How do I combine LINQ expressions into one?

Question

I\'ve got a form with multiple fields on it (company name, postcode, etc.) which allows a user to search for companies in a database. If the user enters values in more than

Answer 1

You can use Enumerable.Aggregate combined with Expression.AndAlso. Here's a generic version:

Expression> AndAll(
    IEnumerable>> expressions) {

    if(expressions == null) {
        throw new ArgumentNullException("expressions");
    }
    if(expressions.Count() == 0) {
        return t => true;
    }
    Type delegateType = typeof(Func<,>)
                            .GetGenericTypeDefinition()
                            .MakeGenericType(new[] {
                                typeof(T),
                                typeof(bool) 
                            }
                        );
    var combined = expressions
                       .Cast()
                       .Aggregate((e1, e2) => Expression.AndAlso(e1, e2));
    return (Expression>)Expression.Lambda(delegateType, combined);
}

Your current code is never assigning to combined:

expr => Expression.And(combined, expr);

returns a new Expression that is the result of bitwise anding combined and expr but it does not mutate combined.