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<T>(QueryNodeVisitor<T> 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<TSource> : QueryNodeVisitor<TSource>
    where TSource: class
{ 
    List<FilterValue> filterValueList = new List<FilterValue>();
    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<object> visitor = new MyVisitor<object>();
options.Filter.FilterClause.Expression.Accept(visitor);

When it has traversed the tree your

visitor.filterValueList

should contain the filters in your desired format. Im sure more work is needed but if you can get this rolling I think you can figure it out.

Answer 2

Building on what Jen S says, you can traverse the AST tree that is returned by the FilterClause.

For instance, you can retrieve the FilterClause from the controller's query options:

public IQueryable<ModelObject> GetModelObjects(ODataQueryOptions<ModelObject> queryOptions)        
    {
        var filterClause = queryOptions.Filter.FilterClause;

You can then traverse the resultant AST tree with code like the following (borrowed from this article):

var values = new Dictionary<string, object>();
TryNodeValue(queryOptions.Filter.FilterClause.Expression, values);

The function called is like so:

public void TryNodeValue(SingleValueNode node, IDictionary<string, object> values)
    {
        if (node is BinaryOperatorNode )
        {
            var bon = (BinaryOperatorNode)node;
            var left = bon.Left;
            var right = bon.Right;

            if (left is ConvertNode)
            {
                var convLeft = ((ConvertNode)left).Source;

                if (convLeft is SingleValuePropertyAccessNode && right is ConstantNode)
                    ProcessConvertNode((SingleValuePropertyAccessNode)convLeft, right, bon.OperatorKind, values);
                else
                    TryNodeValue(((ConvertNode)left).Source, values);                    
            }

            if (left is BinaryOperatorNode)
            {
                TryNodeValue(left, values);
            }

            if (right is BinaryOperatorNode)
            {
                TryNodeValue(right, values);
            }

            if (right is ConvertNode)
            {
                TryNodeValue(((ConvertNode)right).Source, values);                  
            }

            if (left is SingleValuePropertyAccessNode && right is ConstantNode)
            {
                ProcessConvertNode((SingleValuePropertyAccessNode)left, right, bon.OperatorKind, values);
            }
        }
    }

    public void ProcessConvertNode(SingleValuePropertyAccessNode left, SingleValueNode right, BinaryOperatorKind opKind, IDictionary<string, object> values)
    {            
        if (left is SingleValuePropertyAccessNode && right is ConstantNode)
        {
            var p = (SingleValuePropertyAccessNode)left;

            if (opKind == BinaryOperatorKind.Equal)
            {
                var value = ((ConstantNode)right).Value;
                values.Add(p.Property.Name, value);
            }
        }
    }

You then can go through the list dictionary and retrieve your values:

 if (values != null && values.Count() > 0)
        {
            // iterate through the filters and assign variables as required
            foreach (var kvp in values)
            {
                switch (kvp.Key.ToUpper())
                {
                    case "COL1":
                        col1 = kvp.Value.ToString();
                        break;
                    case "COL2":
                        col2 = kvp.Value.ToString();
                        break;
                    case "COL3":
                        col3 = Convert.ToInt32(kvp.Value);
                        break;
                    default: break;
                }
            }
        }

This example is fairly simplistic in that it only accounts for "eq" evaluations, but for my purposes it worked well. YMMV. ;)

Answer 3

In .NET, there's a library available that will do this for you. Writing your own regex runs the risk of missing some edge case.

Using NuGet, bring in Microsoft.Data.OData. Then, you can do:

using Microsoft.Data.OData.Query;

var result = ODataUriParser.ParseFilter(
  "name eq 'Facebook' or name eq 'Twitter' and subscribers gt 30",
  model,
  type);

result here will be in the form of an AST representing the filter clause.

(To get the model and type inputs, you could parse your $metadata file using something like this:

using Microsoft.Data.Edm;
using Microsoft.Data.Edm.Csdl;

IEdmModel model = EdmxReader.Parse(new XmlTextReader(/*stream of your $metadata file*/));
IEdmEntityType type = model.FindType("organisation");

)