如何避免 C# while 和 do-while 循环中的代码重复？

Question

I have a loop inside a C# method that has the following structure.

do
{
     getUserInput();
     if (inputIsBad)
     {
          doSomethingElse();
     } 
} while (inputIsBad);

alternately, with a while loop:

getUserInput();
while (inputIsBad)
{
     doSomethingElse();
     getUserInput();
}

But both methods use redundant code: the do-while has both an if statement and while loop checking the same condition; the while loop calls getUserInput() both before and inside the loop.

Is there a simple, non-redundant, non- ad hoc way to do what these method patterns do, either generally or in C# specifically, that only involves writing each basic component once?

Answer 1

Assuming that getUserInput(..) can be converted into a expression yielding a boolean value*..

while (getUserInput()
    && isBadInput()) {
  doSomethingElse();
}

// Prompts for user input, returns false on a user-abort (^C)
private bool getUserInput() { .. }

Other variations (presumed without non-local state) shown in comments.

*Trivially, it can always be written as a wrapping function - see Local Functions , introduced in C#7. (There are other methods for the same effect, some of which I consider 'too clever'.)

// local function
bool getUserInputAlwaysTrue() {
   getUserInput(); // assume void return
   return true;
}

while (getUserInputAlwaysTrue()
    && isBadInput()) {
  doSomethingElse();
}

This can be followed to pushing out the logic further, in some cases. The general premise holds: getUserInput() is always invoked prior to the next isBadInput() .

// local function or member method
// Prompt for user input, returning true on bad input.
bool getCheckedUserInput() {
   getUserInput(); // assume void return
   return isBadInput();
}

while (getCheckedUserInput()) {
  doSomethingElse();
}

Answer 2

do
{
     getUserInput();

     if (!inputIsBad) break;

     doSomethingElse();

} while (true);

Answer 3

I would use a boolean variable, which you need to declare outside the body of the loop. That way you only need to run the inputIsBad check once. I have turned it into a method as well, since that seems more logical.

bool badInput = true;  // Assume bad until checked -- failsafe.
do
{
  getUserInput();
  badInput = inputIsBad();
  if (badInput)
  {
    doSomethingElse();
  } 
} while (badInput);

Answer 4

Building on user2864740's answer:

Assume getUserInput() can be converted into a function which returns true if the input is good and bad otherwise. Assuming its original return type wasn't boolean or void, return its original return value via an out or ref parameter depending on the case, eg

int originalReturnValue;

while (!getUserInput(out originalReturnValue))
{
     doSomethingElse();
} 

...

bool getUserInput<T>(out T output)
{
// method body
}