jqgrid with asp.net webmethod and json working with sorting, paging, searching and LINQ -- but needs dynamic operators

Question

Consider this extension method, that converts a string into a MemberExpression:

public static class StringExtensions
{
    public static MemberExpression ToMemberExpression(this string source, ParameterExpression p)
    {
        if (p == null)
            throw new ArgumentNullException("p");

        string[] properties = source.Split('.');

        Expression expression = p;
        Type type = p.Type;

        foreach (var prop in properties)
        {
            var property = type.GetProperty(prop);
            if (property == null)
                throw new ArgumentException("Invalid expression", "source");

            expression = Expression.MakeMemberAccess(expression, property);
            type = property.PropertyType;
        }

        return (MemberExpression)expression;
    }
}

The method below converts the strings that you have into an Lambda Expression, that you can use to filter a Linq query. It is a generic method, with T as the domain entity.

    public virtual Expression<Func<T, bool>> CreateExpression<T>(string searchField, string searchString, string searchOper)
    {
        Expression exp = null;
        var p = Expression.Parameter(typeof(T), "p");

        try
        {
            Expression propertyAccess = searchField.ToExpression(p);

            switch (searchOper)
            {
                case "bw":
                    exp = Expression.Call(propertyAccess, typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) }), Expression.Constant(searchString));
                    break;
                case "cn":
                    exp = Expression.Call(propertyAccess, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), Expression.Constant(searchString));
                    break;
                case "ew":
                    exp = Expression.Call(propertyAccess, typeof(string).GetMethod("EndsWith", new Type[] { typeof(string) }), Expression.Constant(searchString));
                    break;
                case "gt":
                    exp = Expression.GreaterThan(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                    break;
                case "ge":
                    exp = Expression.GreaterThanOrEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                    break;
                case "lt":
                    exp = Expression.LessThan(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                    break;
                case "le":
                    exp = Expression.LessThanOrEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                    break;
                case "eq":
                    exp = Expression.Equal(propertyAccess, Expression.Constant(searchString.ToType(propertyAccess.Type), propertyAccess.Type));
                    break;
                case "ne":
                    exp = Expression.NotEqual(propertyAccess, Expression.Constant(searchString, propertyAccess.Type));
                    break;
                default:
                    return null;
            }

            return (Expression<Func<T, bool>>)Expression.Lambda(exp, p);
        }
        catch
        {
            return null;
        }
    }

So, you can use it like this:

db.TBL_USERs.Where(CreateExpression<TBL_USER>("LAST_NAME", "Costa", "eq"));