NReco.LambdaParser Values Comparison

NReco.LambdaParser uses ValueComparer when comparing values by default. But it also allows you to provide your implementation or configure its option to get the desired behavior. You can customize the ValueComparer with the following two options.

NullComparison

It determines how comparison with null is handled, and it also provides two options:

• MinValue: null is treated as a minimum possible value for any type - like .NET IComparer
• Sql: null is not comparable with any type, including another null - like in SQL

Let's consider the following simple example, which uses the default options.

public static void Example1()
{
	var varContext = new Dictionary<string, object>()
	{
		{ "obj",  new string("test string") },
		{ "nullVar", null }
	};

	List<string> expressions = new List<string>()
	{
		"null == nullVar",
		"5 > nullVar",
		"testObj != null",
		"nullVar>5",
	};

	var lambdaParser = new LambdaParser();

	foreach (var expression in expressions)
	{
		var result = lambdaParser.Eval(expression, varContext);
		Console.WriteLine("{0} = {1}", expression, result);
	}
}

Let's run the above code and you will see the following output.

null == nullVar = True
5 > nullVar = True
testObj != null = False
nullVar>5 = False

Now let's set the ValueComparer.NullComparison value to ValueComparer.NullComparisonMode.Sql and pass it as a parameter to the LambdaParser constructor as shown below.

public static void Example2()
{
	var varContext = new Dictionary<string, object>()
	{
		{ "obj",  new string("test string") },
		{ "nullVar", null }
	};

	List<string> expressions = new List<string>()
	{
		"null == nullVar",
		"5 > nullVar",
		"testObj != null",
		"nullVar > 5",
	};

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

	foreach (var expression in expressions)
	{
		var result = lambdaParser.Eval(expression, varContext);
		Console.WriteLine("{0} = {1}", expression, result);
	}			
}

Let's run the above code and you will see the following output.

null == nullVar = False
5 > nullVar = False
testObj != null = True
nullVar>5 = False

SuppressErrors

It allows you to avoid convert exceptions. If an error appears during comparison, exception is not thrown, which means that values are not comparable and will return false.