Value & Reference Types

Every type in C# is either a value type (copied when you assign it) or a reference type (shared when you assign it). This one difference explains many confusing Unity errors, like why transform.position.x = 5 doesn't compile.

Which is which?

Seeing the difference

// Value type: b gets its own copy
Vector3 a = new Vector3(1f, 2f, 3f);
Vector3 b = a;
b.x = 100f;
Debug.Log(a.x); // 1   (a is unchanged)

// Reference type: both variables point to the same object
List<int> listA = new List<int> { 1, 2, 3 };
List<int> listB = listA;
listB.Add(4);
Debug.Log(listA.Count); // 4   (it's the same list)

Why transform.position.x = 5 fails

transform.position is a property that returns a Vector3. Because Vector3 is a struct, you get back a copy. Changing x on a temporary copy would do nothing, so the compiler stops you with error CS1612.

// Error CS1612: Cannot modify the return value of 'Transform.position' because it is not a variable
transform.position.x = 5f;

// Fix: copy, change, assign back
Vector3 pos = transform.position;
pos.x = 5f;
transform.position = pos;

The same applies to rb.linearVelocity, material.color and any other struct property.

Passing to methods

void MakeBig(Vector3 v)        { v *= 10f; }         // changes a local copy only
void MakeBig(ref Vector3 v)    { v *= 10f; }         // changes the caller's variable
void Rename(GameObject go)     { go.name = "Big"; }  // changes the shared object

Vector3 size = Vector3.one;
MakeBig(size);        // size is still (1,1,1)
MakeBig(ref size);    // size is now (10,10,10)
Rename(gameObject);   // the real GameObject is renamed

Strings are special

string is a reference type, but it's immutable: you can never change a string, only make a new one. So it behaves like a value type in practice.

string a = "Hello";
string b = a;
b += " World";   // creates a NEW string; a is still "Hello"

Making your own struct

Use a struct for small bundles of data that you treat as a single value (like a coordinate or a damage number). Use a class for everything else.

[System.Serializable] // lets Unity show it in the Inspector
public struct DamageInfo
{
    public float amount;
    public Vector3 hitPoint;
    public bool isCritical;

    public DamageInfo(float amount, Vector3 hitPoint, bool isCritical)
    {
        this.amount = amount;
        this.hitPoint = hitPoint;
        this.isCritical = isCritical;
    }
}

// Usage
var info = new DamageInfo(25f, hit.point, Random.value < 0.1f);
target.TakeDamage(info);

What happens at runtime

Common Mistakes

• Modifying a struct inside a list: points[0].x = 5 fails for List<Vector3> for the same reason as transform.position.x. Copy, change, assign back: var p = points[0]; p.x = 5; points[0] = p; (Arrays allow it directly: pointsArray[0].x = 5 works.)
• Expecting a copy of a class. List<Item> backup = inventory; is not a backup; it's the same list. Use new List<Item>(inventory).
• Big structs. Structs are copied every time you pass them, so keep them small (a handful of fields).

See also

• Null & Nullable Types
• Vector3
• Collections

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