Guid - In

A T extension method to determines whether the object is equal to any of the provided values.

Try it

public static void Main()
{
    Guid guid = Guid.NewGuid();

    Guid[] guids = new Guid[]
    {
        Guid.NewGuid(),
        Guid.NewGuid(),
        Guid.NewGuid(),
        guid
    };

	// C# Extension Method: Guid - In
    if(guid.In(guids))
    {
        Console.WriteLine("{0} exists in the list.", guid);
    }
    else
    {
        Console.WriteLine("{0} doesn't exists in the list.", guid);
    }
}

View Source 
using System;

public static partial class Extensions
{
    /// <summary>
    ///     A T extension method to determines whether the object is equal to any of the provided values.
    /// </summary>
    /// <param name="this">The object to be compared.</param>
    /// <param name="values">The value list to compare with the object.</param>
    /// <returns>true if the values list contains the object, else false.</returns>
    /// ###
    /// <typeparam name="T">Generic type parameter.</typeparam>
    public static bool In(this Guid @this, params Guid[] values)
    {
        return Array.IndexOf(values, @this) != -1;
    }
}