ArrayExtensions.GetElementSize.cs

using System;
using System.Runtime.InteropServices;

public static class ArrayExtensions_Dotnet
{
#if NET5_0_OR_GREATER
    public static unsafe int GetElementSize(this Array array)
    {
        fixed (void* baseAddress = &MemoryMarshal.GetArrayDataReference(array))
        {
            // NOTE: UnsafeAddrOfPinnedArrayElement <- <pinned-array-address> + <element-index> * <element-size>
            nint elementAddress = Marshal.UnsafeAddrOfPinnedArrayElement(array, index: 1);
            return (int)(elementAddress - (nint)baseAddress);
        }

    }
#endif
}

public static class ArrayExtensions_Unity
{
    public static unsafe int GetElementSize(this Array array)
    {
        ulong handle = 0;
        try
        {
            nint baseAddress = (nint)UnsafeUtility.PinGCArrayAndGetDataAddress(array, out handle);

            // NOTE: UnsafeAddrOfPinnedArrayElement <- <pinned-array-address> + <element-index> * <element-size>
            nint elementAddress = Marshal.UnsafeAddrOfPinnedArrayElement(array, index: 1);
            return (int)(elementAddress - baseAddress);
        }
        finally
        {
            UnsafeUtility.ReleaseGCObject(handle);
        }
    }
}

public static class ArrayExtensions_UnityDotnet
{
    public static int GetElementSize(this Array array)
    {
        GCHandle handle = default;
        try
        {
            handle = GCHandle.Alloc(array, GCHandleType.Pinned);
            nint baseAddress = handle.AddrOfPinnedObject();

            // NOTE: UnsafeAddrOfPinnedArrayElement <- <pinned-array-address> + <element-index> * <element-size>
            nint elementAddress = Marshal.UnsafeAddrOfPinnedArrayElement(array, index: 1);
            return (int)(elementAddress - baseAddress);
        }
        finally
        {
            if (handle.IsAllocated)
            {
                handle.Free();
            }
        }
    }
}