C# System.MissingMethodException:方法'；StringBuilder.AppendFormat'；找不到

我一直在遵循Derek Banas的教程，到目前为止它做得很好，但是，我不能正确使用StringBuilder。它说AppendFormat在它显然必须存在时找不到，因为它是自动完成方法

代码

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您正在执行哪个版本的.NET framework？是的。正如saravana所问的，提及您的.net版本；）我如何在MonoDevelop中找到它？在这个ide里有点像个傻瓜，我也在运行LinuxY'know。应该安装.net core ey1.0.4是我刚下载的.net core版本

using System;
using System.Globalization;
using System.Text;

namespace Test
{
    class MainClass
    {
        public static void Main(string[] args)
        {

            StringBuilder sb = new StringBuilder("Random Text");
            StringBuilder sb2 = new StringBuilder("This is some more Random Text", 256);

            Console.WriteLine("Capacity : {0}",
                              sb.Capacity);
            Console.WriteLine("Capacity : {0}",
                              sb2.Capacity);
            Console.WriteLine("Length : {0}",
                              sb.Length);
            sb2.AppendLine("\nMore important text!");

            CultureInfo enUS =
                CultureInfo.CreateSpecificCulture("en-US");
            string bestCust = "Bob Smith";
            sb2.AppendFormat(enUS, "Best Customer : {0}", bestCust);
            Console.WriteLine(sb2.ToString());

        }

    }
}