Why is this an issue?

Because serialization constructors allocate and initialize objects, security checks that are present on regular constructors must also be present on a serialization constructor. Failure to do so would allow callers that could not otherwise create an instance to use the serialization constructor to do this.

This rule raises an issue when a type implements the System.Runtime.Serialization.ISerializable interface, is not a delegate or interface, is declared in an assembly that allows partially trusted callers and has a constructor that takes a System.Runtime.Serialization.SerializationInfo object and a System.Runtime.Serialization.StreamingContext object which is not secured by a security check, but one or more of the regular constructors in the type is secured.

Noncompliant code example

using System;
using System.IO;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Formatters.Binary;
using System.Security;
using System.Security.Permissions;

[assembly: AllowPartiallyTrustedCallersAttribute()]
namespace MyLibrary
{
    [Serializable]
    public class Foo : ISerializable
    {
        private int n;

        [FileIOPermissionAttribute(SecurityAction.Demand, Unrestricted = true)]
        public Foo()
        {
           n = -1;
        }

        protected Foo(SerializationInfo info, StreamingContext context) // Noncompliant
        {
           n = (int)info.GetValue("n", typeof(int));
        }

        void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
        {
           info.AddValue("n", n);
        }
    }
}

Compliant solution

using System;
using System.IO;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Formatters.Binary;
using System.Security;
using System.Security.Permissions;

[assembly: AllowPartiallyTrustedCallersAttribute()]
namespace MyLibrary
{
    [Serializable]
    public class Foo : ISerializable
    {
        private int n;

        [FileIOPermissionAttribute(SecurityAction.Demand, Unrestricted = true)]
        public Foo()
        {
           n = -1;
        }

        [FileIOPermissionAttribute(SecurityAction.Demand, Unrestricted = true)]
        protected Foo(SerializationInfo info, StreamingContext context)
        {
           n = (int)info.GetValue("n", typeof(int));
        }

        void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
        {
           info.AddValue("n", n);
        }
    }
}

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