15.1 End points and reachability
Every statement has an end point. In intuitive terms, the end point of a
statement is the location that
immediately follows the statement. The execution rules for composite
statements (statements that contain
embedded statements) specify the action that is taken when control reaches
the end point of an embedded
statement. [Example: For example, when control reaches the end point of a
statement in a block, control is
transferred to the next statement in the block. end example]
If a statement can possibly be reached by execution, the statement is said
to be reachable. Conversely, if
there is no possibility that a statement will be executed, the statement is
said to be unreachable.
[Example: In the example
void F() {
Console.WriteLine("reachable");
goto Label;
Console.WriteLine("unreachable");
Label:
Console.WriteLine("reachable");
}
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the second invocation of Console.WriteLine is unreachable because there is
no possibility that the
statement will be executed. end example]
A warning is reported if the compiler determines that a statement is
unreachable. It is specifically not an
error for a statement to be unreachable.
[Note: To determine whether a particular statement or end point is
reachable, the compiler performs flow
analysis according to the reachability rules defined for each statement.
The flow analysis takes into account
the values of constant expressions (§14.15) that control the behavior of
statements, but the possible values of
non-constant expressions are not considered. In other words, for purposes
of control flow analysis, a nonconstant
expression of a given type is considered to have any possible value of that
type.
In the example
void F() {
const int i = 1;
if (i == 2) Console.WriteLine("unreachable");
}
the boolean expression of the if statement is a constant expression because
both operands of the
== operator are constants. As the constant expression is evaluated at
compile-time, producing the value
false, the Console.WriteLine invocation is considered unreachable. However,
if i is changed to be a
local variable
void F() {
int i = 1;
if (i == 2) Console.WriteLine("reachable");
}
the Console.WriteLine invocation is considered reachable, even though, in
reality, it will never be
executed. end note]
The block of a function member is always considered reachable. By
successively evaluating the reachability
rules of each statement in a block, the reachability of any given statement
can be determined.
[Example: In the example
void F(int x) {
Console.WriteLine("start");
if (x < 0) Console.WriteLine("negative");
}
the reachability of the second Console.WriteLine is determined as follows:
? The first Console.WriteLine expression statement is reachable because the
block of the F method is
reachable (§15.2).
? The end point of the first Console.WriteLine expression statement is
reachable15.2 because that
statement is reachable (§15.6 and §15.2).
? The if statement is reachable because the end point of the first
Console.WriteLine expression
statement is reachable (§15.6 and §15.2).
? The second Console.WriteLine expression statement is reachable because
the boolean expression of
the if statement does not have the constant value false.
end example]
There are two situations in which it is a compile-time error for the end
point of a statement to be reachable:
? Because the switch statement does not permit a switch section to ?fall
through? to the next switch
section, it is a compile-time error for the end point of the statement list
of a switch section to be
reachable. If this error occurs, it is typically an indication that a break
statement is missing.
? It is a compile-time error for the end point of the block of a function
member that computes a value to be
reachable. If this error occurs, it typically is an indication that a
return statement is missing.
