最近再磨練基本功的時候複習了常用到的「轉型」，找一找資料後統整了一下才比較搞懂這些差異。

轉型別基本上分為下列五種：

1. Convert(轉型)

   Convert.ToInt32("5.5");

2. Parse(強制轉型)

   (int)(5.5);
   int.Parse("5.5");

3. TryParse(安全轉型)

   int i = 0;
   int.TryParse("5.5", out i)

4. 運算子 – as

   CustomModel model = obj as CustomModel

5. 運算子 – is

   CustomModel model = obj is CustomModel

根據參考資料差異如下：

1. Convert 是將值轉換成最接近的 32 位元帶正負號的整數

   Console.WriteLine(Convert.ToInt32("4.5")); //result = 4
   Console.WriteLine(Convert.ToInt32("4.51"));//result = 5
   Console.WriteLine(Convert.ToInt32("5.4")); //result = 5
   Console.WriteLine(Convert.ToInt32("5.5")); //result = 6

2. Parse 則是無條件捨去( long、float、double、或 decimal )

   Console.WriteLine((int)(4.5)); //result = 4
   Console.WriteLine((int)(4.51));//result = 4
   Console.WriteLine((int)(5.4)); //result = 5
   Console.WriteLine((int)(5.5)); //result = 5
   

3. TryParse 會回傳一布林值來判斷是否轉換成功

   string s1 = "1234"; 
   string s2 = "1234.65"; 
   string s3 = null; 
   string s4 = "123456789123456789123456789123456789123456789"; 
   bool success = false;    
   int result = 0; 
   success = Int32.TryParse(s1, out result); //-- success => true;  result => 1234 
   success = Int32.TryParse(s2, out result); //-- success => false; result => 0 
   success = Int32.TryParse(s3, out result); //-- success => false; result => 0 
   success = Int32.TryParse(s4, out result); //-- success => false; result => 0

4. as 若轉型失敗會回傳null，且只能用於參考類型，不能應用於值類型(除非是Nullable的值類型)。

   CustomModel model = obj as CustomModel;
   if(model  != null)
   {
       //轉型成功
   }
   else
   {
       //轉型失敗
   }

5. is 若轉型失敗則會跳Exception

   try
   {
       CustomModelt = obj is CustomModel;
   }
   catch
   {
       //轉型失敗
   }

2017/09/24 更新

(Net Core 2.0)效能上 Parse > TryParse > Convert。

測試程式碼：

class Program
{
    static void Main(string[] args)
    {
        Stopwatch sw = new Stopwatch();

        sw.Start();
        for (int i = 0; i < 10000000; i++)
        {
            Convert.ToInt32(i.ToString());
        }
        sw.Stop();

        Console.WriteLine(string.Format("Convert.ToInt32 = {0} ms", sw.ElapsedMilliseconds));
        sw.Reset();

        sw.Start();
        for (int i = 0; i < 10000000; i++)
        {
            int.Parse(i.ToString());
        }
        sw.Stop();

        Console.WriteLine(string.Format("int.Parse = {0} ms", sw.ElapsedMilliseconds));
        sw.Reset();

        sw.Start();
        for (int i = 0; i < 10000000; i++)
        {
            int.TryParse(i.ToString(), out int tmp);
        }
        sw.Stop();

        Console.WriteLine(string.Format("int.TryParse = {0} ms", sw.ElapsedMilliseconds));
        sw.Reset();

        object model = new TestModel
        {
            Tmp = 100
        };

        sw.Start();
        for (int i = 0; i < 10000000; i++)
        {
            var tmp = model as TestModel;
        }
        sw.Stop();

        Console.WriteLine(string.Format("as = {0} ms", sw.ElapsedMilliseconds));
        sw.Reset();

        sw.Start();
        for (int i = 0; i < 10000000; i++)
        {
            var tmp = model is TestModel;
        }
        sw.Stop();

        Console.WriteLine(string.Format("is = {0} ms", sw.ElapsedMilliseconds));
        sw.Reset();

        Console.ReadKey();
    }

    public class TestModel
    {
        public int Tmp { get; set; }
    }
}

參考：

1. Convert(轉型)與Int32(強制轉型)的差別、效能
2. [C#]Convert.ToInt32()與int.Parse()的差別
3. [C#]Convert、Parse、TryParse 差異
4. [C#]Effective C# 條款三： 運算子is或as優於強制轉型