[ThreadStatic]
private static byte[] _threadSafeTempByteBuffer;

/* later */

byte[] tempBytes = _threadSafeTempByteBuffer;
if (tempBytes == null)
{
    tempBytes = _threadSafeTempByteBuffer = new byte[sizeof(uint)];
}

do
{
    rng.GetBytes(tempBytes);
    result = mask & Convert.ToUInt32(tempBytes);
} while (result > range);