You can look at the reference source and see that it has a few extra things to worry about, notably parameter validation and endianness worries:
public static unsafe short ToInt16(byte[] value, int startIndex) {
if( value == null) {
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.value);
}
if ((uint) startIndex >= value.Length) {
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.startIndex, ExceptionResource.ArgumentOutOfRange_Index);
}
if (startIndex > value.Length -2) {
ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
}
Contract.EndContractBlock();
fixed( byte * pbyte = &value[startIndex]) {
if( startIndex % 2 == 0) { // data is aligned
return *((short *) pbyte);
}
else {
if( IsLittleEndian) {
return (short)((*pbyte) | (*(pbyte + 1) << 8)) ;
}
else {
return (short)((*pbyte << 8) | (*(pbyte + 1)));
}
}
}
}