c# - 向字符串类添加扩展方法 - C#

Question

不确定我在这里做错了什么。无法识别扩展方法。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using StringExtensions;


namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            RunTests();
        }

        static void RunTests()
        {
            try
            {
                ///SafeFormat
                SafeFormat("Hi There");

                SafeFormat("test {0}", "value");

                SafeFormat("test missing second value {0} - {1}", "test1");

                SafeFormat("{0}");

                //regular format
                RegularFormat("Hi There");

                RegularFormat("test {0}", "value");

                RegularFormat("test missing second value {0} - {1}", "test1");

                RegularFormat("{0}");

                ///Fails to recognize the extension method here
                string.SafeFormat("Hello");

            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
            Console.ReadLine();
        }

        private static void RegularFormat(string fmt, params object[] args)
        {
            Console.WriteLine(String.Format(fmt, args));
        }

        private static void SafeFormat(string fmt, params object[] args)
        {
            string errorString = fmt;

            try
            {
                errorString = String.Format(fmt, args);
            }
            catch (System.FormatException) { } //logging string arguments were not correct
            Console.WriteLine(errorString);
        }

    }

}

namespace StringExtensions
{
    public static class StringExtensionsClass
    {
        public static string SafeFormat(this string s, string fmt, params object[] args)
        {
            string formattedString = fmt;

            try
            {
                formattedString = String.Format(fmt, args);
            }
            catch (System.FormatException) { } //logging string arguments were not correct
            return formattedString;
        }
    }
}

Answer 1

您正在尝试在 type 字符串上调用它。您需要在字符串实例 上调用它，例如

"{0}".SafeFormat("Hello");

诚然，这不会做您想要的，因为 SafeFormat 方法实际上完全忽略了第一个参数 (s)。它应该看起来像这样:

    public static string SafeFormat(this string fmt, params object[] args)
    {
        string formattedString = fmt;

        try
        {
            formattedString = String.Format(fmt, args);
        }
        catch (FormatException) {} //logging string arguments were not correct
        return formattedString;
    }

然后你可以调用:

"{0} {1}".SafeFormat("Hi", "there");

扩展方法的要点在于它们看起来像扩展类型上的实例 方法。您不能在扩展类型上创建看似静态方法的扩展方法。