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Thank you for reading. I have a question that is breaking my mind... I do have a table in my database and I was thinking on filtering the records using WHERE (linq), the only thing is that I would like to filter using a function that return true/false according to what it finds... example:
var myrecords = db.Locations.Where(w => {
**//some code here...
//some code here...
return false;**
});
According to my understanding, it requires a Func<location, bool> that takes a location (in this case, wherever the type is) and returns a bool in order to include or not the record... the code above fails saying: "A lambda expression with a statement body cannot be converted to an expression tree"
Now, if I create the Func and then call it inside the parenthesis, then I can include multi-line...
Func<Location, bool> processingFunc = location =>
{
//some code...
//some code...
return true;
};
var x = db.Locations.Where(location => processingFunc(location));
my question is: If the FUNC "processingFunc" I created is the same as the code block, why I can't do the block where the WHERE is? am I missing something here? in other words, if I create a separated FUNC, I can add múltiple lines of code and return the desired boolean, but if I try to do it in the WHERE parenthesis then I cant...
Thank you for your help :-)
The error message is telling you that Linq-2-Entites cannot take your created function and generate T-SQL that can be submitted to the database engine to be executed against a database.
Yes... but why is accepting the external FUNC then?? thank for your help!
I don't know. I am just telling you what the error message means in the context of how you are using it that is causing the exception. You cannot do it, and you need to take another approach.
I understand what you say... what makes me think is, the method is expecting a function that takes an instance of an object, and must return true/false.
According to my understanding of c#, it should not matter what the function do inside or how it does the things, the important thing is that it must return a boolean value, just a matter of convenience, they use the lambda but otherwise, they should do it providing an external function... both solutions should work but they do not... will that happen in other situations with c# also?
According to my understanding of c#, it should not matter what the function do inside or how it does the things, the important thing is that it must return a boolean value, just a matter of convenience, they use the lambda but otherwise, they should do it providing an external function... both solutions should work but they do not... will that happen in other situations with c# also?
No, Linq using lambda expressions are not panacea solutions. If the EF engine cannot generate the T-SQL becuase you want to use a function within Linq-2-Entities using a lambda expression query, it can't do it. How does Linq-2-Entites know what's in a function you created in order to generate the T-SQL? EF can't look inside a given function or even a C# function you decided to use in a Linq-2-Entities query.
I see your point, all of these lambda expressions must be converted in the end to t-sql... but we are missing the point here, I'm not saying it should accept any type of code, etc... my point here is that if I want to make an anonymous block that returns a boolean I can't do it BUT if I point to an external FUNC it do work, what difference makes if I create an anonymous block of code or an external func if both do the same? the questions is not related to what LINQ can do or not, or if this can translate the final code to T-sql, its more related to C#.
In my point of view, I should be allowed to provide an anonymous function or if I like, an anonymous function using a lambda expression, for me it is weird that if both things do the same, it accepts to receive an external function and not the anonymous code block...
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In fact I think there's a little bit more to it. What Linq is expecting is something it can visit and transform to SQL. In order to do that it does not expect just a lambda inside the where but something that can be transformed into a Linq Expression Tree.
Lambdas with a body are by definition not transformable to an Expression Tree, and hence the error.
When you create the external function and use it inside the where the result is not conceptually the same: Of course, you have been able to get a single line lambda which can now be transformed into an Expression Tree but there is a difference: Linq is now unaware of the content of the external function and so he is not going to be able to visit it and therefore, no SQL equivalent. Due to this, the where is not going to be executed on SQL Server, but in the client. In other words, Linq will retrieve all the records in your query and then filter client side.
In lots of cases this can be useful but take care!
Hope it helps.
I want to make an anonymous block that returns a boolean I can't do it BUT if I point to an external FUNC it do work, what difference makes if I create an anonymous block of code or an external func if both do the same?
You can call a method that helps build the Linq projection for an anaymous type or a known type within the projection for a property or an object within the type, which is a projection of the results returned from the Linq query.
Hello,
Try the following, wrap an Expression around the Func
Expression<Func<Locations, bool>> processingFunc = p => p.City == "Portland";
var myrecords = db.Locations.Where(processingFunc).Count();