public class ExampleClass
{
    private static string[] words = new string[] { "Word1","Word2","Word3","Word4","Word5" };

    public static bool IsExist(string Word)
    {
        return words.Any(w => w==Word);
    }
}

ExampleClass.IsExist("Word1"); //Returns true
ExampleClass.IsExist("WordNotExist"); //Returns false

ExampleClass.IsExist["Word1"]; //Returns true
ExampleClass.IsExist["WordNotExist"]; //Returns false

public class ExampleClass
{
    public class IsExistHelper
    {
        private static string[] words = new string[] { "Word1", "Word2", "Word3", "Word4", "Word5" };

        public bool this[string Word]
        {
            get
            {
                return words.Any(w => w == Word);
            }
        }
    }

    public static IsExistHelper IsExist { get; } = new IsExistHelper();
}

public static class ExampleClass
{

    public class InnerIsExist
    {
        private string[] words = new string[] { "Word1", "Word2", "Word3", "Word4", "Word5" };

        public bool this[string word]
        {
            get
            {
                return words.Contains(word);
            }
        }
    }

    public static InnerIsExist IsExist { get; } = new IsExistClass();
}

var doesItContain = ExampleClass.IsExist["b"]; // false

public class ExampleClass
{
    private string[] words = new string[] { "Word1", "Word2", "Word3", "Word4", "Word5" };

    public bool this[string Word]
    {
        get { return words.Any(w => w == Word); }
    }
}

var _ = new ExampleClass();
var isTrue = _["Word1"] == true

public static bool this[string Word]
{
    get { return words.Any(w => w==Word); }
}