Consider the following simple immutable class that uses the [BsonConstructor] attribute to configure class map based serialization:

public class C
{
    private readonly int _x;

    [BsonConstructor]
    public C(int x)
    {
        _x = x;
    }

    [BsonElement]
    public int X
    {
        get { return _x; }
    }
}

The constructor argument "x" is matched with the property "X" (using case insensitive matching).

Now consider the following variation of the constructor which compiles correctly but doesn't work for deserialization:

[BsonConstructor]
public C(int y)
{
    _x = y;
}

In this case, the constructor argument "y" can't be matched with any property, so this is an error.

Currently the driver is throwing a NullReferenceException when a constructor parameter can't be matched with a property. It should throw a more informative exception.