以下是一个可能的示例，用于从字符串中提取特定的子串并组合成一个新的字符串：

```csharp
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ComplexCode
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = "This is a test string for demonstrating complex code in C#. The test string will be used to extract specific substrings and concatenate them into a new string.";

            List<string> substrings = new List<string>();
            substrings.Add("test");
            substrings.Add("used to extract");
            substrings.Add("concatenate them");

            string output = "";
            foreach (string substring in substrings)
            {
                int index = input.IndexOf(substring);
                if (index >= 0)
                {
                    output += input.Substring(index, substring.Length) + " ";
                }
            }

            Console.WriteLine("Input string: " + input);
            Console.WriteLine("Substrings to extract: " + string.Join(", ", substrings));
            Console.WriteLine("Output string: " + output);
            Console.ReadKey();
        }
    }
}
```
上述代码创建了一个名为 ComplexCode 的C#控制台应用程序。该程序包含一个 Main 方法，调用 IndexOf方法在输入字符串中搜索子串并提取它们。最后，使用 string.Join方法将提取的子串连接成新字符串，并在控制台输出该新字符串。
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