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Oftentimes you’ll find yourself using classes you can’t modify. Whether they’re basic data types or part of an existing framework, you’re stuck with the functions that are provided. That being said, C# provides a nifty trick to appending functions to classes! These are known as Extension Methods.

Extension methods are fairly simple to create and are frequently used as syntactic sugar. A practical example can be seen with Unity’s Transform class. Let’s say you want to set only the xvariable of Transform.position.

using UnityEngine;
using System.Collections;
 
public class Player : MonoBehaviour 
{
    void Update () 
    {
        //Set new x position to 5
        transform.position = new Vector3(5f, transform.position.y, transform.position.z);
    }
}

In this case Transform.position gives you an error if you only try to assign its x member variable, so you have to assign the entire Vector3. An extension method such as SetPositionX() could be appended to the Transform class and help make this code more readable.

In order to create extension methods you have to create a static class. In addition, an extension method declaration must be declared static and have the first parameter be of the type that you’re writing the method for, preceded by the this keyword.

using UnityEngine;
using System.Collections;
 
//Must be in a static class 
public static class Extensions
{
    //Function must be static
    //First parameter has "this" in front of type
    public static void SetPositionX(this Transform t, float newX)
    {
        t.position = new Vector3(newX, t.position.y, t.position.z);
    }
}

Now you can go back to your other script and replace our old code with the new extension method.

using UnityEngine;
using System.Collections;
 
public class Player : MonoBehaviour 
{
    void Update () 
    {
        //Set new x position to 5
        transform.SetPositionX(5f);
    }
}

NOTE: Extension methods can only be called on an instance of a class, not on the class itself.

Here are a few more extension methods to get you started, as well as an example script that utilizes a few of them.

Extensions:

using UnityEngine;
using System.Collections;
 
public static class Extensions
{
    public static void SetPositionX(this Transform t, float newX)
    {
        t.position = new Vector3(newX, t.position.y, t.position.z);
    }
 
    public static void SetPositionY(this Transform t, float newY)
    {
        t.position = new Vector3(t.position.x, newY, t.position.z);
    }
 
    public static void SetPositionZ(this Transform t, float newZ)
    {
        t.position = new Vector3(t.position.x, t.position.y, newZ);
    }
 
    public static float GetPositionX(this Transform t)
    {
        return t.position.x;
    }
 
    public static float GetPositionY(this Transform t)
    {
        return t.position.y;
    }
 
    public static float GetPositionZ(this Transform t)
    {
        return t.position.z;
    }
 
    public static bool HasRigidbody(this GameObject gobj)
    {
        return (gobj.rigidbody != null);
    }
 
    public static bool HasAnimation(this GameObject gobj)
    {
        return (gobj.animation != null);
    }
 
    public static void SetSpeed(this Animation anim, float newSpeed)
    {
        anim[anim.clip.name].speed = newSpeed; 
    }
}

Example Script:

using UnityEngine;
using System.Collections;
 
public class Player : MonoBehaviour 
{
    void Update () 
    {
        //move x position 5 units
        float currentX = transform.GetPositionX();
        transform.SetPositionX(currentX + 5f);
 
        if(gameObject.HasRigidbody())
        {
            //Do something with physics!
        }
 
        if(gameObject.HasAnimation())
        {
            //Double the animation speed!
            gameObject.animation.SetSpeed(2f);
        }
    }
}

NOTE: Static classes do NOT extend MonoBehaviour.

ANOTHER NOTE: If you define your extension methods inside of a namespace, you have to declare the use of that namespace in order to bring them into scope.

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public static class Extensions { public static bool TryGetKey<K, V>(this IDictionary<K, V> instance, V value, out K key) { foreach (var entry in instance) { if (!entry.Value.Equals(value)) { continue; } key = entry.Key; return true; } key = default(K); return false; } }