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C# Futures: Null Check Operator


In the June 24th C# Language Design Meeting, Microsoft made a subtle change to the parameter null checking syntax. By changing it from Type parameterName! to Type parameterName!! they effectively introduced a new ‘null check operator’.

This eliminates the problem of the single-bang having two similar, but opposite meanings.

void MethodA(string a!)

string a = b!;

In the above example, a! would mean “verify this isn’t null” while b! means “assume this isn’t null without checking”.

Before settling on the double-bang, there were other contenders for the syntax.

void M(string param!)
void M(string param!!)
void M(string! param)
void M(string !param)
void M(checked string param)
void M(string param ?? throw)
void M(string param is not null)
void M(notnull string param)
void M(null checked string param)
void M(bikeshed string param)
void M([NullChecked("Helper")] string param)
/* contract precondition forms */
void M(string param) Requires.NotNull(param)
void M(string param) when param is not null

For those wondering about the ‘bikeshed’ keyword, it refers to Parkinson's Law of Triviality where in a committee is likely to focus the majority of their attention on minor details, say the bike shed outside of a nuclear reactor, while ignoring the hard questions such as which reactor core to use. One has to assume the note-taker for the June 17 C# Language Design Meeting was getting a bit frustrated.

For the C# 9 timeframe, the null check operator (!!) will still only be applicable to parameters. But developers and language designers are already speculating on where else it could be used.

To illustrate, consider this function and statement.

string? Foo();
string x1 = Foo();

This would result in a compiler warning if nullable reference types are enabled. But the developer may know that Foo() won’t return a null in this context. So the developer silences the exception with a bang (!):

string x2 = Foo()!;

This says, “Assign the result of Foo() to x2. Assume that Foo() won’t return a null.”

That may be acceptable today, but if Foo() changes in the future it may start returning unexpected nulls. To address this, a future version of C# may allow a double-bang (!!):

string x3 = Foo()!!;

This changes the meaning to be, “Throw an exception if Foo() returns a null, otherwise assign the result to x3.”

Initial thoughts would be the exception thrown would probably be an InvalidOperationException with the currently executing expression being included in the exception’s message. This is problematic, however, as the type of the exception should indicate if the error was made by the caller, the library, or the environment. An InvalidOperationException implies the caller made a mistake, which in most cases this is more of “library author made the wrong assumption” scenario.

Ed Ball asks,

Could the !! operator translate into Value access for Nullable<T>?

void MethodOne(string? text, DateTime? dateTime)
    MethodTwo(text!!.Substring(1)); // throws InvalidOperationException if null
    MethodThree(dateTime!.Value.AddHours(1)); // throws InvalidOperationException if null
    MethodThree(dateTime!!.AddHours(1)); // throws InvalidOperationException if null

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