有时候我们还要提供功能更强大的IFormattable接口来解决问题。该接口包含了一个重载的ToString()方法,它允许我们为类型指定某种格式信息。当我们需要为类型创建不同形式的字符串输出时,这个接口非常有用。
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Code
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IFormattable Members#region IFormattable Members
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//所支持的格式:
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//用n表示name.
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//用r表示revenue。
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//用p表示contact phone
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//同时支持组合格式
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//"G"表示通用格式
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string System.IFormattable.ToString(string format,IFormatProvider formatProvider)
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{
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if(formatProvider != null)
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{
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ICustomFormatter fmt = formatProvider.GetFormat(this.GetType()) as ICustomFormatter;
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if(fmt != null)
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{
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return fmt.Format(format,this,formatProvider);
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}
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}
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switch(format)
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{
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case "r":
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return _revenue.ToString();
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case "p":
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return _contactPhone;
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case "nr":
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return string.Format("{0,20},{1,10:c}",_name,_revenue);
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case "n":
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case "G":
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default:
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return _name;
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}
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}
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#endregion添加该函数,使得Customer类型的客户可以定制Customer类型的表示:
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IFormattable c1 = new Customer();
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Console.WriteLine("Customer record:{0}",c1.ToString("nrp",null)); 这里需要注意的是1.必须提供通用格式"G";2.必须提供两种形式的空""和null;3.以上两点在重写时必须与Object.ToString()返回的字符串相同。
当类的使用者期望一种类的作者没有考虑到的格式,我们就要使用IFormatProvider接口啦。我们需要创建两个类:一个实现IFormatProvider接口——该类有一个方法:GetFormat(),返回一个实现了ICustomFormatter接口的对象;另一个实现ICustomFormatter接口——该类用于创建自定义的输出格式。
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Code
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//IFormatProvider示例
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public class CustomFormatter : IFormatProvider
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{
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IFormatProvider Members#region IFormatProvider Members
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//IFormatProvider仅包含一个方法。一般情况下,只有ICustomFormatter被实现。
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public object GetFormat(Type formatType)
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{
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if(formatType == typeof(ICustomFormatter))
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return new CustomFormatProvider();
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return null;
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}
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#endregion
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//一个嵌套类,为Customer类提供定制格式
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private class CustomerFormatProvider : ICustomFormatter
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{
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ICustomFormatter Members#region ICustomFormatter Members
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public string Format(string format,object arg,IFormatProvider formatProvider)
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{
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//以下为类的使用者新期望的格式,只需在此实现
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Customer c = arg as Customer;
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if(c == null)
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return arg.ToString();
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return string.Format("{0,50},{1,15},{2,10:c}",c.Name,c.ContactPhone,c.Revenue);
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}
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#endregion
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}
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}只有在极少数情况下,我们需要提供比较复杂的输出格式时,才应该实现IFormattable接口。一般只要重写ToString()方法就可以满足需要。