C#•2y 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•2y 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•2y ago

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

Developful•2y ago

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

AndyBOP•2y ago

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

Developful•2y 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•2y 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•2y ago

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

AndyBOP•2y ago

Go on...

jiniux•2y 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•17mo ago

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

AndyBOP•17mo 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•17mo ago

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

AndyBOP•17mo ago

I'm using .NET8

jiniux•17mo ago

you should be fine

AndyBOP•17mo ago

Looks simpler, and no try/catch needed?

jiniux•17mo ago

nope it's just a cast

AndyBOP•17mo ago

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

jiniux•17mo 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•17mo 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•17mo 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•17mo 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•17mo ago

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

AndyBOP•17mo 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•17mo ago

what about this.. what?

hiyosilver•17mo ago

This is some code containing several words and special characters.

Accord•17mo 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?

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