I recently came across this syntax:

var projection = myCollection.Select(static f => f.MyProperty);

and it's the first time I'd seen this static predicate syntax in a Select statement.

That said, I've yet to find any solid documentation on the benefits/drawbacks of this. Can somebody enlighten me?

Thanks!

CodePudding user response：

According to the C# documentation,

Beginning with C# 9.0, you can apply the static modifier to a lambda expression to prevent unintentional capture of local variables or instance state by the lambda

In other news, it just prevents you from accessing the object the lambda was defined in (this and base)

EG, this is legal:

return Collection.Select(f =>
{
    if (this.Type == "Thing")
        return f.Value;
    return 0;
}).Sum();

And this is not:

//                       static declared here...
//                       vvvvvv 
return Collection.Select(static f =>
{
//      ...but this instance is referenced here!
//      vvvv
    if (this.Type == "Thing")
        return f.Value;
    return 0;
}).Sum();

Summary Benchmarks

public long Benchmark() => Collection.Select(static f => f.Value).Sum();

TL;DR:

No difference in performance (mean differences are within stderror!), while reducing GC pressure. No harm no foul! I'd suggest using this if your code is heavily object oriented to avoid hanging onto "dead" objects through a misbehaving lambda.