using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var text = string.Format("key1:value1{0}key2:value2{0}{0}key3:value3:value3a{0}{0}{0}{0}{0}{0}", Environment.NewLine);

        var dictionary = text.Split(new []{ Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries)

          .Select(line => line.Split(':'))

          .ToDictionary(lineParts => lineParts[0], lineParts => string.Join(":", lineParts.Skip(1)));

        foreach (var item in dictionary)

            Console.WriteLine($"{item.Key} - {item.Value}");

    }

}