Question

I have a method, which must be called no more than once, for example, Dispose. Now, I realize It as next:

private bool _isAlive = true;

public void Dispose()
{
    if (this._isAlive)
    {
        this._isAlive = false;
        //Do Something
    }
}

But It is not thread-safe because there is a gap between comprasion and setting flag _isAlive to false. So, It is possible that more than one thread execute //Do Something code.

Is there thread-safe variant of it?

Answer 1

use (updated according to comments):

private long _isSomeMethodExecuted = 0;

public void Dispose()
{
 if ( Interlocked.Read ( ref this._isSomeMethodExecuted ) != 0 )
      return;

 if (Interlocked.Increment (ref this._isSomeMethodExecuted) == 1) //check if method is already executed
 {
        //Main code of method

 }
// leave the decrement out - this leads to 
// this method being callable exactly once as in the lifetime of the object
// Interlocked.Decrement (ref this._isSomeMethodExecuted);
}

For refrences see http://msdn.microsoft.com/en-us/library/zs86dyzy.aspx

UPDATE (as per comment from @LukeH):

A single CompareExchange call is simpler/better:

public void Dispose() 
{ 
if (Interlocked.CompareExchange(ref _isSomeMethodExecuted, 1, 0) == 0) 
{ /* main code of method */ } 
}