[C#] Convert(轉型)、Parse(強制轉型)、TryParse(安全轉型)與運算子轉型的差別、效能

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：

Convert(轉型)
```
Convert.ToInt32("5.5");
```
Parse(強制轉型)
```
(int)(5.5);
int.Parse("5.5");
```
TryParse(安全轉型)
```
int i = 0;
int.TryParse("5.5", out i)
```
運算子 – as
```
CustomModel model = obj as CustomModel
```
運算子 – is
```
CustomModel model = obj is CustomModel
```

根據參考資料差異如下：

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

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

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

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
}
```

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：

[C#] Convert(轉型)、Parse(強制轉型)、TryParse(安全轉型)與運算子轉型的差別、效能

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