If statement to be executed once

Try:

bool doneOnce = false;
while(true)
   { 
   DoSomethingA();

   if(!doneOnce && something)
      {
      DoSomethingB();
      doneOnce = true;
      }

   DoSomethingC();
   }

Just an advice: Learn better coding style.
Coding style is the way you write code, a good coding style makes your code easier to read, and in large code base, it matters.
Never try to save a keystroke by grouping things on same line, particularly never put code after a '{' , it makes code more difficult to read.
Some professional programmer editors (like Eclipse) have a code formatting feature.

while(true)
{
    do something     (A)
    if(something)
    {
        do something  (B)
    }
    do other things   (C)
}

or

while(true) {
    do something     (A)
    if(something) {
        do something  (B)
    }
    do other things   (C)
}

Advice: Learn to indent properly your code, it show its structure and it helps spotting structures mistakes.
Indentation style - Wikipedia[^]

Professional programmer's editors have this feature and others ones such as parenthesis matching and syntax highlighting.
Notepad++ Home[^]
ultraedit[^]
Enabling Open Innovation & Collaboration | The Eclipse Foundation[^]