The ```LinkedList<T>``` generic class in the ```System.Collections.Generic``` namespace provides the **```AddBefore()``` method**. This method can be used to add a node before 
 a specified node of a linked list in C#.

# Syntax
```
public LinkedListNode<T> AddBefore (LinkedListNode<T> node, T value);
```
- As input, this takes a node on a linked list and the `T` value. It will return a new ```LinkedListNode<T>``` that contains the passed input value, which is inserted before the passed input node.

- It also has an overload available that takes a ```LinkedListNode<T>``` as input instead of the value `T`.

# Things to note
- This method is an $O(1)$ operation.

- ```LinkedList<T>``` accepts null as a valid reference type value.

- ```LinkedList<T>``` allows duplicate values as well.

- It throws an exception if the specified node is null.

- It throws an exception if the specified node is in a different linked list.

# Code
In the code below, we created a linked list of strings and added the names of a few months to it. The linked list contains three strings: `January`, `March`, and `May`.

We have saved the references of the linked list node, with `March` and `May` values to be used later as an input parameter to the ```AddBefore()``` method.

We have also created `Print()`, a helper method to display the linked list nodes.

Now we call the ```AddBefore()``` method while passing the linked list node for `March`, then we string the new node values on this linked list. The `February` string is the new node value. This adds  `February` to the linked list before `March`.  We can see that `February` and `March` are next to each other in the output.

We  call the ```AddBefore()``` method again, while passing the linked list node 
 for `May`  and string `April` as the new node value. This adds a new node, the value `April`, before `May`.

Please note that the ```LinkedList<T>``` class is maintained internally as a doubly-linked list and ```AddBefore()``` is an $O(1)$ operation.

using System;
using System.Collections.Generic;

class LinkedListAddBefore
{
    static void Main()
    {
        LinkedList<string> monthList = new LinkedList<string>();
        monthList.AddLast("January");
        LinkedListNode<string> marchNode = monthList.AddLast("March");
        LinkedListNode<string> mayNode = monthList.AddLast("May");
        Console.WriteLine("LinkedList Elements");
        Print(monthList);

        monthList.AddBefore(marchNode, "February");
        Console.WriteLine($"LinkedList Elements After AddBefore({marchNode.Value},'Feburary')");
        Print(monthList);

        monthList.AddBefore(mayNode, "April");
        Console.WriteLine($"LinkedList Elements After AddBefore({mayNode.Value},'April')");
        Print(monthList);
    }

    private static void Print(LinkedList<string> list)
    { 
        foreach (var node in list)
        {
            Console.Write(node + ", ");
        }
        Console.WriteLine("\n");        
    }
}

How to add an element before a node in a linked list in C#

Use LinkedList<T>.AddBefore() in C# to insert a node before another efficiently. It supports null and duplicates, but the node must belong to the same linked list.