LINQ to SQL indirect variable - sql

Question

First, may I apologise if this question is too basic for this forum. I am very new to this and am struggling with a number of basics - but am persevering!
I have a problem in that I want to create a LINQ to SQL query with a WHERE clause with an indirect reference to one of the columns in my database. For example, if I had some code that looked something like this:
var PLMatches = from PLMat in db1.PLAccountHeaders
where PLMat.CompanyAlphaId.Equals(CoId)
&& dbField.Equals(Limit)
select PLMat;
such that dbField would be a variable containing the name of the database field. So, if the value of dbField was"PLMat.ItemCode" it would happily go away and return all instances of records with an ItemCode equal to the value of Limit and if value of dbField was"PLMat.ItemName" it would happily go away and return all instances of records with an ItemName equal to the value of Limit and so on.
I would really appreciate some help on this both to answer a very specific problem and I am sure it will enhance my basic understanding.
Many thanks

Answer 1

i dont think that what you are asking for is possible in LINQ, you can however use the Dynamic Linq Library.
here's the link
You download the zip, unzip it and then just reference it in your solution.
you can then do something like this:
// the "normal" LINQ way:
var query = from x in ctx.People
where x.city == "Boston" && x.age > 18
orderby x.ID
select x;
// the Dynamic Linq way:
var query = database.People
.Where("city = Boston AND age > 18")
.OrderBy("id")
.Select("New(Person as Name, Age)");

Answer 2

Another way (besides dynamic linq) is composing your query conditionally. Like so:
var PLMatches = from PLMat in db1.PLAccountHeaders
where PLMat.CompanyAlphaId.Equals(CoId)
select PLMat;
if (dbField == "ItemCode")
PLMatches = PLMatches.Where(m => m.ItemCode == Limit);
else if (dbField == "ItemName")
PLMatches = PLMatches.Where(m => m.ItemName == Limit);
else if (dbField == ...
The code looks a bit repetitive, but I always prefer to do it this way if the number of possible conditions is not too large. The advantages are, first, that the code integrity check is at compile time and not at runtime (as with dynamic linq) and, second, it is very clear what the code is about. (The second advantage also goes for dynamic linq, but less so for a third alternative: building expressions in code).
You can compose linq queries this way because of deferred execution. PLMatches = PLMatches.Where... only changes the query, but nothing is executed yet.