C# 7: What are discards and how to use them

Let’s assume that you wish to call a method that has a return value and also accepts an out variable, but you do not wish to use the contents of the out variable that will be returned.
So far we were creating a dummy variable that will later not be used, or discarded.
With C# 7 you can now use Discards

Discards are local variables which you can assign a value to them and that value cannot be read (discarded). In essence they are “write-only” variables.

These discards do not have names, but rather they are represented with a _ (underscore).

So let’s go with the following example.
Assume we have a ConcurrentQueue of integers, from which we wish to dequeue something, without actually using that something.

What would we do?

int outDummy;
if(m_myQueue.TryDequeue(out outDummy))
{
   //do something here
}
else
{
   //do something else here
}

Now, with C# 7 we can utilize discards.

if(m_myQueue.TryDequeue(out _))
{
   //do something here
}
else
{
   //do something else here
}

And the value that has been dequeued will not and can not be used.

For example the following piece of code

int x = 0;
if(m_myQueue.TryDequeue(out _))
{
   x = _;
}

will not compile, nor will it appear in IntelliSense.

Please remember though, that since _ is a contextual keyword, if you declare a variable with the name _ the variable will be used.

int x = 0;
int _;
if(m_myQueue.TryDequeue(out _))
{
   x = _;
}

In the above code, the value that will removed from the queue will be assigned to the variable x, as in the above case the underscore is used as a variable and not as a discard.

Conclusion

The discards in C# enables a way to ignore some local variables, it is a design time feature.
At runtime a variable may be required and the compiler may generate a name for it.
Since the _ keyword is a contextual keyword, you need to set a code policy to avoid declaring local variables with the name _ to reduce confusions.
This feature is compatible with previous versions of .NET platforms as it does not require a CLR change.