Check if a variable is in an ad-hoc list of values

You could achieve this by using the List.Contains method:

if(new []{1, 2, 3}.Contains(x))
{
    //x is either 1 or 2 or 3
}

int x = 1;
if((new List<int> {1, 2, 3}).Contains(x))
{
}

If it's in an IEnumerable<T>, use this:

if (enumerable.Any(n => n == value)) //whatever

Else, here's a short extension method:

public static bool In<T>(this T value, params T[] input)
{
    return input.Any(n => object.Equals(n, value));
}

Put it in a static class, and you can use it like this:

if (x.In(1,2,3)) //whatever

public static bool In<T>(this T x, params T[] set)
{
    return set.Contains(x);
}

...

if (x.In(1, 2, 3)) 
{ ... }

Required reading: MSDN Extension methods

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