How do the In and Out attributes work in .NET?

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This is a remarkably poorly documented feature of .NET Remoting. It doesn’t have anything to do with whether your class is [Serializable] or derived from MarshalByRefObject. At issue here is how the argument is marshaled across the AppDomain boundary. The call itself is made under the hood by Remoting. Arrays do not automatically get marshaled back after the call, clearly a performance optimization. Only the [Out] attribute is required, [In] is implied. I could not find any relevant documentation about this in MSDN, just a blog post from somebody that ran into the same issue (scroll down to “Using OutAttribute in Remoting”).

Some code to play with:

using System;
using System.Runtime.InteropServices;

class Program {
    static void Main(string[] args) {
        var ad = AppDomain.CreateDomain("second");
        var t = (Test)ad.CreateInstanceAndUnwrap(typeof(Test).Assembly.FullName, typeof(Test).FullName);
        var b = new byte[] { 1 };
        t.Read(b);
        System.Diagnostics.Debug.Assert(b[0] == 2);
    }
}

class Test : MarshalByRefObject {
    public void Read([Out]byte[] arg) {
        arg[0] *= 2;
    }
}

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