List<string> Convert(string text) {
    string[] splitStrings = { Environment.NewLine, "\r\n", "\n" };
    string[] textSplit = text.Split(splitStrings, StringSplitOptions.RemoveEmptyEntries);
    var list = new List<string>();
    for (int i = 0; i < textSplit.Length; i++) {
        // check if there's new line at the beginning of the text
        // if there is add empty string to the list
        if (i == 0 && text.StartsWith(Environment.NewLine)) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\r\n")) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\n")) {
            list.Add("");
        }
        // add splitted text to the list
        list.Add(textSplit[i]);

        if (i < textSplit.Length - 1) {
            // for every element in the list except the last element add empty string to the list
            list.Add("");
        } else {
            // check if there's new line at the end of the text
            // if there is add an empty string to the list
            if (text.EndsWith(Environment.NewLine)) {
                list.Add("");
            } else if (text.EndsWith("\r\n")) {
                list.Add("");
            } else if (text.EndsWith("\n")) {
                list.Add("");
            }
        }
    }
    return list;
}

List<string> Convert(string text) {
    string[] splitStrings = { Environment.NewLine, "\r\n", "\n" };
    string[] textSplit = text.Split(splitStrings, StringSplitOptions.RemoveEmptyEntries);
    var list = new List<string>();
    for (int i = 0; i < textSplit.Length; i++) {
        // check if there's new line at the beginning of the text
        // if there is add empty string to the list
        if (i == 0 && text.StartsWith(Environment.NewLine)) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\r\n")) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\n")) {
            list.Add("");
        }
        // add splitted text to the list
        list.Add(textSplit[i]);

        if (i < textSplit.Length - 1) {
            // for every element in the list except the last element add empty string to the list
            list.Add("");
        } else {
            // check if there's new line at the end of the text
            // if there is add an empty string to the list
            if (text.EndsWith(Environment.NewLine)) {
                list.Add("");
            } else if (text.EndsWith("\r\n")) {
                list.Add("");
            } else if (text.EndsWith("\n")) {
                list.Add("");
            }
        }
    }
    return list;
}

using System.Collections.Generic;

List<string> Convert(string text) {
    string[] splitStrings = { Environment.NewLine, "\r\n", "\n" };
    string[] textSplit = text.Split(splitStrings, StringSplitOptions.RemoveEmptyEntries);
    var list = new List<string>();
    for (int i = 0; i < textSplit.Length; i++) {
        if (i == 0 && text.StartsWith(Environment.NewLine)) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\r\n")) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\n")) {
            list.Add("");
        }

        list.Add(textSplit[i]);

        if (i < textSplit.Length - 1) {
            list.Add("");
        } else {
            if (text.EndsWith(Environment.NewLine)) {
                list.Add("");
            } else if (text.EndsWith("\r\n")) {
                list.Add("");
            } else if (text.EndsWith("\n")) {
                list.Add("");
            }
        }
    }
    return list;
}

Console.WriteLine("----");
var test1 = Convert("First line\r\nAnd more \r\n in new line\r\n");
foreach (var item in test1) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test2 = Convert("First line\r\nAnd more \r\n in new line");
foreach (var item in test2) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test3 = Convert("First line\r\nAnd more \n in new line");
foreach (var item in test3) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test4 = Convert("\nFirst line\r\nAnd more \r\n in new line\r\n");
foreach (var item in test4) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test5 = Convert("\nFirst line\r\nAnd more " + Environment.NewLine + "in new line\r\n");
foreach (var item in test5) {
    Console.WriteLine(item);
}
Console.WriteLine("----");

using System.Collections.Generic;

List<string> Convert(string text) {
    string[] splitStrings = { Environment.NewLine, "\r\n", "\n" };
    string[] textSplit = text.Split(splitStrings, StringSplitOptions.RemoveEmptyEntries);
    var list = new List<string>();
    for (int i = 0; i < textSplit.Length; i++) {
        if (i == 0 && text.StartsWith(Environment.NewLine)) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\r\n")) {
            list.Add("");
        } else if (i == 0 && text.StartsWith("\n")) {
            list.Add("");
        }

        list.Add(textSplit[i]);

        if (i < textSplit.Length - 1) {
            list.Add("");
        } else {
            if (text.EndsWith(Environment.NewLine)) {
                list.Add("");
            } else if (text.EndsWith("\r\n")) {
                list.Add("");
            } else if (text.EndsWith("\n")) {
                list.Add("");
            }
        }
    }
    return list;
}

Console.WriteLine("----");
var test1 = Convert("First line\r\nAnd more \r\n in new line\r\n");
foreach (var item in test1) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test2 = Convert("First line\r\nAnd more \r\n in new line");
foreach (var item in test2) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test3 = Convert("First line\r\nAnd more \n in new line");
foreach (var item in test3) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test4 = Convert("\nFirst line\r\nAnd more \r\n in new line\r\n");
foreach (var item in test4) {
    Console.WriteLine(item);
}
Console.WriteLine("----");
var test5 = Convert("\nFirst line\r\nAnd more " + Environment.NewLine + "in new line\r\n");
foreach (var item in test5) {
    Console.WriteLine(item);
}
Console.WriteLine("----");