How to parse OData $filter with regular expression in C#?

Question

Hi I\'m wondering what the best approach would be to parse an OData $filter string in C#, for example

/API/organisations?$filter=\"name eq \'Facebook\' or name eq \'

Answer 1

I think you are supposed to travserse the AST with the interface provided using the visitor pattern.

Consider you have this class that represents a filter

public class FilterValue
{
    public string ComparisonOperator { get; set; }
    public string Value { get; set; }
    public string FieldName { get; set; }
    public string LogicalOperator { get; set; }
}

So, how do we "extract" the filters that comes with the OData parameters to your class?

Well the FilterClause object have an Expression property which is a SingleValueNode wich inherits from a QueryNode. The QueryNode have the Accept method who takes a QueryNodeVisitor.

    public virtual T Accept(QueryNodeVisitor visitor);

Right, so you must implement your own QueryNodeVisitor and do your stuff. Below is a non finished example (I dont override all possible visitors).

public class MyVisitor : QueryNodeVisitor
    where TSource: class
{ 
    List filterValueList = new List();
    FilterValue current = new FilterValue();
    public override TSource Visit(BinaryOperatorNode nodeIn)
    {
        if(nodeIn.OperatorKind == Microsoft.Data.OData.Query.BinaryOperatorKind.And 
            || nodeIn.OperatorKind == Microsoft.Data.OData.Query.BinaryOperatorKind.Or)
        {
            current.LogicalOperator = nodeIn.OperatorKind.ToString();
        }
        else
        {
            current.ComparisonOperator = nodeIn.OperatorKind.ToString();
        }
        nodeIn.Right.Accept(this);
        nodeIn.Left.Accept(this);
        return null;
    }
    public override TSource Visit(SingleValuePropertyAccessNode nodeIn)
    {
        current.FieldName = nodeIn.Property.Name;
        //We are finished, add current to collection.
        filterValueList.Add(current);
        //Reset current
        current = new FilterValue();
        return null;
    }

    public override TSource Visit(ConstantNode nodeIn)
    {
        current.Value = nodeIn.LiteralText;
        return null;
    }

}

Then, fire away :)

MyVisitor