c# 扩展方法奇思妙用基础篇七：IsBetween 通用扩展

相信大家在看了本文的题目之后，马上就能写出 IsBetween 扩展来：

public static bool IntIsBetween(this int i, int lowerBound, int upperBound, 
    bool includeLowerBound = false, bool includeUpperBound = false)
{
    return (includeLowerBound && i == lowerBound) ||
        (includeUpperBound && i == upperBound) ||
        (i > lowerBound && i < upperBound);
}

大家的写法可能不尽相同，但效果都是一样的，以上可能是最精简的写法了吧，类似可以写出其它数据类型 IsBetween 来：

public static bool IntIsBetween(this long i, long lowerBound, long upperBound,
bool includeLowerBound = false, bool includeUpperBound = false)
{
    return (includeLowerBound && i == lowerBound) ||
        (includeUpperBound && i == upperBound) ||
        (i > lowerBound && i < upperBound);
}
public static bool DoubleIsBetween(this float i, float lowerBound, float upperBound,
        bool includeLowerBound = false, bool includeUpperBound = false)
{
    return (includeLowerBound && i == lowerBound) ||
        (includeUpperBound && i == upperBound) ||
        (i > lowerBound && i < upperBound);
}

不过这样写下去可是很罗嗦的，.net 中的数值类型太多了，除了上面几个还有：short、long、ushort、uint、ulong、double、decimal…

此外，我们还有其它要类型如：string、DateTime 等也要进行 IsBetween 判断，一个一个写来不优雅。

泛型版本 IsBetween 扩展

.Net 中 IComparable<T> 接口 定义了值类型或类的通用比较方法，借助于此接口，写一个通用的 IsBetween 就容易多了：

public static bool IsBetween<T>(this T t, T lowerBound, T upperBound,
    bool includeLowerBound = false, bool includeUpperBound = false)
        where T: IComparable<T>
{
    if(t == null) throw new ArgumentNullException("t");

    var lowerCompareResult = t.CompareTo(lowerBound);
    var upperCompareResult = t.CompareTo(upperBound);

    return (includeLowerBound && lowerCompareResult == 0) ||
        (includeUpperBound && upperCompareResult == 0) ||
        (lowerCompareResult > 0 && upperCompareResult < 0);
}

这个是相当通用的，支持 .net 中的所有数值类型，也支持 string、DateTime，因为它们都实现了 IComparable<T> 接口，下面是一些调用代码：

//int
bool b0 = 3.IsBetween(1, 5);
bool b1 = 3.IsBetween(1, 3, includeUpperBound: true);
bool b2 = 3.IsBetween(3, 5, includeLowerBound: true);
//double
bool b3 = 3.14.IsBetween(3.0, 4.0);
//string
bool b4 = "ND".IsBetween("NA", "NC");
//DateTime
bool b5 = new DateTime(2011, 2, 17).IsBetween(new DateTime(2011, 1, 1), new DateTime(2011, 3, 1));

带 IComparer<T> 参数的 IsBetween 扩展

上面的扩展很通用，但还不够好，比如下面的 Person 类：

public class Person
{
    public string Name { get; set; }
    public DateTime Birthday { get; set; }
}

没有实现任何 IComparable<T> 接口，但我们需要确定某人是否界于另外两人这间，根据生日或其它条件判断。

使用 .Net 中另外一个关于通用比较的接口 IComparer<T> 接口，我们再创建一个扩展：

public static bool IsBetween<T>(this T t, T lowerBound, T upperBound, IComparer<T> comparer, 
    bool includeLowerBound = false, bool includeUpperBound = false)
{
    if (comparer == null) throw new ArgumentNullException("comparer");

    var lowerCompareResult = comparer.Compare(t, lowerBound);
    var upperCompareResult = comparer.Compare(t, upperBound);

    return (includeLowerBound && lowerCompareResult == 0) ||
        (includeUpperBound && upperCompareResult == 0) ||
        (lowerCompareResult > 0 && upperCompareResult < 0);
}

这个扩展需要一个 IComparer<T> 的实例，我们先来实现人员生日的“比较器” ：

public class PersonBirthdayComparer : IComparer<Person>
{
    public int Compare(Person x, Person y)
    {
        return Comparer<DateTime>.Default.Compare(x.Birthday, y.Birthday);
    }
}

人员生日 IsBetween 示例代码如下：

var p1= new Person{ Name = "鹤冲天",  Birthday  = new DateTime(1990, 1, 1)};
var p2= new Person{ Name = "鹤中天",  Birthday  = new DateTime(2000, 1, 1)};
var p3= new Person{ Name = "鹤微天",  Birthday  = new DateTime(2010, 1, 1)};
bool b6 = p2.IsBetween(p1, p3, new PersonBirthdayComparer());

类似，我们还可以对人员进行身高、体重、人品等其它方面的 IsBetween 判断，只需要传入不同的 comparer。

针对 IComparable<T> 接口的 IsBetween 扩展

之前一篇文章中，我曾说过要针对接口扩展，我们何不针对 IComparable<T> 接口 进行扩展呢：

public static bool IsBetween<T>(this IComparable<T> t, T lowerBound, T upperBound,
    bool includeLowerBound = false, bool includeUpperBound = false)
{
    if (t == null) throw new ArgumentNullException("t");

    var lowerCompareResult = t.CompareTo(lowerBound);
    var upperCompareResult = t.CompareTo(upperBound);

    return (includeLowerBound && lowerCompareResult == 0) ||
        (includeUpperBound && upperCompareResult == 0) ||
        (lowerCompareResult > 0 && upperCompareResult < 0);
}

这个扩展的应用场景可能不多，这里举一个方便大家理解，假定我们自定义了一个大整数类形：

public class BigInt: IComparable<int>, IComparable<double>
{
    //...
    public int CompareTo(int other)
    {
        //...
    }
    public int CompareTo(double other)
    {
        //...
    }
}

因为它实 IComparable<int> 和 IComparable<double>，所以可以和 int 和 double 进行比较，则可以如下使用：

BigInt bi = new BigInt();
bool b8 = bi.IsBetween(10, 20);
bool b9 = bi.IsBetween(1.424E+12, 2.3675E+36);

( .Net 中已经有了大整数类型，请参见：BigInteger 结构，不过没有实现 IComparable<int> 和 IComparable<double> )

其它比较扩展

有了上面的 IsBetween 扩展，再写其它比较扩展就易如反掌了，比如下面几个：

• LessThan
• LessOrEquals
• GreatOrEquals
• GreatThan