NGWS SDK Documentation  

This is preliminary documentation and subject to change.
To comment on this topic, please send us email at ngwssdk@microsoft.com. Thanks!

8.1 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. For example, when control reaches the end point of a statement in a block, control is transferred to the next statement in the block.

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.

In the example

void F() {
   Console.WriteLine("reachable");
   goto Label;
   Console.WriteLine("unreachable");
   Label:
   Console.WriteLine("reachable");
}

the second Console.WriteLine invocation is unreachable because there is no possibility that the statement will be executed.

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.

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 (§7.15) 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 non-constant 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. The constant expression is evaluated at compile-time, producing the value false, and the Console.WriteLine invocation is therefore 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 it will in reality never be executed.

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.

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:

There are two situations in which it is an error for the end point of a statement to be reachable: