Use STRSTARTS in fluent filter

I have troubles figuring out how to represent the following query in dotnetrdf using fluent query. The SPARQL Query:

PREFIX a: <http://www.example.com/ex1#>
PREFIX rdf: < http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT ?object
WHERE {
a:Branch1 rdf:type ?object
FILTER(!STRSTARTS(STR(?object), "http://www.w3.org/2002/07/owl#"))
}

This is what I achieved so far, I couldn't figure the filter predicate.

var prefixes = new NamespaceMapper(true);
prefixes.AddNamespace("rdf", new Uri("http://www.w3.org/1999/02/22-rdf-syntax-ns#"));
prefixes.AddNamespace("a", new Uri("http://www.example.com/ex1#"));

var objVar = "object";
var queryBuilder =
    QueryBuilder
    .Select(new string[] { objVar })
    .Where(
        (triplePatternBuilder) =>
        {
            triplePatternBuilder
                .Subject(element)
                .PredicateUri("rdf:type")
                .Object(objVar);
        })
    .Filter(f => 
                // What should be added here?
            );

queryBuilder.Prefixes = prefixes;

Thanks,

Unfortunately the STRSTARTS function hasn't been mapped to the fluent interface yet, but there is a way around that. You can build a regular ISparqlExpression in the Filter action and wrap it in a new BooleanExpression:

var queryBuilder =
    QueryBuilder
    .Select(new string[] { objVar })
    .Where(
        (triplePatternBuilder) =>
        {
           triplePatternBuilder
              .Subject(element)
              .PredicateUri("rdf:type")
              .Object(objVar);
        })
        .Filter(f =>
           new BooleanExpression(
              new StrStartsFunction(
                 new StrFunction(new VariableTerm(objVar)),
                    f.Constant("http://www.w3.org/2002/07/owl#").Expression
           ))
        );

NOTE: You may need to add VDS.RDF.Query.Builder.Expressions and VDS.RDF.Nodes to your imports for the above to compile.