Shortest way of checking if Double is "NaN"

As MSDN says, NaN means that result is undefined. With infinities result is defined:

A method or operator returns NaN when the result of an operation is undefined. For example, the result of dividing zero by zero is NaN, as the following example shows. (But note that dividing a non-zero number by zero returns either PositiveInfinity or NegativeInfinity, depending on the sign of the divisor.)

So, it's not good idea to tread infinities as NaN. You can write extension method to check if value is not NaN or infinity:

// Or IsNanOrInfinity
public static bool HasValue(this double value)
{
    return !Double.IsNaN(value) && !Double.IsInfinity(value);
}

You no longer need an extension from SergeyBerezovskiy answer.

double has IsFinite() method to check if a double is a finite number (is not NaN or Infinity):

double.IsFinite(d)

See source code in .Net Framework and .Net Core

If you'd like the value of double to always be a number, you could use this FiniteOrDefault extension. It is of course inspired by Sergey Berezovskiy's answer.

public static bool HasValue(this double value)
{
    return !double.IsNaN(value) && !double.IsInfinity(value);
}

/// <summary>
/// Returns zero when double is NaN or Infinte
/// </summary>
public static double FiniteOrDefault(this double value)
{
    return value.HasValue() ? value : default;
}

With that, code like the following can be much more readable:

Rect dimensions = new Rect
{
    X = Canvas.GetLeft(Control).FiniteOrDefault(),
    Y = Canvas.GetTop(Control).FiniteOrDefault(),
    Width = Control.ActualWidth.FiniteOrDefault(),
    Height = Control.ActualHeight.FiniteOrDefault()
};

There are three special values in the Double type, which is based on IEEE standard 754. One is Positive Infinity, another is Negative Infinity, and the last is Not-a-Number (NaN). All that the Double.IsNaN method does is check to see if the value in the variable is this special NaN value.