NET-2280 New Rule Idea: Do not use `is not { }`

[alex-meseldzija-sonarsource]

Description

Do not use is not { }.

This code is confusing, and 99% of the time can be refactored into more readable and less obfuscated code.

In the (very)rare case that this is cleaner, it can be accepted.

Noncompliant code snippet

if(SomeMethod() is not {  } token)  // Noncompliant
{
    DoSomethingWithoutToken();
}
DoSomethingWithToken(token);

Compliant code snippet

if(SomeMethod() is {  } token)  // Compliant
{
    DoSomethingWithToken(token);
}
 DoSomethingWithoutToken();

[github-actions]

Internal ticket NET-2280

[MaryGeorgiou]

I'll try to do this 🫡

[jilles-sg]

I commonly use this with a return or continue statement in the if, like

if (SomeMethod1() is not { } token1) {
    return null;
}
InBetweenCode();
if (SomeMethod2() is not { } token2) {
    return null;
}
DoSomethingWithTokens(token1, token2);
MoreCodeHere();

With is { } token1 and is { } token2, the rest of the code would be indented two more levels. In that case, it would be less bad to spend separate lines on variable definitions, like

var token1 = SomeMethod1();
if (token1 is null) {
    return null;
}
InBetweenCode();
var token2 = SomeMethod2();
if (token2 is null) {
    return null;
}
DoSomethingWithTokens(token1, token2);
MoreCodeHere();

but that's still more lines so I prefer is not { }.