How to convert a structure to a byte array in C#?

I know this is really late, but with C# 7.3 you can do this for unmanaged structs or anything else that's unmanged (int, bool etc...):

public static unsafe byte[] ConvertToBytes<T>(T value) where T : unmanaged {
        byte* pointer = (byte*)&value;

        byte[] bytes = new byte[sizeof(T)];
        for (int i = 0; i < sizeof(T); i++) {
            bytes[i] = pointer[i];
        }

        return bytes;
    }

Then use like this:

struct MyStruct {
        public int Value1;
        public int Value2;
        //.. blah blah blah
    }

    byte[] bytes = ConvertToBytes(new MyStruct());

Almost all of the answers here use Marshal.StructureToPtr, which might be good for P/Invoke but it is very slow, and doesn't even always represent the actual raw content of the value. @Varscott128's answer is much better but it also contains an explicit byte copying, which is not necessary.

For unmanaged structs (structs without managed references) all you need is to reinterpret the allocated result array so a simple assignment does the trick (works even for huge structs):

.NET (Core) Solution:

If you can use the Unsafe class, then the solution is really easy. The unsafe modifier is required only due to sizeof(T).

public static unsafe byte[] SerializeValueType<T>(in T value) where T : unmanaged
{
    byte[] result = new byte[sizeof(T)];
    Unsafe.As<byte, T>(ref result[0]) = value;
    return result;
}

// Note: Validation is omitted for simplicity
public static T DeserializeValueType<T>(byte[] data) where T : unmanaged
    => return Unsafe.As<byte, T>(ref data[0]);

.NET Framework/Standard Solution:

public static unsafe byte[] SerializeValueType<T>(in T value) where T : unmanaged
{
    byte[] result = new byte[sizeof(T)];
    fixed (byte* dst = result)
        *(T*)dst = value;
    return result;
}

// Note: Validation is omitted for simplicity
public static unsafe T DeserializeValueType<T>(byte[] data) where T : unmanaged
{
    fixed (byte* src = data)
        return *(T*)src;
}

See the complete code with validations here.

Remarks:

The OP's example contains a string, which is a reference type so the solution above cannot be used for that. And if you can't use generic methods for some reason things start to get more complicated, especially for .NET Framework (but non-generic size calculation is a pain also on the Core platform). If performance does not matter, then you can revert to Marshal.SizeOf and StructureToPtr as suggested by several other answers, or feel free to use the BinarySerializer.SerializeValueType method from my library that I linked also for the examples above (NuGet).

I've come up with a different approach that could convert any struct without the hassle of fixing length, however the resulting byte array would have a little bit more overhead.

Here is a sample struct:

[StructLayout(LayoutKind.Sequential)]
public class HelloWorld
{
    public MyEnum enumvalue;
    public string reqtimestamp;
    public string resptimestamp;
    public string message;
    public byte[] rawresp;
}

As you can see, all those structures would require adding the fixed length attributes. Which could often ended up taking up more space than required. Note that the LayoutKind.Sequential is required, as we want reflection to always gives us the same order when pulling for FieldInfo. My inspiration is from TLV Type-Length-Value. Let's have a look at the code:

public static byte[] StructToByteArray<T>(T obj)
{
    using (MemoryStream ms = new MemoryStream())
    {
        FieldInfo[] infos = typeof(T).GetFields(BindingFlags.Public | BindingFlags.Instance);
        foreach (FieldInfo info in infos)
        {
            BinaryFormatter bf = new BinaryFormatter();
            using (MemoryStream inms = new MemoryStream()) {

                bf.Serialize(inms, info.GetValue(obj));
                byte[] ba = inms.ToArray();
                // for length
                ms.Write(BitConverter.GetBytes(ba.Length), 0, sizeof(int));

                // for value
                ms.Write(ba, 0, ba.Length);
            }
        }

        return ms.ToArray();
    }
}

The above function simply uses the BinaryFormatter to serialize the unknown size raw object, and I simply keep track of the size as well and store it inside the output MemoryStream too.

public static void ByteArrayToStruct<T>(byte[] data, out T output)
{
    output = (T) Activator.CreateInstance(typeof(T), null);
    using (MemoryStream ms = new MemoryStream(data))
    {
        byte[] ba = null;
        FieldInfo[] infos = typeof(T).GetFields(BindingFlags.Public | BindingFlags.Instance);
        foreach (FieldInfo info in infos)
        {
            // for length
            ba = new byte[sizeof(int)];
            ms.Read(ba, 0, sizeof(int));

            // for value
            int sz = BitConverter.ToInt32(ba, 0);
            ba = new byte[sz];
            ms.Read(ba, 0, sz);

            BinaryFormatter bf = new BinaryFormatter();
            using (MemoryStream inms = new MemoryStream(ba))
            {
                info.SetValue(output, bf.Deserialize(inms));
            }
        }
    }
}

When we want to convert it back to its original struct we simply read the length back and directly dump it back into the BinaryFormatter which in turn dump it back into the struct.

These 2 functions are generic and should work with any struct, I've tested the above code in my C# project where I have a server and a client, connected and communicate via NamedPipeStream and I forward my struct as byte array from one and to another and converted it back.

I believe my approach might be better, since it doesn't fix length on the struct itself and the only overhead is just an int for every fields you have in your struct. There are also some tiny bit overhead inside the byte array generated by BinaryFormatter, but other than that, is not much.