When to use out parameters in C#

The out keyword

In C# you can use the out keyword to define output parameters in your method signature. For example, the Int32.TryParse method in the System namespace has the following signature:

public static bool Int32.TryParse(string s, out int result)

From this you can read out that the TryParse method returns a boolean but also provides a result as an out parameter. The boolean will indicate whether it was possible to parse an integer value from the string s and store it in the result variable.

You can call the TryParse method like this:

if (Int32.TryParse(str, out var result))
{
    Console.WriteLine($"The result is: {result}");
}
else
{
    Console.WriteLine("Parsing failed");
}

When is it a good idea to use out parameters?

Most of the time the use of out parameters adds complexity to your program. It will make the source code harder to follow since anyone reading the code will have to juggle several return values instead of just one.

One of the most important jobs as a programmer is to minimize complexity, hence you should avoid using out parameters and as much as possible write simple methods that returns a single, obvious, result. However, there are a couple of use cases when out parameters can be useful.

The Try-Parse Pattern

I have already showed the Try-Parse Pattern in action when showing the Int32.TryParse method above. The pattern is useful in situations where the operation might fail and you want to avoid the overhead of dealing with exceptions. Throwing and catching exceptions is very bad for performance and using the Try-Parse Pattern can be a good alternative.

Returning multiple values

There might be situations where you want a method to be able to return multiple values. You could create a separate struct just for holding these values and let the method return an instance of that struct, also known as a Value object. However, if the usage is very limited, introducing a separate struct just for holding a few values might add more complexity than just introducing a few out parameters. If in doubt, create both implementations and use your best judgement on which one to keep.

Additional resources

Out parameter modifier
Exceptions and Performance (Try-Parse Pattern)
Value object