c# - Generic Linq to Entities filter method that accepts filter criteria and properties to be filtered

Question

I've looked into many generic linq filtering questions and their answers here in SO but none of them satisfy my needs so I thought I should create a question.

I've created many of what I call "filter provider" classes, one for each entity class in my model, to provide a simplistic search for my application. I didn't want to go into more advanced solutions like Lucene.Net because a basic filtering with matching score would suffice.

Inside each one of these provider classes there are multiple methods that will receive the filtering terms and query specific properties, returning a score for each match based on the relevance of the property. Most methods will filter multiple properties at once, but not all.

Here are two of these methods:

private IQueryable<Retailer> MatchHighRelevanceFields(string searchTerm, IQueryable<Retailer> retailers)
{
    var results = retailers.Where(r =>
        (r.CompanyName != null && r.CompanyName.ToUpper().Contains(searchTerm))
        || (r.TradingName != null && r.TradingName.ToUpper().Contains(searchTerm))
    );

    return results;
}

private IQueryable<Retailer> MatchMediumRelevanceFields(string searchTerm, IQueryable<Retailer> retailers)
{
    var results = retailers.Where(r =>
        (r.Address.Street != null && r.Address.Street.ToUpper().Contains(searchTerm))
        || (r.Address.Complement != null && r.Address.Complement.ToUpper().Contains(searchTerm))
    );

    return results;
}

These methods are replicated ad nauseum throughout each provider class and I hope I could replace them for a single method that would receive the properties to be included in the query.

Something like:

public static IQueryable<T> Match<T>(string searchTerm, IQueryable<T> data, Expression<Func<T, string>> filterProperties)
{
    var results = **build the query for each property in filterProperties**

    return results;
}

But I really can't figure it out. I tried using reflection but it only worked with Linq to Objects and I need a solution for Linq to Entities.

Answer 1

So to solve this problem we need a few puzzle pieces first. The first puzzle piece is a method that can take an expression that computes a value, and then another expression that computes a new value taking the same type the first returns, and creates a new expression that represents the result of passing the result of the first function as the parameter to the second. This allows us to Compose expressions:

public static Expression<Func<TFirstParam, TResult>>
    Compose<TFirstParam, TIntermediate, TResult>(
    this Expression<Func<TFirstParam, TIntermediate>> first,
    Expression<Func<TIntermediate, TResult>> second)
{
    var param = Expression.Parameter(typeof(TFirstParam), "param");

    var newFirst = first.Body.Replace(first.Parameters[0], param);
    var newSecond = second.Body.Replace(second.Parameters[0], newFirst);

    return Expression.Lambda<Func<TFirstParam, TResult>>(newSecond, param);
}

This relies on the following tool to replace all instances of one expression with another:

public static Expression Replace(this Expression expression,
    Expression searchEx, Expression replaceEx)
{
    return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
}
internal class ReplaceVisitor : ExpressionVisitor
{
    private readonly Expression from, to;
    public ReplaceVisitor(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node)
    {
        return node == from ? to : base.Visit(node);
    }
}

We'll also need a tool to help us OR two predicate expressions together:

public static class PredicateBuilder
{
    public static Expression<Func<T, bool>> True<T>() { return f => true; }
    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    public static Expression<Func<T, bool>> Or<T>(
        this Expression<Func<T, bool>> expr1,
        Expression<Func<T, bool>> expr2)
    {
        var secondBody = expr2.Body.Replace(
            expr2.Parameters[0], expr1.Parameters[0]);
        return Expression.Lambda<Func<T, bool>>
              (Expression.OrElse(expr1.Body, secondBody), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(
        this Expression<Func<T, bool>> expr1,
        Expression<Func<T, bool>> expr2)
    {
        var secondBody = expr2.Body.Replace(
            expr2.Parameters[0], expr1.Parameters[0]);
        return Expression.Lambda<Func<T, bool>>
              (Expression.AndAlso(expr1.Body, secondBody), expr1.Parameters);
    }
}

Now that we have this we can use Compose on each property selector to map it from the property results to whether or not that property value is non-null and contains the search term. We can then OR all of those predicates together to get a filter for your query:

public static IQueryable<T> Match<T>(
    IQueryable<T> data,
    string searchTerm,
    IEnumerable<Expression<Func<T, string>>> filterProperties)
{
    var predicates = filterProperties.Select(selector =>
            selector.Compose(value => 
                value != null && value.Contains(searchTerm)));
    var filter = predicates.Aggregate(
        PredicateBuilder.False<T>(),
        (aggregate, next) => aggregate.Or(next));
    return data.Where(filter);
}