C#•16mo ago

❔ Is `where` still used?

I'm looking at creating some functions to convert byte[] to Structures and back, and all examples I see are very old. They use the keyword "where" which I've never seen before. Is this still current or has it been depricated? e.g.

private static byte[] StructToBytes<T>(T data) where T : struct
{
    byte[] rawData = new byte[Marshal.SizeOf(data)];
    GCHandle handle = GCHandle.Alloc(rawData, GCHandleType.Pinned);
    try
    {
        IntPtr rawDataPtr = handle.AddrOfPinnedObject();
        Marshal.StructureToPtr(data, rawDataPtr, false);
    }
    finally
    {
        handle.Free();
    }

    return rawData;
}

private static byte[] StructToBytes<T>(T data) where T : struct
{
    byte[] rawData = new byte[Marshal.SizeOf(data)];
    GCHandle handle = GCHandle.Alloc(rawData, GCHandleType.Pinned);
    try
    {
        IntPtr rawDataPtr = handle.AddrOfPinnedObject();
        Marshal.StructureToPtr(data, rawDataPtr, false);
    }
    finally
    {
        handle.Free();
    }

    return rawData;
}

26 Replies

Developful•16mo ago

the where keyword (it's not deprecated) constraints T, in this case, to be a struct. for example I could also constraint T to be part of a class named Node for that, i'd just type where T : Node so if you try to pass in something that is not a struct, it'll give you an error

AndyBOP•16mo ago

Copy, TY. So are you saying the code would still work without the where but using it will help in debugging?

Developful•16mo ago

in some cases it would work, in some it wouldn't but no, it wouldnt work

AndyBOP•16mo ago

Ok. For some reason, I hadn't seen that keyword before.

Developful•16mo ago

because if you remove the struct constraint, if the code expects a struct anywhere within the functions it wont be sure that T is a struct, and therefore will give you an error

AndyBOP•16mo ago

Would you say this is still a valid way to copy bytes into a fixed structure, or are there more modern (.NET 8) ways to do this? Assuming the struct is formed correctly with LayoutKind.Sequential and so on...

jiniux•16mo ago

you could use a span to avoid the heap allocation of the byte array

AndyBOP•16mo ago

Go on...

jiniux•16mo ago

https://stackoverflow.com/questions/50285147/how-get-a-spanbyte-view-of-a-struct-without-the-unsafe-keyword take a look here

Stack Overflow

How get a Span view of a struct without the unsafe keyword

How can a Span<byte> view (reinterpret cast) be created from a single struct value with no copying, no allocations, and without the unsafe keyword.
I can currently only accomplish this usi...

jiniux•16mo ago

keep in mind that a span is a reference to existing memory

AndyBOP•16mo ago

TY. It says "partial solution", is this valid code?

using System.Runtime.InteropServices;

public Span<byte> StructToBytes<T>(ref T data) where T : unmanaged
{
    Span<T> dataSpan = MemoryMarshal.CreateSpan(ref data, 1);
    return MemoryMarshal.Cast<T, byte>(dataSpan);
}

using System.Runtime.InteropServices;

public Span<byte> StructToBytes<T>(ref T data) where T : unmanaged
{
    Span<T> dataSpan = MemoryMarshal.CreateSpan(ref data, 1);
    return MemoryMarshal.Cast<T, byte>(dataSpan);
}

Hadn't heard of MemoryMarshal before.

jiniux•16mo ago

for what i understood this works if you're not targeting netstandard2.0

AndyBOP•16mo ago

I'm using .NET8

jiniux•16mo ago

you should be fine

AndyBOP•16mo ago

Looks simpler, and no try/catch needed?

jiniux•16mo ago

nope it's just a cast

AndyBOP•16mo ago

Very cool. Just have to use Span<byte> instead of byte[] when reading the data. TY!

jiniux•16mo ago

though i don't know how safe is this code because i don't know how it is used so make good judgment whether to use byte[] or Span<byte>

AndyBOP•16mo ago

When working with hardware or embedded devices, It's a very common part of C/C++ software where you read byte data from a device and then manipulate it with a structure. C# being managed doesn't seem to have this built-in ability, so just looking to create some generic functions. Appreciate the help!

jiniux•16mo ago

there are many options to use C# in an "unmanaged way", you should take a look to the unsafe keyword, the classes MemoryMarshal, Unsafe etc... so just be careful that span is like a pointer, if the memory it points to is invalid, it will trigger an undefined behavior

Thinker•16mo ago

It's definitely not deprecated in any way. Type parameter constraints (where T :) are used all around for various purposes, everything from in your example constraining types arguments to only value types, to constraining to types implementing certain interfaces. The numeric interfaces introduced in .NET 7 hinges on constraints to be useful in the first place. Constraints are alive and well. eg.

T Sum<T>(IEnumerable<T> xs) where T : INumber<T>
{
    var result = T.Zero;
    foreach (var x in xs) result += x;
    return result;
}

var ints = new[] { 1, 2, 3 };
Console.WriteLine(Sum(ints)); // 6

var floats = new[] { 1.5f, 2.2f, 0.3f, };
Console.WriteLine(Sum(floats)); // 3.0

T Sum<T>(IEnumerable<T> xs) where T : INumber<T>
{
    var result = T.Zero;
    foreach (var x in xs) result += x;
    return result;
}

var ints = new[] { 1, 2, 3 };
Console.WriteLine(Sum(ints)); // 6

var floats = new[] { 1.5f, 2.2f, 0.3f, };
Console.WriteLine(Sum(floats)); // 3.0

TheBoxyBear•16mo ago

And you can safely pass references of T where the constraint type is expected without casting.

AndyBOP•16mo ago

Thanks everyone. TIL about “where” ! What about this?

public Span<byte> StructToBytes<T>(ref T data) where T: struct
{
   return MemoryMarshal.AsBytes<T>(data);
}

public Span<byte> StructToBytes<T>(ref T data) where T: struct
{
   return MemoryMarshal.AsBytes<T>(data);
}

Developful•16mo ago

what about this.. what?

hiyosilver•16mo ago

This is some code containing several words and special characters.

Accord•16mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Is `where` still used?