List<T>.Contains() / IndexOf()

using System.Collections.Generic;

// Returns true if the specified element exists in the list.
list.Contains(T item)

// Returns the index of the first occurrence of the specified element. Returns -1 if not found.
list.IndexOf(T item)

// Starts searching from startIndex.
list.IndexOf(T item, int startIndex)

// Returns the index of the last occurrence of the specified element.
list.LastIndexOf(T item)

using System;
using System.Collections.Generic;

List<string> languages = new List<string> { "C#", "Java", "Python", "Java", "Go" };

// Check for existence using Contains().
Console.WriteLine(languages.Contains("Python")); // True
Console.WriteLine(languages.Contains("Swift"));  // False

// A typical pattern for using Contains() with an if statement.
if (languages.Contains("C#"))
{
    Console.WriteLine("C# is in the list.");
}

// Get the index of the first occurrence using IndexOf().
Console.WriteLine(languages.IndexOf("Java"));  // 1
Console.WriteLine(languages.IndexOf("Swift")); // -1 (not found, so -1)

// Get the index of the last occurrence using LastIndexOf().
Console.WriteLine(languages.LastIndexOf("Java")); // 3

// Check for duplicates using IndexOf() and LastIndexOf().
int first = languages.IndexOf("Java");
int last = languages.LastIndexOf("Java");
if (first != last)
{
    Console.WriteLine("Java appears more than once.");
}

Both Contains() and IndexOf() scan the list from the beginning, so they take time proportional to the number of elements (O(n)). If you need frequent lookups, consider using Dictionary<TKey, TValue> or HashSet<T> instead. These support O(1) lookups.

For sorting and iterating over a list, see List<T>.Sort() / Reverse() / ForEach().