C＃在文件中查找变量？

我正在建立数据库，而我的系统存在一个小问题：我只能读取整个文件（不能读取变量），但可以将变量写入文件中，但是我也需要能够编辑文件。我可以编辑它，但是我想自己编辑部分，而不必打开整个文件进行编辑/仅在部分中替换整个文件中的给定字符串。

我想问用户他们将更改文件中的哪一部分，但是我的脚本更改了赋予整个文件的所有字符串。

所以这就是我想要的：

选择文件->选择要编辑的节/行->输入数据->将数据保存到行

该文件如下所示：

Name: Kim jr.

Age: 1980/05/20

Gender: Male

Status: Single

Weight: 46.2 Kg

Friends: Unknown

Notes: 
This is a dummy file, used for testing.

这是我的代码当前的样子：

Console.Clear();

string[] fileArray = Directory.GetFiles(@"C:\Nova_Files\Database"); //slecets the files in the folder

if (fileArray.Length < 1)
{
    Console.WriteLine();
    Console.WriteLine("There are no files in the database for deleting");
    Console.ReadLine();
}
else
{
    Console.WriteLine("Here is a list of the files in the database, please select a file to edit.\n");

    foreach (string name in fileArray) //shows the files in the folder.
    {
        Console.WriteLine(name);
    }

    Console.WriteLine();
    userValue = Console.ReadLine();

    string filename = userValue;

    foreach (string x in fileArray)
    {
        if (x.Contains(filename))
        {
            Console.WriteLine();
            Console.WriteLine(filename + " Was selected for editing");
            Console.WriteLine();
            Console.WriteLine("Press anykey to continue");
            Console.ReadLine();
            Console.Clear();
            Console.WriteLine("The current editing " + filename);
            Console.WriteLine("The File [" + filename + "] Contains This:\n");

            string text = System.IO.File.ReadAllText(@"C:\Nova_Files\Database\" + filename);

            Console.WriteLine(text);
            Console.WriteLine();
            Console.WriteLine("What do you want to edit?");
            userValue = Console.ReadLine();

            string needOfEdit = userValue;

            Console.WriteLine();
            Console.WriteLine("What will you replace " + needOfEdit + " With?");
            userValue = Console.ReadLine();

            string replacement = userValue;

            File.WriteAllText(@"C:\Nova_Files\Database\" + filename, File.ReadAllText(@"C:\Nova_Files\Database\" + filename).Replace(needOfEdit, replacement));
            text = System.IO.File.ReadAllText(@"C:\Nova_Files\Database\" + filename);
            Console.WriteLine(text);
            Console.Read();