C # 7.0 discard parameter ambiguity
From https://blogs.msdn.microsoft.com/dotnet/2017/03/09/new-features-in-c-7-0/ :
We also allow to "strip" output parameters as _ so that you can ignore parameters you don't need:
p.GetCoordinates(out var x, out _); // I only care about x
Consider the following code:
void Foo(out int i1, out int i2)
{
i1 = 1;
i2 = 2;
}
int _;
Foo(out var _, out _);
Console.WriteLine(_); // outputs 2
Questions:
Why is the "discard" option displayed in this context?
Also, there should be no "already defined in scope" error for out var _
?
int i;
Foo(out var i, out i);
Console.WriteLine(i); // Error: A local variable or function named 'i'
// is already defined in this scope
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Here's a GitHub issue that describes the behavior of the discards , relevant parts:
However, semantically we want this to create an anonymous variable and shadow any true variable (such as a parameter or field) from the enclosing scope named _.
...
We must be careful with these changes so that any program that uses _ as an identifier and is legal today continues to compile with the same value under these revised rules.
_
means "discard this parameter" if you declare it and use it
- as an identifier in a "notation" (declaration expression) that includes parameters
out
and deconstruction of tuples and similar - as an identifier according to the pattern (
switch
...case int _
) - obviously also for declaring anonymous delegates that must take parameters to match the type of the delegate, but where you don't need the actual parameter.
out var _
is one of them in this category.
However int _;
, it is a separate statement, and thus it means that you "care" about this variable and therefore do not discard.
Thus, you get a normal variable with the name _
, which is still legal . This gets the value 2 from the method call, as expected.
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