Convert byte array to generic type value in C#

Question

I would like to have a helper method for converting byte array to generic type value:

public static T BytesToValue<T>(byte[] bytes)
{
    int pos = 0;
    T result = default;
    foreach (byte b in bytes)
    {
        //Cannot convert from type byte to T error here
        result |= ((T)b) << pos;
        pos += 8;
    }
    return result;
}

The problem is that the compiler gives the error.

The method will primarily be used for getting int and long values and performance is very critical.

How can this be fixed?

Are you trying to implement something like BitConverter.ToInt64(byte[], int)? — dymanoid
– dymanoid, Commented Oct 6, 2019 at 17:01
This isn't going to work. You also can't apply arithmetic operators to values with generic types, so the << won't work either. — user555045
– user555045, Commented Oct 6, 2019 at 17:02
May be the answer on this topic helps: stackoverflow.com/questions/33616621/… — hardaxan
– hardaxan, Commented Oct 6, 2019 at 17:20
dymanoid, yes. I'm trying to implement an alternative to BitConverter becuase the latter is slow compared to bitwise operations. — yaugenka
– yaugenka, Commented Oct 6, 2019 at 18:16
@yaugenka I'd be very surprised. On my hardware, your method takes 7 ns to execute and BitConverter.ToInt64 takes 3 ns. BitConverter.ToInt64 uses unsafe context (so no bound-check) and loop unrolling, there's no way your version is going to be faster — Kevin Gosse
– Kevin Gosse, Commented Oct 6, 2019 at 18:45

score 1 · Accepted Answer · 2019-10-06 18:53:17Z

1

Bitwise operators can't be used on generic type parameters.

Even this simple cast does not compile:

result = (T)b;

But we can write this that compiles (usefull for other case):

result = (T)Convert.ChangeType(b, typeof(T));

So this does not compile:

result |= ( (T)Convert.ChangeType(b, typeof(T)) ) << pos;

edited Oct 6, 2019 at 18:53

answered Oct 6, 2019 at 17:06

user12031933

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6 Comments

yaugenka Over a year ago

If I use ((int)b) << pos it works, so why would ((T)b) << pos not work when T is of type int?

user12031933 Over a year ago

T (generic type parameter) can be any type. We can only operate on integer numbers because of the architecture of the processor that uses bytes as units. Applying bitwise operators on references is not allowed as it will change the memory address of an instance of a type unless overloading these operators to act on an embedded value. Hence bitwise operators can't be used on generic type parameters that can match any reference of an instance of a type. And C# uses immutable for programmers references, not pointers. ((int)b) << pos compiles because it is a integer number value type.

yaugenka Over a year ago

Then the suggested Convert.ChangeType should not work either, should it? It gives "Operand << cannot be applied to operands of type T and int" error with it.

user12031933 Over a year ago

Indeed. Answer updated to be more clear about what I tried to say.

yaugenka Over a year ago

If I use ulong type for the bitwise operation itself and then cast the result to a desired type (which is either Int or Long in my case) as suggested, will performance suffer? Or should I better have two separate methods for Int and Long convertions? The reason I ask this is that performance is really critical. I have gigabytes of bynary 24 and 40-bit data which need to be converted to Int and Long respectively quite often.

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