DateTime - In

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

Try it

public static void Main()
{
    DateTime date = new DateTime(2018, 12, 25);
	
	DateTime [] values = {new DateTime(2018, 12, 25), new DateTime(2019, 2, 28), new DateTime(2019, 2, 24), new DateTime(2017, 5, 12)};

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

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 DateTime @this, params DateTime[] values)
    {
        return Array.IndexOf(values, @this) != -1;
    }
}