# Check for compile-time constraints


Use the [`IsConstantExpression`](/engine/6000.6/script-reference/unity/burst/compilerservices/constant/isconstantexpression.md) intrinsic to check if a given expression is constant at compile-time:

```lang-cs
using static Unity.Burst.CompilerServices.Constant;

var somethingWhichWillBeConstantFolded = math.pow(42.0f, 42.0f);

if (IsConstantExpression(somethingWhichWillBeConstantFolded))
{
    // Burst knows that somethingWhichWillBeConstantFolded is a compile-time constant
}
```

This is useful to check if a complex expression is always constant folded. You can use it for optimizations for a known constant value. For example, if you want to implement a `pow`-like function for integer powers:

```lang-cs
using static Unity.Burst.CompilerServices.Constant;

public static float MyAwesomePow(float f, int i)
{
    if (IsConstantExpression(i) && (2 == i))
    {
        return f * f;
    }
    else
    {
        return math.pow(f, (float)i);
    }
}
```

The `IsConstantExpression` check means that Burst always removes the branch if `i` isn't constant, because the if condition is false. This means that if `i` is constant and is equal to 2, you can use a more efficient simple multiply instead.

The result of `IsConstantExpression` intentionally depends on the result of the optimizations being run. Therefore the result can change based on whether a function gets inlined or not. For example, in the previous case: `IsConstantExpression(i)` is false on its own, because `i` is a function argument which is not constant. However, if `MyAwesomePow` gets inlined with a constant value for `i`, then it evaluates true. If `MyAwesomePow` ends up not being inlined, then `IsConstantExpression(i)` remains false.

> **Note:**
>
> Constant folding only takes place during optimizations. If you've disabled optimizations, the intrinsic returns false.

## Additional resources

* [`[IsConstantExpression]` API reference](ScriptRef:Unity.Burst.CompilerServices.Constant.IsConstantExpression*)
* [Burst intrinsics overview](/engine/6000.6/manual/scripting/compilation-and-code-reload/script-compilation/burst/csharp-burst-intrinsics.md)