error CS1061: 'X' does not contain a definition for 'Y'

Read the Console line first

The message names both types: X is the type of the variable before the dot, Y is the member you asked for. The file and line are at the start of the line, for example Assets/Scripts/Player.cs(18,12).

Common causes

1. The variable is a GameObject, not the component

   GameObject has no velocity, health or TakeDamage. Those live on components. Get the component first.

   Example

   C# Copy

   public GameObject enemy;
   
   void Hit()
   {
       enemy.TakeDamage(10); // CS1061: GameObject has no TakeDamage
   }

   6 lines

2. The name is spelled or capitalised differently

   C# is case-sensitive. transform.Position, rigidbody.Velocity and gameObject.SetActive(True) all fail. Unity APIs use PascalCase for methods and camelCase for properties such as position and velocity.

3. An extension method's namespace is missing

   LINQ methods such as Where, Select and First are extension methods in System.Linq. Without using System.Linq; at the top, the compiler reports CS1061 for them.

4. The member was removed or renamed in a newer Unity

   Code written for an older version can call APIs Unity has since removed. Check the Unity 6 upgrade pages, or paste the script into the script checker.

The fix

public GameObject enemy;

void Hit()
{
    if (enemy.TryGetComponent(out Health health))
        health.TakeDamage(10);
}

// Or keep a reference of the component type in the first place:
// [SerializeField] private Health enemyHealth;

Stop it coming back

Type serialized fields as the component you need (Health, Rigidbody) rather than GameObject, so the Inspector only accepts objects that have it and the compiler knows its members. Let your IDE autocomplete Unity APIs instead of typing them from memory.

Questions

Why does GetComponent not appear when I type a dot after my variable?

If the variable is a component, GetComponent is there; if autocomplete shows nothing at all, your IDE is not connected to Unity's project files. Set your editor in Preferences > External Tools and choose Regenerate project files.

The code compiled in an older Unity version. Why not now?

Unity removes APIs some versions after marking them obsolete. Paste the script into the Unity 6 script checker to see which calls changed and what replaces them.

How is CS1061 different from CS0103?

CS1061 is about a member after a dot: the type exists, but it has no member with that name. CS0103 is about a name on its own that the compiler cannot find at all.

Scripts that avoid it

• Health SystemReusable health component with damage, healing, invincibility frames, and events.
• 3D Player ControllerFirst/third person character controller with smooth movement, jumping, and slope handling using CharacterController.

Related errors

• NullReferenceException: Object reference not set to an instance of an object
• MissingReferenceException: The object has been destroyed but you are still trying to access it
• IndexOutOfRangeException: Index was outside the bounds of the array
• ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection
• error CS0246: The type or namespace name could not be found

Got a different error? Paste it into the error lookup.