To use data in an expression, the data has to be passed in a ParserContext object. The context object is essentially a dictionary of key-value pairs. The keys are the variables that will be available in the expression.
For example:
var parser = new CSharpParser(); var context = new ParserContext(); context["x"] = 10; context["y"] = 5; var result = parser.Evaluate<int>("x+y", context); // returns 15
By default, the ParserContext uses strict C# rules for evaluating expression. This means that variables are case sensitive and non-boolean expressions can't be used as booleans. The context can be configured to be more relaxed by specifying a ParserContextBehavior enum parameter.
ParserContextBehavior has the following flags:
Rules for converting an expression to a boolean:
| Flag | |
|---|---|
NullIsFalse |
null is treated as false |
NotNullIsTrue |
not null is treated as true |
NotZeroIsTrue |
any numeric value which is not zero is treated as true |
ZeroIsFalse |
any numeric value which is zero is treated as false |
EmptyStringIsFalse |
an empty string is treated as false |
NonEmptyStringIsTrue |
a non-empty string is treated as true |
EmptyCollectionIsFalse |
an empty collection is treated as false |
Other flags:
| Flag | |
|---|---|
ReturnNullWhenNullReference |
Expression returns null when a null value is dereferenced |
CaseInsensitiveVariables |
Variable names are case insensitive |
Combinations
| Flag | |
|---|---|
Falsy |
NullIsFalse | NotNullIsTrue | NotZeroIsTrue | ZeroIsFalse | EmptyStringIsFalse | NonEmptyStringIsTrue | EmptyCollectionIsFalse |
Easy |
Falsy | ReturnNullWhenNullReference |
It's very common to use the most relaxed evaluation rules for expressions. The FlexContext class is a subclass of ParserContext with a default behavior of Easy, which means that any expression is convertible to a boolean and any null references in an expression will result in null to be returned instead of throwing an exception.