Fixed a issue when comparing null values with <, <=, >, >= operators

[OPunktSchmidt]

This operators should return false if one or both operands are null

Example:

The previous version would return 'true' for expression: (new System.TimeSpan(15,0,0) > null) Now it returns false. That is now the exactly result which original c# code would return.

[VitaliyMF]

Topic about comparison with nulls can be rather discussive - especially in case what should be returned if both operands are null. Currently if variable "a"=null expression "a==null" returns true and this is how comparison works in C#.

I can explain why "new System.TimeSpan(15,0,0) > null" returns true; this happens because ValueComparer treats "null" as "MinValue" for ">" and "<" comparisons.

I think that there are no one "only-true" answer how comparison with null should work for ALL possible usage scenarios and currently NReco.LambdaParser has comparison logic "as is" and it should be preserved for backward compatibility.

This means that I cannot accept your pull request; instead of that I've created new issue #10 that should solves this problem in a more generic way.

[OPunktSchmidt]

Yes your are absoluty right. This problem should be solved in a more generic way. Thanks for creating an issue. Until then I will continue using my fork :)

[VitaliyMF]

@OPunktSchmidt Oliver I've just pushed issue #10 changes. Now you cna force SQL-like comparison for nulls in the following way:

var lambdaParser = new LambdaParser(
  new ValueComparer() { NullComparison = ValueComparer.NullComparisonMode.Sql }); 

Could you build NReco.LambdaParser from 'master' and try to use it in your project? If everything is ok just let me know and I'll publish it on nuget.

[OPunktSchmidt]

It seems to work everything I need. Looking forward for a nuget package. Thank you.

[VitaliyMF]

Version 1.0.6 published on nuget