c#中string拼接的效率分析

通过unity profiler测试的代码，及运行结果

using UnityEngine;
using UnityEngine.Profiling;
using System.Text;

#if UNITY_5_5_OR_NEWER
using TProfiler = UnityEngine.Profiling.Profiler;
#else
using TProfiler = UnityEngine.Profiler;
#endif

public namespace Test {

    /// string concat test
    public class TestString : MonoBehaviour {
        // Update is called once per frame
        void Update ()
        {
            // loop 10000times
            for(int i = 0; i < 10000; ++i) {

                TestString();
            }

            TestString10000();
        }

        // object new once
        private StringBuilder _sb = new StringBuilder();

        /// concat 10000 times
        private void TestString10000() {

            // +
            TProfiler.BeginSample("string.(10000+)");
            string s1 = string.Empty;
            for(int i = 0; i < 10000; ++i) {
                s1 = s1 + ((int)Time.time).ToString();
            }
            TProfiler.EndSample();


            // StringBuilder Append
            TProfiler.BeginSample("StringBuilder.10000Append");
            _sb.Clear();
            for(int i = 0; i < 10000; ++i) {
                _sb.Append((int)Time.time);
            }
            string s2 = _sb.ToString();
            TProfiler.EndSample();
        }

        /// concat 3 parts
        private void TestString() {

            System.Random random = new System.Random();

            // +
            TProfiler.BeginSample("string.(+)");
            string s1 = "txt_pre_" + random.Next().ToString() + "_end";
            TProfiler.EndSample();

            // Concat
            TProfiler.BeginSample("string.Concat");
            string s2 = string.Concat("txt_pre_", random.Next(), "_end");
            TProfiler.EndSample();

            // Concat ToString()
            TProfiler.BeginSample("string.Concat & int.ToString");
            string s3 = string.Concat("txt_pre_", ((int)Time.time).ToString(), "_end");
            TProfiler.EndSample();

            // format
            TProfiler.BeginSample("string.Format");
            string s4 = string.Format("txt_pre_{0}_end", (int)Time.time);
            TProfiler.EndSample();

            // format ToString()
            TProfiler.BeginSample("string.Format & int.ToString");
            string s5 = string.Format("txt_pre_{0}_end", ((int)Time.time).ToString());
            TProfiler.EndSample();

            // StringBuilder AppendFormat
            TProfiler.BeginSample("StringBuilder.AppendFormat & int.ToString");
            _sb.Clear();
            _sb.AppendFormat("txt_pre_{0}_end", ((int)Time.time).ToString());
            string s6 = _sb.ToString();
            TProfiler.EndSample();

            // StringBuilder Append
            TProfiler.BeginSample("StringBuilder.Append");
            _sb.Clear();
            _sb.Append("txt_pre_");
            _sb.Append((int)Time.time);
            _sb.Append("_end");
            string s7 = _sb.ToString();
            TProfiler.EndSample();

            // StringBuilder Append ToString()
            TProfiler.BeginSample("StringBuilder.Append & int.ToString");
            _sb.Clear();
            _sb.Append("txt_pre_");
            _sb.Append(((int)Time.time).ToString());
            _sb.Append("_end");
            string s8 = _sb.ToString();
            TProfiler.EndSample();

            // StringBuilder AppendNumber
            TProfiler.BeginSample("StringBuilder.AppendNumber");
            _sb.Clear();
            _sb.Append("txt_pre_");
            _sb.AppendNumber((int)Time.time);
            _sb.Append("_end");
            string s9 = _sb.ToString();
            TProfiler.EndSample();
        }
    }
}

运行结果： 

结果分析及结论： 

1、string.Format和StringBuider.AppendFormat()

可读性好 
堆内存开销高，因为Format需要额外的内存分配 
运行效率低 
建议： 
【少用，慎用】 

2、StringBuilder.Append

可读性一般 
堆内存开销低 
运行效率较高 
建议： 
大量字符串拼接时【必须使用】，内存及运行效率优化效果十分明显 
少量字符串拼接时【酌情使用】，如调用频度 

3、+和Concat

可读性较好 
堆内存开销低 
运行效率较高 
在很多情况下，+会被优化为Concat 
建议： 
大量字符串拼接时【禁止使用】，内存及运行效率开销非常大，无法忍受 
少量字符串拼接时【建议使用】 
另外，对于string拼接时的int等基本数据类型，养成ToString()的习惯，避免Box操作带来的额外内存开销。

一个通用的int(Enum)转string的缓存方案

/// Sample for SimpleDictCache
    ///    private static SimpleDictCache<EnumType> nameCache;
    /// 指挥官头像icon
    //    public static string GetName(EnumType id)
    //    {
    //        if (nameCache == null)
    //            nameCache = new SimpleDictCache<EnumType>();
    //
    //        return nameCache.Get((int)id, id, (o) => {
    //            return o.ToString(); // 此处可以替换为复杂的获取string的实现
    //        });
    //    }
    /// 通用Dictionary<int, string>缓存
    /// 比如enum对应的string可以用cache方式避免反复ToString()操作
    class SimpleDictCache<T>
    {
        public delegate string CacheValue(T id);
    
        private Dictionary<int, string> data;
    
        public string Get(int k, T id, CacheValue cv)
        {
            if (data == null)
                data = new Dictionary<int, string>();
    
            if (data.ContainsKey(k))
            {
                return data[k];
            }
    
            if (cv != null)
                data.Add(k, cv(id));
            else
                data.Add(k, string.Empty);
    
            return data[k];
        }
    }