如何：將字串轉換為數值 (C# 程式設計手冊)

這些範例會示範許多不同的方法，讓您將 string 轉換為 int。舉例來說，從命令列引數取得數值輸入時，這種轉換就很有用。將字串轉換為其他數值型別還有一些類似的方法，例如 float 或 long。下表會列出這些方法。

範例

這個範例會呼叫 ToInt32(String) 方法，將輸入由 string 轉換為 int。程式可以攔截這個方法會擲回的兩個最常見的例外狀況， FormatException 和 OverflowException。如果數字可以遞增而不會造成整數儲存位置溢位，那麼程式會將結果增加 1 並列印輸出。

static void Main(string[] args)
{
    int numVal = -1;
    bool repeat = true;

    while (repeat == true)
    {
        Console.WriteLine("Enter a number between −2,147,483,648 and +2,147,483,647 (inclusive).");

        string input = Console.ReadLine();

        // ToInt32 can throw FormatException or OverflowException.
        try
        {
            numVal = Convert.ToInt32(input);
        }
        catch (FormatException e)
        {
            Console.WriteLine("Input string is not a sequence of digits.");
        }
        catch (OverflowException e)
        {
            Console.WriteLine("The number cannot fit in an Int32.");
        }
        finally
        {
            if (numVal < Int32.MaxValue)
            {
                Console.WriteLine("The new value is {0}", numVal + 1);
            }
            else
            {
                Console.WriteLine("numVal cannot be incremented beyond its current value");
            }
        }
        Console.WriteLine("Go again? Y/N");
        string go = Console.ReadLine();
        if (go == "Y" || go == "y")
        {
            repeat = true;
        }
        else
        {
            repeat = false;
        }
    }
    // Keep the console open in debug mode.
    Console.WriteLine("Press any key to exit.");
    Console.ReadKey();    
}
// Sample Output:
// Enter a number between -2,147,483,648 and +2,147,483,647 (inclusive).
// 473
// The new value is 474
// Go again? Y/N
// y
// Enter a number between -2,147,483,648 and +2,147,483,647 (inclusive).
// 2147483647
// numVal cannot be incremented beyond its current value
// Go again? Y/N
// Y
// Enter a number between -2,147,483,648 and +2,147,483,647 (inclusive).
// -1000
// The new value is -999
// Go again? Y/N
// n
// Press any key to exit.

將 string 轉換為 int 的另一個方法是透過 System.Int32 結構 (Struct) 的 Parse 或 TryParse 方法。ToUInt32 方法會在內部使用 Parse。如果字串的格式無效，Parse 會擲回例外狀況，而 TryParse 不會擲回例外狀況但傳回 false。下列範例會示範 Parse 和 TryParse 成功與不成功的呼叫。

int numVal = Int32.Parse("-105");
Console.WriteLine(numVal);
// Output: -105

// TryParse returns true if the conversion succeeded
// and stores the result in the specified variable.
int j;
bool result = Int32.TryParse("-105", out j);
if (true == result)
    Console.WriteLine(j);
else
    Console.WriteLine("String could not be parsed.");
// Output: -105

try
{
    int m = Int32.Parse("abc");
}
catch (FormatException e)
{
    Console.WriteLine(e.Message);
}
// Output: Input string was not in a correct format.

string inputString = "abc";
int numValue;
bool parsed = Int32.TryParse(inputString, out numValue);

if (!parsed)
    Console.WriteLine("Int32.TryParse could not parse '{0}' to an int.\n", inputString);

// Output: Int32.TryParse could not parse 'abc' to an int.

請參閱

工作

HOW TO：判斷字串是否表示數值 (C# 程式設計手冊)

參考

型別 (C# 程式設計手冊)