public string GetWebPage(string address)
{
lock (this.ThisLock)
{
// <-- Can assume cache is valid.
if (this.cachedAddress == address)
{
return(this.cachedWebPage);
// <-- Must guarantee that cache is valid because
// the operation returns and releases the lock.
}
// <-- Must guarantee that cache is valid here because
// the operation releases the lock.
}
string webPage = slow.GetWebPage(address);
lock (this.ThisLock)
{
// <-- Can assume cache is valid.
// <-- Cache is no longer valid because the operation
// changes one of the values.
this.cachedAddress = address;
this.cachedWebPage = webPage;
// <-- Cache is valid again here.
// <-- Must guarantee that cache is valid because
// the operation releases the lock.
}
return(webPage);
}
public string GetWebPage(string address)
{
// <-- Can assume that cache is valid.
if (this.cachedAddress == address)
{
return(this.cachedWebPage);
}
// <-- Must guarantee that the cache is valid, because
// the operation can be called again before we return.
string webPage = slow.GetWebPage(address);
// <-- Can assume cache is valid.
// <-- Cache is no longer valid because we are changing
// one of the values.
this.cachedAddress = address;
this.cachedWebPage = webPage;
// <-- Cache is valid again here.
return(this.cachedWebPage);
// <-- Must guarantee that cache is valid because we are returning.
}