extracting file content

Try like below:

// Get all the lines which contains the value as "3456"
string[] lines = (from rec in System.IO.File.ReadAllLines(@"C:\Users\Public\TestFolder\WriteLines2.txt")
where rec.Contains("3456")
select rec).ToArray();
// check if it contains in First column
foreach(string temp in lines)
{
string[] tempArr = temp.Split(new char[]{' '}, StringSplitOptions.RemoveEmptyEntries);
if (tempArr[0] == "3456")
{
// Apply your own logic
}
}