error CS1061: 'X' does not contain a definition for 'Y' and no accessible extension method 'Y' accepting a first argument of type 'X' could be found

The exact message

error CS1061: 'X' does not contain a definition for 'Y' and no accessible extension method 'Y' accepting a first argument of type 'X' could be found

What it means

You called a method or read a property that does not exist on the type you called it on. The message names both: the first quoted name is the type, the second is the member you asked for. The most useful thing about it is that the type is stated explicitly, which frequently reveals that the variable is not the type you assumed it was.

Causes and fixes

Ranked by how often they actually occur, most common first.

1. 1.The member name is misspelt or has the wrong casing

   AddListner for AddListener, enable for enabled, GetComponant for GetComponent. C# is case sensitive and Unity's API mixes conventions, with fields such as enabled in lower case and methods in Pascal case, so this is easy to get wrong and quick to rule out.

   The fix: Retype the member and let autocomplete complete it. If autocomplete does not offer what you expect, the problem is the type rather than the spelling, which is the next cause.

   Source: Unity Support: What is CS1061?

2. 2.The variable is not the type you think it is

   The classic Unity case is OnCollisionEnter, whose parameter is a Collision rather than a Collider or a GameObject. Collision has no GetComponent, so calling it there fails even though the same call works everywhere else. Any API that hands you a wrapper object rather than the object itself produces this, and the message tells you exactly which type you actually have.

   The fix: Read the type in the message and go through the right property. Collision exposes the other object as collision.gameObject and its collider as collision.collider, and both of those do have GetComponent.

   Collision wraps the object; Collider is the object's component.

   using UnityEngine;
   
   public class Projectile : MonoBehaviour
   {
       void OnCollisionEnter(Collision collision)
       {
           // Wrong: Collision has no GetComponent.
           // collision.GetComponent<Health>();
   
           // Right: go through the GameObject or the Collider.
           if (collision.gameObject.TryGetComponent(out Health health))
           {
               health.Apply(-10);
           }
       }
   
       void OnTriggerEnter(Collider other)
       {
           // A trigger hands you the Collider directly, so this one does work.
           other.GetComponent<Health>();
       }
   }
   
   public class Health : MonoBehaviour
   {
       public void Apply(int delta) { }
   }

   Source: Unity Discussions: CS1061 on Collision does not contain a definition for GetComponent, Unity Scripting API: Collision

3. 3.The member exists but is private

   A member declared without an access modifier is private in C#, so it is invisible outside its own class. The compiler reports it as not existing rather than as inaccessible, which is misleading: you can see it in the file, so the message reads as wrong. Fields you added for the Inspector with [SerializeField] are the usual case, because that attribute does not change accessibility.

   The fix: Make the member public if other classes genuinely need it, or better, expose only what is needed through a public method or a property with a private setter. [SerializeField] on a private field is the right way to show something in the Inspector without exposing it to other code.

   Serialised in the Inspector, readable from outside, writable only from within.

   using UnityEngine;
   
   public class Player : MonoBehaviour
   {
       [SerializeField] private int health = 100;
   
       // Visible to other scripts without letting them assign it directly.
       public int Health => health;
   
       public void Damage(int amount) => health = Mathf.Max(0, health - amount);
   }

   Source: Unity Support: What is CS0122?

4. 4.The extension method's namespace is not imported

   The second half of the message, about extension methods, is the part people skip, and it is often the answer. LINQ operators such as Any, Where and Select are extension methods that only exist when System.Linq is imported. Without it the compiler reports the member as absent from the type rather than telling you an import is missing.

   The fix: Add the using directive for the namespace that supplies the extension. System.Linq covers the LINQ operators; a package's own extensions are named on its documentation page.

   Without the System.Linq import, every call below reports CS1061.

   using System.Collections.Generic;
   using System.Linq;
   using UnityEngine;
   
   public class Squad : MonoBehaviour
   {
       [SerializeField] private List<Health> members = new();
   
       public bool AnyAlive() => members.Any(m => m != null);
   
       public Health Weakest() => members
           .Where(m => m != null)
           .OrderBy(m => m.Health)
           .FirstOrDefault();
   }

   Source: Unity Support: What is CS1061?

5. 5.The API was renamed or removed in a newer Unity version

   Code copied from an older tutorial can reference members that no longer exist. Rigidbody.velocity became linearVelocity in Unity 6, and the legacy Input class is absent entirely in projects configured to use only the new Input System. The code was correct when it was written, which makes this hard to spot by reading it.

   The fix: Check the member on the Scripting API page for your Unity version; removed members are usually documented with their replacement. Where the difference is the Input System, Project Settings > Player > Active Input Handling controls which API is available.

   Source: Unity Scripting API: Rigidbody.linearVelocity

How to prevent it

Unity version differences

CS1061 is a standard C# compiler error, identical across Unity versions. Which members exist is what varies: Rigidbody.velocity was renamed to linearVelocity in Unity 6, and the legacy Input class is unavailable when Active Input Handling is set to Input System Package only.

Related errors

• CS0246: CS0246Why Unity reports CS0246, ranked by how often each cause occurs, including assembly definitions, Editor folders and knock-on errors from elsewhere.
• Script class cannot be foundWhy Unity refuses to add a script component, ranked from compile errors and name mismatches to namespaces and abstract classes, with the fix for each.

Related tools

• Input MappingEvery keycode, gamepad button, and mouse input with Unity, Unreal, and Godot code.
• DOTween ReferenceSearchable DOTween cheat sheet with copy-paste Unity C# code for every common tween method.

Sources

• Unity Support: What is CS1061?
• Unity Scripting API: Collision
• Unity Discussions: CS1061 on Collision does not contain a definition for GetComponent
• Unity Support: What is CS0122?
• Unity Scripting API: Rigidbody.linearVelocity