Why does my IJob throw on writing to NativeArray?

You marked it [ReadOnly]. The safety system catches the write. Remove the attribute, or use [WriteOnly] for output-only buffers; the safety system then disallows reads.

What's the benefit of [ReadOnly] / [WriteOnly]?

They let multiple parallel jobs share NativeArrays without serializing. Multiple jobs can read the same [ReadOnly] in parallel; only writers conflict.

Can a single field be both?

No — pick one. Read-modify-write workflows need a regular (RW) field, which serializes job execution against any other accessor of the buffer.

Fix: Jobs ReadOnly Attribute Throws On Write

Quick answer: Remove [ReadOnly] or replace with [WriteOnly] for output buffers. Pick the attribute that matches actual access pattern.

You declare an output array [ReadOnly] by reflex. Inside Execute you write to it. Burst safety throws.

The Fix

public struct ApplyVelocity : IJobParallelFor {
    [ReadOnly]  public NativeArray<float3> Velocities;  // read-only input
    [WriteOnly] public NativeArray<float3> OutPositions; // write-only output
                public float Dt;

    public void Execute(int i) {
        OutPositions[i] = Velocities[i] * Dt;
    }
}

For RW (read then write same index), drop both attributes — safety serializes the buffer against other consumers.

Verifying

Schedule job. No exception. Two parallel jobs reading the same Velocities run concurrently. Two jobs writing to OutPositions serialize.

“Right attribute. Right access. Safety happy.”

Related Issues

For NativeArray dispose, see dispose timing. For Burst determinism, see determinism.

Mark intent. Safety green.