How to Execute String formatted LINQ query in c# MVC3

Question

     public ActionResult GenerateReport(FormCollection Form)
        {
            var type_id = Form["type_id"];           
            var Start_Date = Form["Start_Date"];

            StringBuilder sb = new StringBuilder();          
            sb.Append("db.contracts.Where(");
            if (!type_id.Equals(null))
            {
                sb.Append("a => a.type_id == type_id");
            }

            if (!Start_Date.Equals(null))
            {
                sb.Append("&& a => a.Start_Date == Start_Date");
            }
            sb.Append(");");
           //here I need to run the query!!
            return View(cm);
        }

How can i execute this LINQ query in C#, I just want to run the query and get the results..

Please help me...

Thanks in Advance...

@GertArnold, you can compile strings using the C# CodeDom classes at runtime, you dont have to emit things.. See GekaLlaur's answer for reference (too many edits, my answers irrelevant now too =() — Dead.Rabit
– Dead.Rabit, Commented May 28, 2013 at 14:57
@Dead.Rabit Yeah, all right, it's probably overkill either way unless this is some kind of special assignment. But I think the OP is only looking for a way to add predicates dynamically because the data source part of the query is know at run time (apparently). (BTW I removed my previous comment just before yours :) ) — Gert Arnold
– Gert Arnold, Commented May 28, 2013 at 15:01
GertArnold I think you're right, though I have implemented this feature a few times @DimitarDimitrov though mostly in developer tools (internal log pages for example) — Dead.Rabit
– Dead.Rabit, Commented May 28, 2013 at 15:07

Community · Accepted Answer · 2017-05-23 12:11:19Z

1

EDIT: this answer works with the original question content, how to execute this LINQ expression from a string:

var expression = @"( from a in Assignments
    where a.assignmentID == 1 && a.assignmentID == 4 )
    select new AssignmentModel
    {
        Title = a.Title,
        Description = a.Description
    }).ToList()";

though this answer should work with the generated string in the current question, only you would be injecting db.contracts rather than Assignments as I explain below.

The alternative to using the compilation classes as @GekaLlaur suggests is to compile the string into an Expression tree, which is possibly more appropriate here as you're compiling LINQ expressions.

The solution found in this SO post (Chosen Solution) looks as though it should work out of the box for you, only using var p = Expression.Parameter(typeof(List<AssignmentModel>), "Assignments"); to describe your parameter. I would test it with your specific example but I don't already have the libraries (linked here).

edited May 23, 2017 at 12:11

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answered May 28, 2013 at 14:52

Dead.Rabit

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Dead.Rabit Over a year ago

Just an added note, compiling code (any code) from strings at runtime is the definition of a security hole.. be very very careful to ensure that any parsed strings meet your expectations and can't run riot on the host PC.

Community · Accepted Answer · 2017-05-23 12:11:20Z

1

Don't do this. Use a Predicate Builder instead!

Here is a great implementation from C# in a nutshell: http://www.albahari.com/nutshell/predicatebuilder.aspx

The predicate builder uses a free library called linqkit: http://www.albahari.com/nutshell/linqkit.aspx

Here is another SO question that addresses this topic and how the whole thing works How does PredicateBuilder work

edited May 23, 2017 at 12:11

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answered May 31, 2013 at 13:47

amhed

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