The type 'UnityEngine.Vector3' cannot be declared const

Question

Hello! It's been a w$$anonymous$$le since I last programmed in C#, and I've run into t$$anonymous$$s error when compiling one of my scripts:

Here is the code that is causing it:

public const Vector3 up = new Vector3(0, 0, 1);

I wasn't aware of any C# mechanic that could allow specific types to explicitly prevent the const specification. I can't find anyt$$anonymous$$ng on that matter either on the forums, Answers, nor Google. Would anyone care to enlighten me? I wish there was an error code database I could look into.

Thanks!

Answer 1

Best Answer

Answer by Mike 3 · Apr 24, 2011 at 01:31 AM

From http://msdn.microsoft.com/en-us/library/ms173119.aspx

Only the C# built-in types (excluding System.Object) may be declared as const.

You may want to look into readonly instead - very similar, but allows writing at construction time

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microp · Apr 24, 2011 at 02:11 AM 0

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Wow. Coming from a C++ background, I find that quite puzzling. Anyway, thanks a lot!

microp · Apr 24, 2011 at 02:30 AM 0

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Unless there's something else I missed, "readonly" values don't seem like they work with default parameters though, do they?

Mike 3 · Apr 24, 2011 at 02:57 AM 0

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const has a different meaning in c# to c++, but at any rate, no, you can't use readonly for default parameters, but you can't use vectors for them at all anyway

Answer 2

Answer by PatHightree · May 27, 2011 at 12:14 PM

If you change your field into a property with only a getter, you will get the desired behavior.

 public Vector3 up { get { return new Vector3(0, 0, 1); } }

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Wolfram · May 27, 2011 at 01:30 PM 0

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This is the way to go.

ProtoTerminator · Oct 09, 2016 at 08:41 PM 2

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This isn't the best approach, as it will create a new Vector3 every time the property is called. A better approach using the property method would be to make a private setter and set it in the class's constructor or Awake function.

 public Vector3 up { 
     get;
     private set;
  }

Ziplock9000 · Jul 25, 2017 at 01:13 PM 0

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This misses the whole point of trying to use a constant if it's going to be calculating and creating a new Vector 3 each time.

Bunny83 Ziplock9000 · Jul 25, 2017 at 02:01 PM 3

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No, this is like a true constant. Vector3 is a value type. So any time you "pass around" a Vector3 it is copied anyways. new Vector3 does not "create" anything. It just initializes the value type. It doesn't allocate any memory. Vector3.up is literally implemented like this:

 // copied from decompiled UnityEngine.dll
 public static Vector3 up
 {
     get
     {
         return new Vector3(0f, 1f, 0f);
     }
 }

How the actual native code will look like might depend on the order of optimisation but a value type constructor like this is most likely inlined. So in the end there would be no real difference between using new Vector3 or just assigning the value from another variable.

You can even initialize value types manually as long as you initialize all fields:

 Vector3 v;
 v.x = 0;
 v.y = 1;
 v.z = 0;
 // use v here

Tortuap Bunny83 · Feb 22, 2022 at 02:56 PM 0

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Sadly even trivial properties are hardly inlined : see https://docs.unity3d.com/Manual/BestPracticeUnderstandingPerformanceInUnity8.html, chapter "Trivial properties".

_dns_ · Jul 25, 2017 at 02:09 PM 2

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@ProtoTerminator @Ziplock9000 Vector3 is a value type and will be created on the stack to be returned to the caller, not allocated on the heap and then not being garbage collected later. The only thing that happens is the copy of the 3 floats of the Vector3 struct, nothing else (well, there is still a function call that would not be here for a simple type like an int or a float, might be optimized by the compiler though). The "new" syntax can be misleading with value type.

EDIT: arf, @Bunny83 is always faster to reply... I guess I'll have to change name for Turtle83 :)

Bergquist · Sep 15, 2020 at 03:03 PM 0

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This answer is not correct.

The compiler will not accept this as a compile time constant, and cannot be used in replacement of a const where a const is required, for example as an optional parameter.

The type 'UnityEngine.Vector3' cannot be declared const

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