【IT168 技术文档】契约式编程不是一门崭新的编程方法论。C/C++ 时代早已有之。Microsoft 在 .NET 4.0 中正式引入契约式编程库。博主以为契约式编程是一种相当不错的编程思想，每一个开发人员都应该掌握。它不但可以使开发人员的思维更清晰，而且对于提高程序性能很有帮助。值得一提的是，它对于并行程序设计也有莫大的益处。
我们先看一段很简单的，未使用契约式编程的代码示例。
// .NET 代码示例
public class RationalNumber
{
private int numberator;
private int denominator;
public RationalNumber(int numberator, int denominator)
{
this.numberator = numberator;
this.denominator = denominator;
}
public int Denominator
{
get
{
return this.denominator;
}
}
}
public class RationalNumber
{
private int numberator;
private int denominator;
public RationalNumber(int numberator, int denominator)
{
this.numberator = numberator;
this.denominator = denominator;
}
public int Denominator
{
get
{
return this.denominator;
}
}
}
上述代码表示一个在 32 位有符号整型范围内的有理数。数学上，有理数是一个整数 a 和一个非零整数 b 的比，通常写作 a/b，故又称作分数(题外话：有理数这个翻译真是够奇怪)。由此，我们知道，有理数的分母不能为 0 。所以，上述代码示例的构造函数还需要写些防御性代码。通常 .NET 开发人员会这样写：
// .NET 代码示例
public class RationalNumber
{
private int numberator;
private int denominator;
public RationalNumber(int numberator, int denominator)
{
if (denominator == 0)
throw new ArgumentException("The second argument can not be zero.");
this.numberator = numberator;
this.denominator = denominator;
}
public int Denominator
{
get
{
return this.denominator;
}
}
}
public class RationalNumber
{
private int numberator;
private int denominator;
public RationalNumber(int numberator, int denominator)
{
if (denominator == 0)
throw new ArgumentException("The second argument can not be zero.");
this.numberator = numberator;
this.denominator = denominator;
}
public int Denominator
{
get
{
return this.denominator;
}
}
}
下面我们来看一下使用契约式编程的 .NET 4.0 代码示例。为了更加方便的说明，博主在整个示例上都加了契约，但此示例并非一定都加这些契约。
// .NET 代码示例
public class RationalNumber
{
private int numberator;
private int denominator;
public RationalNumber(int numberator, int denominator)
{
Contract.Requires(denominator != 0, "The second argument can not be zero.");
this.numberator = numberator;
this.denominator = denominator;
}
public int Denominator
{
get
{
Contract.Ensures(Contract.Result<int>() != 0);
return this.denominator;
}
}
[ContractInvariantMethod]
protected void ObjectInvariant()
{
Contract.Invariant(this.denominator != 0);
}
}
public class RationalNumber
{
private int numberator;
private int denominator;
public RationalNumber(int numberator, int denominator)
{
Contract.Requires(denominator != 0, "The second argument can not be zero.");
this.numberator = numberator;
this.denominator = denominator;
}
public int Denominator
{
get
{
Contract.Ensures(Contract.Result<int>() != 0);
return this.denominator;
}
}
[ContractInvariantMethod]
protected void ObjectInvariant()
{
Contract.Invariant(this.denominator != 0);
}
}
