build dynamic linq query in where clause

Question

I want to build a dynamic query that able to extend the where clause condition if the value of the object is not empty string. Here is the code

public IEnumerable<Filter> GetFilter(Filter filter)
{
    var y = ConditionalAttribute(filter);
    var query =
        from sub in Subscriptions
        join u in Users
        on sub.UserID equals u.Id
        join od in Order_Details1
        on sub.OD_Id equals od.OD_Id
        join p in Products
        on od.ProductId equals p.ProductId
        where p.Type == "Testing" + y
        select new Filter
        {
            //do something
        };

for the Filter Object, here is the code

public class Filter
{
    public int UserId { get; set; }
    public string FirstName { get; set;}
}

the idea is if the filter.FirstName is not null it will append the where clause like this

public String ConditionalAttribute(Filter filter)
{ 
    if(filter.FirstName != "")
        return "&& u.First_Name = " + filter.FirstName + "";           
}

Is there any way to append where clause by string like the code above? Because I've tried the approach above and it fails thanks

It is not simple or easy, unless you are already using a framework which does the heavy lifting for you.. I guess, you want something like this - dynamiclinq.codeplex.com — Vikas Gupta
– Vikas Gupta, Commented Oct 22, 2014 at 3:58

Code Uniquely · Accepted Answer · 2014-10-22 04:05:39Z

Create as many dynamic terms as you want to use in small methods that return an IQueryable.

public IQueryable ConditionalAttribute(IQueryable query, Filter filter)
{ 
    if(filter.FirstName != "") {
        query = query.Where(x => x.First_Name == filter.FirstName);
    }

    return query;
}

then apply them after the initial LINQ statement as you require:

public IEnumerable<Filter> GetFilter(Filter filter)
{
    var query =
        from sub in Subscriptions
        join u in Users
        on sub.UserID equals u.Id
        join od in Order_Details1
        on sub.OD_Id equals od.OD_Id
        join p in Products
        on od.ProductId equals p.ProductId
        where p.Type == "Testing"
        select new Filter
        {
            //do something
        };

        query = ConditionalAttribute(query, filter);

The statement isn't going to run until you project it into by using .ToList() or FirstOrDefault() or something like that so you can keep chaining onto query in this way as many times as you need.

Andrew Shepherd · Accepted Answer · 2014-10-22 04:29:43Z

I tend to prefer using standard C# syntax rather than the LINQ syntax, but, having said that, I find that LINQ syntax is more elegant when it comes to joining queries.

Here's an approach which utilizes the standard C# syntax to dynamically filter the source containers, then uses LINQ syntax to create the joining query.

public IEnumerable<Filter> GetFilter(Filter filter)
{
    var y = ConditionalAttribute(filter);

    IEnumerable<User> filteredUsers = Users;
    if(!string.IsNullOrEmpty(filter.FirstName))
    {
        filteredUsers = filteredUsers.Where(u => u.First_Name == filter.FirstName);
    }

    var query =
        from sub in Subscriptions
        join u in filteredUsers
        on sub.UserID equals u.Id
        join od in Order_Details1
        on sub.OD_Id equals od.OD_Id
        join p in Products
        on od.ProductId equals p.ProductId
        where p.Type == "Testing" + y
        select new Filter
        {
            //do something
        };

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