本教程展示如何定义转换以及如何在类或结构之间使用转换。
示例文件
请参见“用户定义的转换”示例以下载和生成本教程中讨论的示例文件。
教程
C# 允许程序员在类或结构上声明转换，以便可以使类或结构与其他类或结构或者基本类型相互进行转换。转换的定义方法类似于运算符，并根据它们所转换到的类型命名。
在 C# 中，可以将转换声明为 implicit（需要时自动转换）或 explicit（需要调用转换）。所有转换都必须为 static，并且必须采用在其上定义转换的类型，或返回该类型。
本教程介绍两个示例。第一个示例展示如何声明和使用转换，第二个示例演示结构之间的转换。
示例 1
本示例中声明了一个 RomanNumeral 类型，并定义了与该类型之间的若干转换。
// conversion.cs
using System;
struct RomanNumeral
{
public RomanNumeral(int value)
{
this.value = value;
}
// Declare a conversion from an int to a RomanNumeral. Note the
// the use of the operator keyword. This is a conversion
// operator named RomanNumeral:
static public implicit operator RomanNumeral(int value)
{
// Note that because RomanNumeral is declared as a struct,
// calling new on the struct merely calls the constructor
// rather than allocating an object on the heap:
return new RomanNumeral(value);
}
// Declare an explicit conversion from a RomanNumeral to an int:
static public explicit operator int(RomanNumeral roman)
{
return roman.value;
}
// Declare an implicit conversion from a RomanNumeral to
// a string:
static public implicit operator string(RomanNumeral roman)
{
return("Conversion not yet implemented");
}
private int value;
}
class Test
{
static public void Main()
{
RomanNumeral numeral;
numeral = 10;
// Call the explicit conversion from numeral to int. Because it is
// an explicit conversion, a cast must be used:
Console.WriteLine((int)numeral);
// Call the implicit conversion to string. Because there is no
// cast, the implicit conversion to string is the only
// conversion that is considered:
Console.WriteLine(numeral);
// Call the explicit conversion from numeral to int and
// then the explicit conversion from int to short:
short s = (short)numeral;
Console.WriteLine(s);
}
}
输出
10 Conversion not yet implemented 10
示例 2
本示例定义 RomanNumeral 和 BinaryNumeral 两个结构，并演示二者之间的转换。
// structconversion.cs
using System;
struct RomanNumeral
{
public RomanNumeral(int value)
{
this.value = value;
}
static public implicit operator RomanNumeral(int value)
{
return new RomanNumeral(value);
}
static public implicit operator RomanNumeral(BinaryNumeral binary)
{
return new RomanNumeral((int)binary);
}
static public explicit operator int(RomanNumeral roman)
{
return roman.value;
}
static public implicit operator string(RomanNumeral roman)
{
return("Conversion not yet implemented");
}
private int value;
}
struct BinaryNumeral
{
public BinaryNumeral(int value)
{
this.value = value;
}
static public implicit operator BinaryNumeral(int value)
{
return new BinaryNumeral(value);
}
static public implicit operator string(BinaryNumeral binary)
{
return("Conversion not yet implemented");
}
static public explicit
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operator int(BinaryNumeral binary)
{
return(binary.value);
}
private int value;
}
class Test
{
static public void Main()
{
RomanNumeral roman;
roman = 10;
BinaryNumeral binary;
// Perform a conversion from a RomanNumeral to a
// BinaryNumeral:
binary = (BinaryNumeral)(int)roman;
// Performs a conversion from a BinaryNumeral to a RomanNumeral.
// No cast is required:
roman = binary;
Console.WriteLine((int)binary);
Console.WriteLine(binary);
}
}
输出
10 Conversion not yet implemented
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