C# 为类型输出格式化字符串

有两种方法可以为类型提供格式化字符串输出：

1、让类型继承IFormattable接口，实现IFormattable接口的ToString方法，需要开发者可以预见类型在格式化方面的要求。

调用输出未重写Tostring方法：

    Person person = new Person
    {
        ID = "NO1",
        FirstName = "You",
        LastName = "Xu"
    };
    Console.WriteLine(person);

输出的是类型名称：

  改善1.Person 

为Person类实现IFormattable接口，重写ToString方法：

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

        //实现IFormattable接口的ToString方法
        public string ToString(string format, IFormatProvider formatProvider)
        {
            switch (format)
            {
                case "Ch":
                    return this.ToString();
                case "Eg":
                    return string.Format("{0} {1}", FirstName, LastName);
                default:
                    return this.ToString();
            }
        }

        //重写Object.ToString()
        public override string ToString()
        {
            return string.Format("{0} {1}", LastName, FirstName);
        }
    }

调用输出修改后的代码：

     Person person = new Person
     {
         ID = "NO1",
         FirstName = "You",
         LastName = "Xu"
     };
     Console.WriteLine(person);
     Console.WriteLine(person.ToString("Ch", null));
     Console.WriteLine(person.ToString("Eg", null));

输出结果：

Xu You
Xu You
You Xu

2、为类型自定义格式化器，可以根据需求的变化提供多个格式化器，随时增加或者修改它。

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

    //Person类的格式化器
    class PersonFormatter : IFormatProvider, ICustomFormatter
    {
        //实现IFprmatProvider接口
        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        //实现ICustomFormatter接口
        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            Person person = arg as Person;
            if (person == null)
            {
                return string.Empty;
            }
            switch (format)
            {
                case "Ch":
                    return string.Format("{0} {1}", person.LastName, person.FirstName);
                case "Eg":
                    return string.Format("{0} {1}", person.FirstName, person.LastName);
                case "ChM":
                    return string.Format("{0} {1}：{2}", person.LastName, person.FirstName, person.ID);
                default:
                    return string.Format("{0} {1}", person.LastName, person.FirstName);
            }
        }
    }

调用格式化器代码：

     Person person = new Person
     {
         ID = "NO1",
         FirstName = "You",
         LastName = "Xu"
     };
     Console.WriteLine(person);

     PersonFormatter pFormatter = new PersonFormatter();

     Console.WriteLine(pFormatter.Format("Ch", person,null));
     Console.WriteLine(pFormatter.Format("Eg", person,null));
     Console.WriteLine(pFormatter.Format("ChM", person, null));

输出结果，因为没有重写Object.ToString方法，类型会输出类型名称：

改善1.Person
Xu You
You Xu
Xu You：NO1

在对IFormattable接口的ToString方法实现时，可以结合格式化器，让格式化输出在语法上支持更多的调用方式，修改ToString方法里switch结构的default部分：

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

        //实现IFormattable接口的ToString方法
        public string ToString(string format, IFormatProvider formatProvider)
        {
            switch (format)
            {
                case "Ch":
                    return this.ToString();
                case "Eg":
                    return string.Format("{0} {1}", FirstName, LastName);
                default:
                    ICustomFormatter customFormatter = formatProvider as ICustomFormatter;
                    if (customFormatter == null)
                    {
                        return this.ToString();
                    }
                    return customFormatter.Format(format, this, null);
            }
        }

        //重写Object.ToString()
        public override string ToString()
        {
            return string.Format("{0} {1}", LastName, FirstName);
        }
    }

调用代码：

    Person person = new Person()
    {
        ID = "NO1",
        FirstName = "You",
        LastName = "Xu"
    };
    Console.WriteLine(person);

    PersonFormatter pFormatter = new PersonFormatter();

    Console.WriteLine(pFormatter.Format("Ch", person, null));
    Console.WriteLine(pFormatter.Format("Eg", person, null));
    Console.WriteLine(pFormatter.Format("ChM", person, null));

    Console.WriteLine(person.ToString("Ch", pFormatter));
    Console.WriteLine(person.ToString("Eg", pFormatter));
    Console.WriteLine(person.ToString("ChM", pFormatter));

参考：《编写高质量代码改善C#程序的157个建议》陆敏技