Implicitly Typed Local Variables

Implicitly Typed Local Variables

It happens time and time again: I’ll be at a game jam, mentoring students, or just showing a friend some Unity C# code, and I’ll type something like this:

var controller = GetComponent<CharacterController>();

They’ll go, “whoa wait, isn’t var a JavaScript thing?”

Well the answer is yes… and no, the keyword var exists in both Unity’s C# and JavaScript languages, but they actually mean different things. In C#, it means I’m defining an implicitly typed local variable.

Implicitly Typed Local Variables

When you use var on a local variable in C#, it means that the compiler will attempt to infer the variable’s type based on context. This differs from a dynamic variable in that it will be fast and strictly-typed as if you’d defined the type yourself.

To explain better, consider the following two lines of code:

var controller = GetComponent<CharacterController>();

CharacterController controller = GetComponent<CharacterController>();

Despite which of these two lines of code you use, Unity will compile them identically. One is not faster or slower than the other. Personally, I really like var for this purpose, because I don’t really like having to type long-winded names like “CharacterController” twice to define a single variable.

If you’re a fan of MonoDevelop or Visual Studio’s code hinting, you’ll be happy to know that using the varkeyword will not cause it trouble, it will still give you correct code hints on the fly as you type.

Thoughts on Usage

Some coders refuse to use implicit typing completely, as it can sometimes be unclear to other readers of the code what type a variable actually is. So I’ll suggest a simple guideline I use: only use implicit typing when the type of the variable is obvious in context. For example:

//This is okay, it is obvious here that the type is "Movement Controller"

var moveController = GetComponent<MovementController>();

 

//Less okay, not necessarily obvious what this function is returning

var properties = moveController.GetProperties();

 

//Unless it is obvious, this is probably a better approach

MoveProperties properties = moveController.GetProperties();

 

//This I am actually okay with, "targets" is defined right here in plain sight

GameObject[] targets = GameObject.FindGameObjectsWithTag("Enemy");

var firstTarget = targets[0];

You don’t have to follow this rule, but I think it is fair. At least you’ll now get to know the joy of telling somebody “actually, this is a feature of C#!” when they incredulously ask you why you are using a JavaScript keyword.

Also finally, keep in mind that member variables cannot be implicitly typed, so for example:

using UnityEngine;

using System.Collections.Generic;

 

public class Player : MonoBehaviour

{

    //This is a member variable, so you can't use "var" here

    List<Transform> targets = new List<Transform>();

 

    void Update()

    {

        //You can only use them for local vars, like this!

        var firstTarget = targets[0];

 

        //Or even as iterator variables

        foreach (var target in targets)

        {

 

        }

    }

}

Hope you enjoyed this article. If you have any questions or feedback, leave a comment below!