark FatalError with DoesNotReturnAttribute to help compiler determine that code after if is unreachable in case !t.HasValue otherwise compiler can't verify that code will not actually be called:

[DoesNotReturn]
private void FatalError(int msgId, Span? span, Dictionary<string, object>? vars = null) {
    throw new SyntaxException(Program.Collect(new Problem(msgId, false, vars, span.Value)));
}