Extension Methods

Extension methods let you add your own methods to types you didn't write, such as Transform, Vector3 or GameObject, and call them as if they were built in. DOTween's transform.DOMove(...) is an extension method.

How to write one

1. Create a static class (any name, often ending in "Extensions").
2. Write a static method.
3. Put this before the first parameter. Its type is the type you're extending.

using UnityEngine;

public static class VectorExtensions
{
    // Returns a copy with a different y
    public static Vector3 WithY(this Vector3 v, float y)
    {
        return new Vector3(v.x, y, v.z);
    }

    // Ignore height: useful for top-down distances and directions
    public static Vector3 Flat(this Vector3 v) => new Vector3(v.x, 0f, v.z);
}

// Usage: looks like a Vector3 method
transform.position = transform.position.WithY(0f);
Vector3 dir = (target.position - transform.position).Flat().normalized;

Compare with the long version: Vector3 p = transform.position; p.y = 0f; transform.position = p;

More useful examples

using UnityEngine;

public static class UnityExtensions
{
    // Destroy every child
    public static void DestroyChildren(this Transform t)
    {
        for (int i = t.childCount - 1; i >= 0; i--)
            Object.Destroy(t.GetChild(i).gameObject);
    }

    // Is this layer inside a LayerMask?
    public static bool Contains(this LayerMask mask, int layer)
    {
        return (mask.value & (1 << layer)) != 0;
    }

    // Get a component, or add it if it's missing
    public static T GetOrAdd<T>(this GameObject go) where T : Component
    {
        return go.TryGetComponent(out T c) ? c : go.AddComponent<T>();
    }

    // Pick a random element
    public static T RandomItem<T>(this T[] array)
    {
        return array[Random.Range(0, array.Length)];
    }
}

// Usage
inventoryPanel.DestroyChildren();
if (groundMask.Contains(other.gameObject.layer)) { /* landed */ }
var source = gameObject.GetOrAdd<AudioSource>();
AudioClip clip = footsteps.RandomItem();

What happens at runtime

Common Mistakes

• Method not found. The class must be static, not nested inside another class, and its namespace must be imported with using.
• Expecting it to change a struct. transform.position.WithY(0) returns a new Vector3; assign it back. See Value & Reference Types.
• Calling on a null object. Extension methods can be called on null (no instant error, since it's a static call), so check inside if it matters.
• Name clashes. If the type already has a method with the same name and parameters, the built-in one wins and yours is ignored.

See also

• Generics
• DOTween (built from extension methods)

This page is a simplified, AI-assisted interpretation. For the complete and authoritative reference, see the official Microsoft C# documentation.