[C#]使用IFormattable接口来实现字符串格式化

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开发工具：VS2017

语言：C#

DotNet版本：.Net FrameWork 4.0及以上

一、编写一个Person类，代码如下：

    class Person
    {
        public string FirstName { set; get; }
        public string LastName { set; get; }
    }

并让Person类继承IFormattable，代码如下：

    class Person:IFormattable
    {
        public string FirstName { set; get; }
        public string LastName { set; get; }

        public string ToString(string format, IFormatProvider formatProvider)
        {
           //关键代码，后面给出
        }
    }

这里将会列出需要实现IFormattable的方法ToString(string format, IFormatProvider formatProvider)，这里是关键代码，用来格式字符串，暂时不给出，由后面给出。

二、编写 PersonFormatter类，让其继承IFormatProvider及ICustomFormatter，用于对字符串进行格式化，代码如下：

    class PersonFormatter : IFormatProvider,ICustomFormatter
    {
        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            //Format实现代码
        }

        public object GetFormat(Type formatType)
        {
            //GetFormat实现代码
        }
    }

Format：用于格式化字符串

Format的实现代码如下：

        Person person = arg as Person;
        switch(format)
        {
            case "CH":return $"{person.LastName} {person.FirstName}";
            case "EN":return $"{person.FirstName} {person.LastName}";
            default: return $"{person.LastName} {person.FirstName}";
        }

GetFormat的实现代码如下：

        if (formatType == typeof(ICustomFormatter)) return this;
        return null;

因此，PersonFormatter类的代码如下：

    class PersonFormatter : IFormatProvider,ICustomFormatter
    {
        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            Person person = arg as Person;
            switch(format)
            {
                case "CH":return $"{person.LastName} {person.FirstName}";
                case "EN":return $"{person.FirstName} {person.LastName}";
                default: return $"{person.LastName} {person.FirstName}";
            }
        }

        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter)) return this;
            return null;
        }
    }

三、实现Person类IFormattable接口ToString方法，代码如下：

        ICustomFormatter customFormatter = formatProvider as ICustomFormatter;
        if (customFormatter == null) return this.ToString();
        return customFormatter.Format(format, this, null);

最终Person类代码如下：

    class Person:IFormattable
    {
        public string FirstName { set; get; }
        public string LastName { set; get; }

        public string ToString(string format, IFormatProvider formatProvider)
        {
            ICustomFormatter customFormatter = formatProvider as ICustomFormatter;
            if (customFormatter == null) return this.ToString();
            return customFormatter.Format(format, this, null);
        }
    }

四、使用Peson类的ToString方法，编写以下代码：

        Person p1 = new Person { FirstName = "XY", LastName = "CN" };
        PersonFormatter pf = new PersonFormatter();
        string s1 = p1.ToString("CN", pf);
        Console.WriteLine(s1);
        string s2 = p1.ToString("EN", pf);
        Console.WriteLine(s2);

五、运行结果：

六、附上完整源码：

    class Program
    {
        static void Main(string[] args)
        {
            Person p1 = new Person { FirstName = "XY", LastName = "CN" };
            PersonFormatter pf = new PersonFormatter();
            string s1 = p1.ToString("CN", pf);
            Console.WriteLine(s1);
            string s2 = p1.ToString("EN", pf);
            Console.WriteLine(s2);
        }
    }

    class PersonFormatter : IFormatProvider,ICustomFormatter
    {
        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            Person person = arg as Person;
            switch(format)
            {
                case "CH":return $"{person.LastName} {person.FirstName}";
                case "EN":return $"{person.FirstName} {person.LastName}";
                default: return $"{person.LastName} {person.FirstName}";
            }
        }

        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter)) return this;
            return null;
        }
    }

    class Person:IFormattable
    {
        public string FirstName { set; get; }
        public string LastName { set; get; }

        public string ToString(string format, IFormatProvider formatProvider)
        {
            ICustomFormatter customFormatter = formatProvider as ICustomFormatter;
            if (customFormatter == null) return this.ToString();
            return customFormatter.Format(format, this, null);
        }
    }