C# 9: Simplified Parameter Null Validation

When we last reported on Simplified Parameter Null Validation , this feature was being challenged by several competing proposals ranging from attributes and compilers flags to full scale AOP with IL weaving. All of that has been discarded in favor of a narrowly tailored feature proposal which reduces the amount of code needed to validate non-null parameters to a single character.

void M(string name!) {
    ...
}

void M(string name!) {
    if (name is null) {
        throw new ArgumentNullException(nameof(name));

    }
    ...
}

The rules are pretty straight forward.

==
string? x!

Regarding the constructor rule, Microsoft did consider allowing the parameter check to occur before invoking the base class constructor. This has always been allowed by .NET and is arguably preferable. However, the C# syntax doesn’t expose this capability, and they didn’t want to generate code that couldn’t be created without using this feature.

In the test plan , we see these scenarios for generics

void M<T>(T value!) { } is OK
void M<T>(T value!) where T : struct { } is an error
void M<T>(T value!) where T : unmanaged { } is an error
void M<T>(T value!) where T : notnull { } is OK
void M<T>(T value!) where T : class { } is OK
void M<T>(T value!) where T : SomeStruct { } is an error
void M<T>(T value!) where T : SomeClass { } is OK

One flaw in the design is that it offers no support for validating properties, as the value parameter is only implied. Orthoxerox suggested a work-around where the bang operator is applied to the set keyword in this scenario.

public Foo Foo {get; set!;}

public Bar Bar {
    get { return bar; }
    set! { bar = value; DoSomethingElse(); }
}