c# – 如何避免长生命字符串导致第2代垃圾收集

我试图将字符串编组成一个结构.令人惊讶的是,我仍然得到第2代GC：s.结构似乎仍然保留了对字符串的引用.完整的控制台应用程序任何帮助赞赏.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication
{
    class Program
    {

        [StructLayout(LayoutKind.Sequential)]
        public struct FixedString
        {
            [MarshalAs(UnmanagedType.ByValTStr,SizeConst = 256)]
            private string str;

            public FixedString(string str)
            {
                this.str = str;
            }
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct UTF8PackedString
        {
            private int length;

            [MarshalAs(UnmanagedType.ByValArray,SizeConst = 256)]
            private byte[] str;

            public UTF8PackedString(int length)
            {
                this.length = length;
                str = new byte[length];
            }

            public static implicit operator UTF8PackedString(string str)
            {
                var obj = new UTF8PackedString(Encoding.UTF8.GetByteCount(str));
                var bytes = Encoding.UTF8.GetBytes(str);
                Array.Copy(bytes,obj.str,obj.length);
                return obj;
            }
        }

        const int BufferSize = 1000000;
        const int LoopCount = 10000000;

        static void Main(string[] args)
        {
            Console.WriteLine("{0}t{1}t{2}t{3}t{4}","Type".PadRight(20),"Time","GC(0)","GC(1)","GC(2)");
            Console.WriteLine();
            for (int i = 0; i < 5; i++)
            {
                TestPerformance<string>(s => s);
                TestPerformance<FixedString>(s => new FixedString(s));
                TestPerformance<UTF8PackedString>(s => s);
                Console.WriteLine();
            }
            Console.ReadKey();
        }

        private static void TestPerformance<T>(Func<string,T> func)
        {
            var buffer = new T[BufferSize];
            GC.Collect(2);
            Stopwatch stopWatch = new Stopwatch();
            var initialCollectionCounts = new int[] { GC.CollectionCount(0),GC.CollectionCount(1),GC.CollectionCount(2) };
            stopWatch.Reset();
            stopWatch.Start();
            for (int i = 0; i < LoopCount; i++)
                buffer[i % BufferSize] = func(i.ToString());
            stopWatch.Stop();
            Console.WriteLine("{0}t{1}t{2}t{3}t{4}",typeof(T).Name.PadRight(20),stopWatch.ElapsedMilliseconds,(GC.CollectionCount(0) - initialCollectionCounts[0]),(GC.CollectionCount(1) - initialCollectionCounts[1]),(GC.CollectionCount(2) - initialCollectionCounts[2])
            );
        }
    }
}

编辑：使用执行所需工作的UnsafeFixedString更新代码：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication
{
    class Program
    {
        public unsafe struct UnsafeFixedString
        {
            private int length;

            private fixed char str[256];

            public UnsafeFixedString(int length)
            {
                this.length = length;
            }

            public static implicit operator UnsafeFixedString(string str)
            {
                var obj = new UnsafeFixedString(str.Length);
                for (int i = 0; i < str.Length; i++)
                    obj.str[i] = str[i];                
                return obj;
            }
        }

        const int BufferSize = 1000000;
        const int LoopCount = 10000000;

        static void Main(string[] args)
        {
            Console.WriteLine("{0}t{1}t{2}t{3}t{4}","GC(2)");
            Console.WriteLine();
            for (int i = 0; i < 5; i++)
            {
                TestPerformance(s => s);
                TestPerformance<UnsafeFixedString>(s => s);
                Console.WriteLine();
            }
            Console.ReadKey();
        }

        private static void TestPerformance<T>(Func<string,GC.CollectionCount(2) };
            stopWatch.Reset();
            stopWatch.Start();
            for (int i = 0; i < LoopCount; i++)
                buffer[i % BufferSize] = func(String.Format("{0}",i));
            stopWatch.Stop();
            Console.WriteLine("{0}t{1}t{2}t{3}t{4}",(GC.CollectionCount(2) - initialCollectionCounts[2])
            );
        }
    }
}

我的电脑输出是：

Type                    Time    GC(0)   GC(1)   GC(2)

String                  5746    160     71      19
UnsafeFixedString       5345    418     0       0