using System;
using System.Text.RegularExpressions;

public class Test
{
    public static void Main()
    {
        string text = @"My arbitrary content....

```c#
public class Foo
{
    //Some code
}
```

Some text

```c#
public class Bar
{
    //Some code
}
```

My other arbitrary content....";

        text = Regex.Replace(
              text,
              @"^```c#\r?\n(.*?)```\r?\n", "$1",
              RegexOptions.Singleline | RegexOptions.Multiline);
              
        Console.WriteLine(text);
    }
}