error CS0246: The type or namespace name 'X' could not be found (are you missing a using directive or an assembly reference?)

The exact message

error CS0246: The type or namespace name 'X' could not be found (are you missing a using directive or an assembly reference?)

What it means

The compiler reached a type name it cannot resolve from where you used it. That is broader than a spelling mistake: the type may exist but sit in a namespace you have not imported, in an assembly your code does not reference, or in an assembly that is not compiled for the platform you are building. The name in quotes is what the compiler could not find, and the parenthetical is a hint rather than a diagnosis.

Causes and fixes

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

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

   C# is case sensitive, so Rigidbody2d does not resolve to Rigidbody2D. This is the most common cause by a wide margin, and it is worth eliminating first because it takes seconds. Types that differ only in case or in a trailing digit, such as Collider and Collider2D, account for a large share of these.

   The fix: Check the spelling and casing against the Scripting API. Deleting the name and letting your editor's autocomplete reinsert it is faster than reading it character by character, and it also confirms the type is visible from that file.

   Source: Unity Support: What is CS0246?

2. 2.The namespace is not imported

   The type exists but lives in a namespace the file has not imported, which is what the parenthetical hint is about. Unity's own APIs are split across several: UI components need UnityEngine.UI, TextMeshPro needs TMPro, lists and dictionaries need System.Collections.Generic, and the new Input System needs UnityEngine.InputSystem.

   The fix: Add the using directive at the top of the file. Where you do not know which namespace a type belongs to, the Scripting API page for that type names it in the heading.

   The imports Unity scripts most often turn out to be missing.

   using System.Collections;               // IEnumerator, for coroutines
   using System.Collections.Generic;       // List<T>, Dictionary<TKey, TValue>
   using UnityEngine;
   using UnityEngine.UI;                   // Button, Image, Slider
   using UnityEngine.SceneManagement;      // SceneManager
   using TMPro;                            // TextMeshProUGUI
   using UnityEngine.InputSystem;          // InputAction, PlayerInput
   
   public class HudController : MonoBehaviour
   {
       [SerializeField] private Button startButton;
       [SerializeField] private TextMeshProUGUI scoreLabel;
       private readonly List<string> log = new();
   }

   Source: Unity Support: What is CS0246?

3. 3.Another compile error is hiding this one

   When a script fails to compile, the types it declares do not exist as far as every other script is concerned. A single syntax error in one file therefore produces CS0246 in every file that uses its types, which is why this error often arrives in bulk. The reported errors are symptoms; only one of them is the cause.

   The fix: Sort the Console by the order errors were reported and fix the first one, then let Unity recompile before reading the rest. Fixing CS0246 errors one by one from the top of a list of fifty is wasted effort when a missing brace in one file produced all of them.

   Source: Unity Support: How do I interpret a compiler error?, Unity Discussions: over 100 CS0246 errors appearing for no apparent reason

4. 4.An assembly definition does not reference the other assembly

   An .asmdef file compiles the folder it sits in into a separate assembly, and that assembly can only see types from assemblies it explicitly references. Adding an asmdef to a folder therefore breaks every reference into it from code that has not been told about it. This is the cause most likely to be missed, because the code is correct and unchanged; only the project structure moved.

   The fix: Select the asmdef that reports the error and add the other assembly under Assembly Definition References in the Inspector. Where a package supplies the type, its assembly name is listed on the package's page in the Package Manager.

   Source: Unity Manual: Assembly definitions

5. 5.Editor-only code is being used at runtime

   Scripts in a folder named Editor compile into an Editor-only assembly that does not exist in a build, as do types in the UnityEditor namespace. Code that references them compiles fine in the Editor and then fails at build time with CS0246, which is why this one typically appears the first time you build rather than while developing.

   The fix: Wrap Editor-only usage in #if UNITY_EDITOR so it is compiled out of players, or move the shared type into a runtime assembly. Note that an Editor folder nested inside a folder that has its own asmdef does not get the usual Editor-assembly treatment, which produces the same error for a different reason.

   The directive must wrap both the using and every use of the type.

   using UnityEngine;
   #if UNITY_EDITOR
   using UnityEditor;
   #endif
   
   public class LevelMarker : MonoBehaviour
   {
       public void Ping()
       {
   #if UNITY_EDITOR
           // Compiled out of the player, where UnityEditor does not exist.
           EditorGUIUtility.PingObject(gameObject);
   #endif
       }
   }

   Source: Unity Discussions: build fails with CS0246 on the Editor namespace, Unity Issue Tracker: build fails with an Editor folder inside a folder that has an Assembly Definition

6. 6.A package or plugin is missing, or excluded on this platform

   Types from a package that is not installed, or from a .dll excluded for the current build target in its plugin import settings, do not resolve. Opening a project on another machine without its packages restored, or building for a platform a plugin does not support, produces this. Version control that ignores Packages/manifest.json changes has the same effect.

   The fix: Confirm the package is present in the Package Manager, and for a .dll check its Inspector to see which platforms it is included for. Where the type comes from an asset store package, reimporting it is usually quicker than diagnosing which file is absent.

   Source: Unity Manual: Import and configure plug-ins

How to prevent it

Unity version differences

CS0246 is a standard C# compiler error and its text comes from Roslyn rather than Unity, so it is identical across Unity versions. What changes between versions is which assemblies exist: Unity 2019 moved several APIs into packages, so code written for an older version can produce CS0246 purely because the package is not installed.

Related errors

• CS1061: CS1061Why Unity reports CS1061, from typos and wrong types to private members and renamed APIs, with the fix for each and how to read the message.
• 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.
• Multiple precompiled assembliesWhy Unity reports duplicate precompiled assemblies, why Newtonsoft.Json causes it most often, and how to find and remove the duplicate safely.

Related tools

• Git for Game Dev.gitignore templates for Unity, Unreal, and Godot, Git LFS setup, and team workflows. The Unity template is explained block by block.

Sources

• Unity Support: What is CS0246?
• Unity Manual: Assembly definitions
• Unity Support: How do I interpret a compiler error?
• Unity Discussions: over 100 CS0246 errors appearing for no apparent reason
• Unity Discussions: build fails with CS0246 on the Editor namespace
• Unity Issue Tracker: build fails with an Editor folder inside a folder that has an Assembly Definition
• Unity Manual: Import and configure plug-ins