Having trouble serializing NetTopologySuite FeaturesCollection to GeoJSON

Update

GeoJsonSerializer has been moved to NetTopologySuite.IO.GeoJSON and now has its own static Create() method:

/// <summary>
/// Factory method to create a (Geo)JsonSerializer
/// </summary>
/// <remarks>Calls <see cref="GeoJsonSerializer.CreateDefault()"/> internally</remarks>
/// <returns>A <see cref="JsonSerializer"/></returns>
public new static JsonSerializer Create()
{
    return CreateDefault();
}

Use of the direct constructor has been deprecated:

[Obsolete("Use GeoJsonSerializer.Create...() functions")]
public GeoJsonSerializer() : this(Wgs84Factory) { }

The code in the question should now work as expected.

---

Original Answer

Use the default constructor for the GeoJsonSerializer class:

var jsonSerializer = new NetTopologySuite.IO.GeoJsonSerializer();

That attaches a CoordinateConverter which prevents the problem.

GeoJsonSerializer does not actually have a static Create() method, so you are falling back on the base class's JsonSerializer.Create(). In fact the following would have resulted in a compiler error:

GeoJsonSerializer jsonSerializer = NTS.IO.GeoJsonSerializer.Create();