Question

I have the following code (generates a quadratic function given the a, b, and c)
Func<double, double, double, Func<double, double>> funcGenerator = (a, b, c) => f => f * f * a + b * f + c;
Up until now, lovely.
But then, if i try to declare a variable named a, b, c or f, visual studio pops a "A local variable named 'f' could not be declared at this scope because it would give a different meaning to 'f' which is used in a child scope."
Basically, this fails, and I have no idea why, because a child scope doesn’t even make any sense.

Func<double, double, double, Func<double, double>> funcGenerator =
    (a, b, c) => f => f * f * a + b * f + c;  
var f = 3; // Fails  
var d = 3; // Fine

What’s going on here?

Answer 1

I think what you’re misunderstanding is that the order of declarations does not matter to the C# compiler with respect to scoping rules.

This:

Func<double, double, double, Func<double, double>> funcGenerator =
    (a, b, c) => f => f * f * a + b * f + c;  
var f = 3;
var d = 3;

Is exactly the same as this:

var f = 3;
Func<double, double, double, Func<double, double>> funcGenerator =
    (a, b, c) => f => f * f * a + b * f + c;  
var d = 3;

Scopes aren’t order-sensitive. You have a local variable named f, and you are trying to declare another variable named f inside the lambda. This is illegal according to the C# spec.

Specifically, it would conflict with the ability of lambdas to do variable capturing. For example, this code is legal:

int x = 3;
Func<int> func = () => x + 1;

This is completely legal and executing func() will return 4. That’s why you can’t declare another variable x here inside the lambda – because the lambda actually needs to be capable of capturing the outer x.

Just change the name of one of the f variables.