Code usage 👩‍💻

Using SequelFilter from code is fairly simple.

To match a single object to a filter, you can use:

var matches = ObjectFilter.Matches(
    someObject,
    "Property > 5 AND OtherProperty == 'fred'");

If you want to apply the same filter to multiple objects, it is more efficient to first compile the expression and then apply it:

var filter = SequelFilterParser.Parse(
    "Property > 5 AND OtherProperty == 'fred'");
var matches = filter.Matches(someObject);

You can also filter IEnumerable instances like so:

var filteredResults = myEnumerable
    .Where("Property > 5 AND OtherProperty == 'fred'")
    .ToList();

Object resolution 🔍

When you compile a filter, you receive a delegate of type ExecutableExpression which is defined as:

delegate bool ExecutableExpression(IFieldReferenceResolver fieldReferenceResolver);

Effectively, IFieldReferenceResolver is used to resolve dotted field references. So if during executing the filter needs to get the value for a dotted reference, a call will be made to the Resolve member of IFieldReferenceResolver.

There are a couple of built in IFieldReferenceResolver types. For the examples below we will consider the dotted field reference first.second:

• SingleObjectResolver - for when you want to filter against a single instance. first would be resolved as a property of that instance, and second would be resolved as a property of whatever object was returned by first.
• MultiObjectResolver - for when you want a set of instances that represent the top-level named objects. first would be used to look up an object that was passed in to the constructor of MutliObjectResolver and second would be resolved as a property of that object.
• ExtendedResolver - this is mostly for internal use, and allows you to introduce an extra named top-level object. This facilitates the x => x.Property style syntax of the enumerable language elements, such that each element of the enumerable can be resolved as x (or whatever identifier you choose) in the expression to the right of the ‘goes into’.

Field selectors 👈

Field selectors allow you to create a list of field selections - which enables things like selecting a set of fields for an export.

var selectors = SequelFilterParser.ParseSelectors("Property, OtherProperty");

The return type is IList<FieldSelector> - and each FieldSelector contains both the field name and a delegate for extracting the field value given an IFieldReferenceResolver instance.

Exceptions 🤔

In the main part, SequelFilter can throw two exception types:

• ParseException - this is thrown when the filter clause cannot be parsed. This would be, for example, if the filter text didn’t make sense.
• InvalidOperationException - this is thrown if there is an issue at runtime. This could happen if you tried to compare a Guid to a DateTime, for example.