You can now add the static modifier to local functions to ensure that local function doesn't capture (reference) any variables from the enclosing scope. Doing so generates CS8421, "A static local function can't contain a reference to <variable>."
Typically, any resources declared or used in a function are released when the function exits. However, when a function captures a variable that is declared in an enclosing scope, that resource has a lifetime that extends beyond the function itself. In many scenarios, this isn't an issue. In others, capturing and preventing the release of resources can impact performance. Declaring local functions as static ensures this doesn't happen.
Consider the following code. The local function LocalFunction accesses the variable y, declared in the enclosing scope (the method M). Therefore, LocalFunction can't be declared with the static modifier:
The local iterator method captures the parameters start and end. Add the static modifier to see the compiler generated warning. You'll need to declare arguments to the local function so that those values aren't captured. Make the changes shown below to get the warning removed:
static IEnumerable<int> localCounter(int first, int endLocation)
{
for (int i = first; i < endLocation; i++)
yield return i;
}The sample should compile and run correctly.