How to create LINQ Query from string?

Question

I am new at LINQ and really need a help with some coding.

At the moment, I have a string and a var variables.

string temp = "from product in myEntities.Products where product.Name.Contains(_Name) select product";
var _Products = temp;
LvProducts.DataSource = _Products;
LvProducts.DataBind();

Basically, what I want to do is to be able to create a custom/complicated LINQ query by assigning it into a string beforehand. After done with composing, I assign the string into the var variable. However, this is obviously will not work. Therefore, can anyone assist me on this?

Answer 1

You have a few options:

• Use the the Dynamic Linq libraries to construct you queries on the fly. The best place to get started is by reading ScottGu's blog entry. However, I don't think these libraries support the contains method in your example. Here is a blog post explaining how to add this support.

• Directly execute SQL statements. Check out the MSDN docs for Linq to Sql or Linq to Entities.

  var _Products = myEntities.ExecuteStoreQuery<Product>
  (@"SELECT * FROM Products WHERE [Name] In ('Item1', 'Item2')");
  

• Use Linq's composable behaviour. This might not be the most elegant solution but it works really well if you do not have too many options. You can just construct your query in multiple parts.

  var _Products = from product in myEntities.Products
                  select product
  
  _Products = from product in _Products 
              where product.Name.Contains(_Name)
              select product
  
  if FilterByPrice {
      _Products = from product in _Products 
                  where product.Price > 100 
                  select product
  }
  

Answer 2

You can do this by compiling this Linq within some c# using the CodeDomProvider - Adding scripting functionality to .NET applications - but this is quite heavyweight as a solution. If you want to see more about how to do this, then take a look at LinqPad - http://www.linqpad.net - the author invites you to use the decompiler to see how it works!

If the requirement is just down to simple where clauses than an alternative might be to use Dynamic Linq - see Scott Gu's posts and the sample code from Microsoft - http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx