C#•15mo ago

❔ how to supress warning CS8774

The following code throws a warning, how can i suppress it? Even an ! on the end doesn't affect that warning i want users of this function to know that they have a non-null Potato once M has completed. I know that an 'await' returns a task and that during the execution of the task the potato might be null, but that is standard reasoning for an async function - if i call an async anything, the results might not be there until the Task Completes ...

using System;
using System.Diagnostics.CodeAnalysis;
using System.Threading.Tasks;

public class C {
    
    Object? _potato;
    
    [MemberNotNull(nameof(_potato))]
    public async Task M() {
        _potato= await FetchPotato()!; //warning CS8774: Member '_potato' must have a non-null value when exiting.
    }
    
    public async Task<Object> FetchPotato() {return new Object();}
}

using System;
using System.Diagnostics.CodeAnalysis;
using System.Threading.Tasks;

public class C {
    
    Object? _potato;
    
    [MemberNotNull(nameof(_potato))]
    public async Task M() {
        _potato= await FetchPotato()!; //warning CS8774: Member '_potato' must have a non-null value when exiting.
    }
    
    public async Task<Object> FetchPotato() {return new Object();}
}

https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBAGABAlAOgBEoIBzGAewGcAXKAYxsIGEqATAUwEEYIANgE8aUGgG5MOfCgCskjFIDM+JLla4A3ply7cOvQHlgAKy6M6AflwB9AA5U6EOlQV79GdwG0AslzDAXABOAHKOIQCuAgIAFPxgXFQAZjH2js5UAJSZALoGuggqCAAc+ABsuD4xmVr57rYOTi4AvPgAnLgAYlx0jAAWAArpLtUAhOK4APSTAO4QQTCw5OoAysUA7OsALCCV/oFBuADkaU1UR7hgEfS4fRAAbly4ELjUMAC0MFECuPeCEU8Zn0uDBcFwAB5QBgwciEOoAXzqdUK+FKCDKAB5jGYLAA+Lo9fpDM7VLQIdavLgzXDY8x0ariREYeFAA===

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8 Replies

JakenVeina•15mo ago

you probably need to wrap the call

_potato = (await FetchPotato())!;

_potato = (await FetchPotato())!;

without that, the ! notation is applying to the Task being awaited, not the awaited result

Aart Bluestoke•15mo ago

@JakenVeina adding brackets doesn't fix it 😦

jcotton42•15mo ago

the issue is this is valid

var foo = M();
_potato.ToString(); // NRE
await foo;

var foo = M();
_potato.ToString(); // NRE
await foo;

JakenVeina•15mo ago

huh? how is that the same scenario? ohhhh, right as in, that's the caller's scenario [MemberNotNull] is not interpreted async-ly

Aart Bluestoke•15mo ago

not the pit of success 😦 eg; if i checked the Result property of an Task that is not completed, i'm in undefined behavior; of course this could be NRE - i wouldn't expect invariants of the function to be enforced while the Task is incomplete ... Question still stands then - any way to suppress the warning, or should i just #pragma around it?

JakenVeina•15mo ago

what you would be looking for is something like [MemberNotNull] except async-aware which doesn't exist so, no if you want to suppress the warnings and risk consumers being able to fatally misuse the API, go for it

indigo•15mo ago

There have been some proposals brought up before https://github.com/dotnet/csharplang/discussions/5657 https://github.com/dotnet/csharplang/issues/6888 Not sure how far it's gotten since then though

GitHub

API Proposal: MemberNotNullWhenCompleteAttribute · dotnet csharplan...

Background and Motivation MemberNotNullAttribute indicate that when the method return, the given members are not null. This is perfect when your Initialisation code is fully synchronous. But when u...

GitHub

Require await to apply nullable postconditions to task-returning ca...

Related discussion: #5657 Summary Assume that nullable postcondition attributes such as MemberNotNull only apply when Task-returning calls are directly awaited. Adjust analysis of implementations t...

Accord•15mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ how to supress warning CS8774