The Contains method checks if the specified item is already exists in the List. The following code snippet checks if an item is already exits.  

if (AuthorList.Contains("Mahesh Chand"))
    AuthorList.Remove("Mahesh Chand");

The IndexOf method returns the first index of an item if found in the List.

int idx = AuthorList.IndexOf("Nipun Tomar");

The LastIndexOf method returns the last index of an item if found in the List. 

idx = AuthorList.LastIndexOf("Mahesh Chand");

The following code snippet shows how to use the Contains, the IndexOf and the LastIndexOf methods. 

List<string> AuthorList = new List<string>();
AuthorList.Add("Mahesh Chand");
AuthorList.Add("Praveen Kumar");
AuthorList.Add("Raj Kumar");
AuthorList.Add("Nipun Tomar");
AuthorList.Add("Mahesh Chand");
AuthorList.Add("Dinesh Beniwal");
            
// Contains - Check if an item is in the list  
if (AuthorList.Contains("Mahesh Chand")) 
{
    Console.WriteLine("Author found!");
}        

// Find an item and replace it with new item
int idx = AuthorList.IndexOf("Nipun Tomar");
if (idx >= 0)
{
    AuthorList[idx] = "New Author";
}
Console.WriteLine("\nIndexOf ");
foreach (var author in AuthorList)
{
    Console.WriteLine(author);
}


// Find Last index of             
idx = AuthorList.LastIndexOf("Mahesh Chand");
if (idx >= 0)
{
    AuthorList[idx] = "New Mahesh";
}
Console.WriteLine("\nLastIndexOf ");
foreach (var author in AuthorList)
{
    Console.WriteLine(author);
}

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