C#7: Out Variables

Last time, we started to look at the new features introduced in C#7. Here is a quick refresher of just what those features are:

• Binary literals
• Numeric literal digit separators
• out variables
• throw expressions
• Expression-bodied constructors
• Expression-bodied finalizers
• Expression-bodied property accessors
• Pattern matching
• Local functions
• Generalized async return types
• ref locals and returns
• Tuples

In this post, we will look at one of the simplest additions to the C# language; out variables.

int dummy

How often have you written code like this?

int dummy;

if (int.TryParse(someString, out dummy) && dummy > 0)
{
   // Do something
}

Or this?

double dummy;

if (myDictionary.TryGetValue(key, out dummy))
{
   //Do something
}

Sometimes you use the out value retrieved, sometimes you do not, often you only use it within the scope of the condition. In any case, there is always the variable definition awkwardly hanging out on its own line, looking more important than it really is and leaving space for it to accidentally get used before it has been initialized. Thankfully, C#7 helps us tidy things up by allowing us to combine the variable definition with the argument.

Using the out variable syntax, we can write this:

if (int.TryParse(someString, out int dummy) && dummy > 0)
{
    //Do something
}

In fact, we do not even need to declare the type of the variable explicitly. While often we want to be explicit to make it clear that it matters (and to ensure we get some compile time checking of our assumptions), we can use an implicitly typed variable like this:

if (myDictionary.TryGetValue(someKey, out var dummy))
{
    //Do something
}

In Conclusion

out variables are part of a wider set of features for reducing repetition (in written code and in run-time execution), and saying more with less (i.e. making it easier for us to infer intent from the code without additional commentary). This is a very simply addition to C# syntax, yet useful. Not only does it reduce what we need to type, it also improves code clarity (in my opinion), and reduces the possibility of silly errors like using a variable before it has been initialized, or worse, thinking that it being uninitialized was a mistake and hiding a bug by initializing it.

Until next time, if you would like to tinker with any of the C#7 features I have been covering, I recommend getting the latest LINQPad beta or Visual Studio 2017 RC.