Where do I find the machine epsilon in C#?

It's(on my machine):

   1.11022302462516E-16

You can easily calculate it:

        double machEps = 1.0d;

        do {
           machEps /= 2.0d;
        }
        while ((double)(1.0 + machEps) != 1.0);

        Console.WriteLine( "Calculated machine epsilon: " + machEps );

Edited:

I calcualted 2 times epsilon, now it should be correct.

Just hard-code the value:

const double e1 = 2.2204460492503131e-16;

or use the power of two:

static readonly double e2 = Math.Pow(2, -52);

or use your definition (more or less):

static readonly double e3 = BitConverter.Int64BitsToDouble(BitConverter.DoubleToInt64Bits(1.0) + 1L) - 1.0;

And see Wikipedia: machine epsilon.

The Math.NET library defines a Precision class, which has a DoubleMachineEpsilon property.

You could check how they do it.

According to that it is:

    /// <summary>
    /// The base number for binary values
    /// </summary>
    private const int BinaryBaseNumber = 2;

    /// <summary>
    /// The number of binary digits used to represent the binary number for a double precision floating
    /// point value. i.e. there are this many digits used to represent the
    /// actual number, where in a number as: 0.134556 * 10^5 the digits are 0.134556 and the exponent is 5.
    /// </summary>
    private const int DoublePrecision = 53;

    private static readonly double doubleMachinePrecision = Math.Pow(BinaryBaseNumber, -DoublePrecision);

So it is 1,11022302462516E-16 according to this source.