Print the letter until the next number, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            List<Data_Array> ab = new List<Data_Array>();
            Data_Array d = new Data_Array(0, "A");
            ab.Add(d);
            d = new Data_Array(3, "B");
            ab.Add(d);
            
            Console.WriteLine(ab.Count);
            Console.WriteLine(ab[0+1].num);
            Data_Array c = new Data_Array(ab.Last().num,ab.Last().str);
            foreach(Data_Array b in ab)
            {
                int i = b.num;
                int j;
                if (b.Equals(c))
                  j =   ab.Last().num + ab.Last().num;
                else 
                    j = ab[i+1].num;
                
                for(i = b.num ; i < j  ; i++)
                {
                   Console.WriteLine(i + "-->" + b.str);
                }
            }
        }
        
      
        
       
        public struct Data_Array
        {
            public int num ;
            public string str;
            
            public Data_Array(int num, string str)
            {
                 this.num = num;
                 this.str = str;
            }           
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,17 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 12 Mb, average nr of threads: 3