In a `while` loop, why can't I declare variable in the condition as in a `for` loop? [duplicate]

Answer 1

I believe the reason is that you would be declaring the variable multiple times (each pass of the loop).

Asked and answered in more detail here: Same question

Answer 2

Since the while loop takes a condition, what this would do is declare a new instance of line every time the loop is run, because the condition is evaluated every time through the loop.

It works in a for loop because the initializer (the first of the three semicolon-separated expressions) is run only once, at the start; the condition is the second expression. You would have the same problem trying to declare a variable in the condition expression of a for loop.

Answer 3

You can still use for:

for (string line; (line = sr.ReadLine()) != null; )
    ...

Answer 4

In a for loop:

for(<<expr1>>,<<expr2>>,<<expr3>>) 

The first expression only gets evaluated once.

In a while loop:

while(<<expr1>>)

The expression gets evaluated on each run through.

It doesn't make sense to redefine the same variable multiple times.