This line of code:
db.Set<T>().Max(d => d.GetType().GetProperty("DateTimePropertyName").GetValue(d))
causes this exception:
LINQ to Entities does not recognize the method 'System.Object GetValue(System.Object)' method
How do I make it work?
Your trying to run c# code that's suppose to get translated into SQL somehow and can't.
This is a good guide to start with.
You have two options:
1.Fetch the data from the DB and go through it with LINQ to Objects - Probally the easiest and not the best way to do things since some queries can return large collections.
2.Try to find a better way to do what you are doing. Why would you want this reflection code to run? What's the purpose? Is the DateTimePropertyName non public? If so why? Otherwise something like this should work:
db.Set<T>().Max(d => d.DateTimePropertyName);
By default it assumes that you pass Expression<Func<T,TResult>>, but what you need is to pass Func<T,TResult>:
Func<T, object> f = p => p.GetType().GetProperty("DateTimePropertyName").GetValue(p);
_context.Set<T>().Max(f);
It comes from IEnumerable, so it will hit performance if you table is large
Related
We are investigating using LinQ to query an internal dynamic collection created by Dapper. The question is:
How to execute dynamic LinQ against the collection using Scott Guthrie dynamic linq (or some other technology if possible)? (http://weblogs.asp.net/scottgu/dynamic-linq-part-1-using-the-linq-dynamic-query-library)
This is what we want to do (much simplified):
Use Dapper to return a dynamic collection (here called rows):
rows = conn.Query(“select ACCOUNT, UNIT, AMOUNT from myTable”);
If we use a “static” LinQ query there is no problem. So this works fine:
var result = rows.Where(w => w.AMOUNT > 0);
But we would like to write something similar to this using dynamic Linq:
var result = rows.Where("AMOUNT > 0");
But we can’t get this to work.
The error we get is:
No property or field ‘AMOUNT’ exists in type ‘Object’
(We have tried a lot of other syntax also – but cant get it to work)
Please note: We do not want to use dynamic SQL when Dapper requests data from the database (that is easy). We want to execute many small dynamic Linq statements on the collection that the Dapper query returns.
Can it be that ScottGu dynamic Linq only works with ‘LinQ to SQL’?
Is there some other alternative approach to achieve the same thing?
(Performance is a key issue)
/Erik
conn.Query("...")
returns an IEnumerable<dynamic>. However, "dynamic LINQ" pre-dates dynamic, and presumably nobody has updated it to work with dynamic; it could certainly be done, but it is work (and isn't trivial). Options:
use Query<T> for some T
make the required changes to "dynamic LINQ", and preferably make those changes available to the wider community
I suspect that conn.Query(“select ACCOUNT, UNIT, AMOUNT from myTable”); returns IEnumerable<object>. In order for DLinq to work, you need to have IEnumerable<TheActualType>.
You can try this:
conn.Query<dynamic>("yourQueryString")
.ToList()
//.ToAnonymousList()
.Where("AMOUNT > 0");
If that doesn't work, you could try and use ToAnonymousList, that tries to return the IList<TheActualType.
The title is pretty unclear. But I couldn't find the proper words. Generally Linq works in the below syntax
MyList.Where().Select(x => {MyFunction(x);})
It is good in ordinary conditions but in some situation like in my case. I am creating a tree structure using dictionary. In this if I want to add a set
Set.Foreach(x => {(MyDict[logEvent.level][logEvent.event][logEvent.subevent][logEvent.filePath]).Add(x);});
But it would be nice if I can do like below
(MyDict[logEvent.level][logEvent.event][logEvent.subevent][logEvent.filePath]).Add(MySet.Foreach(x => {return x;}));
Is there any way possible to dothis ?
You can do it, if object stored in Dict has AddRange method which accepts IEnumerable<T>. But you should ski[ ForEach and just pass MySet:
MyDict[logEvent.level][logEvent.event][logEvent.subevent][logEvent.filePath]).AddRange(MySet);
I have the following lines of code in C#
that gets data using DataTables
This is pretty generic and helps me with multiple tables.
object obj = ((DataRowView)editingElement.DataContext).Row[this.SelectedValuePath];
I want to change this statement to a generic LINQ statement so that i can use it with multiple LINQ tables too.
Can somebody help me figure this out?
I don't think you can use LINQ to make the code you wrote nicer or more elegant in any way. I assume that the type of editingElement.DataContext is object, so you'll need to write the cast anyway. If you forget about the casting, your code is just indexed access:
var rows = (DataRowView)editingElement.DataContext;
object obj = rows.Row[this.SelectedValuePath];
LINQ doesn't have any features that would make indexing nicer, so I think this is the best you can get. One possible ugly thing is that you get object as the result and you'll need to cast that to some other type (e.g. CustomerInfo).
If you were using LINQ from the beginning (to populate the data for the DataContext), you could probably write something like this to access the customer:
var rows = (IEnumerable<CustomerInfo>)editingElement.DataContext;
CustomerInfo info = rows.Row[this.SelectedValuePath];
This would be a bit more elegant, because you'd need just a single cast. However, I think that your code is fine and LINQ cannot help you (in this piece of code).
Ok, understand that I come from Cold Fusion so I tend to think of things in a CF sort of way, and C# and CF are as different as can be in general approach.
So the problem is: I want to pull a "table" (thats how I think of it) of data from a SQL database via LINQ and then I want to do some computations on it in memory. This "table" contains 6 or 7 values of a couple different types.
Right now, my solution is that I do the LINQ query using a Generic List of a custom Type. So my example is the RelevanceTable. I pull some data out that I want to do some evaluation of the data, which first start with .Contains. It appears that .Contains wants to act on the whole list or nothing. So I can use it if I have List<string>, but if I have List<ReferenceTableEntry> where ReferenceTableEntry is my custom type, I would need to override the IEquatable and tell the compiler what exactly "Equals" means.
While this doesn't seem unreasonable, it does seem like a long way to go for a simple problem so I have this sneaking suspicion that my approach is flawed from the get go.
If I want to use LINQ and .Contains, is overriding the Interface the only way? It seems like if there way just a way to say which field to operate on. Is there another collection type besides LIST that maybe has this ability. I have started using List a lot for this and while I have looked and looked, a see some other but not necessarily superior approaches.
I'm not looking for some fine point of performance or compactness or readability, just wondering if I am using a Phillips head screwdriver in a Hex screw. If my approach is a "decent" one, but not the best of course I'd like to know a better, but just knowing that its in the ballpark would give me little "Yeah! I'm not stupid!" and I would finish at least what I am doing completely before switch to another method.
Hope I explained that well enough. Thanks for you help.
What exactly is it you want to do with the table? It isn't clear. However, the standard LINQ (-to-Objects) methods will be available on any typed collection (including List<T>), allowing any range of Where, First, Any, All, etc.
So: what is you are trying to do? If you had the table, what value(s) do you want?
As a guess (based on the Contains stuff) - do you just want:
bool x= table.Any(x=>x.Foo == foo); // or someObj.Foo
?
There are overloads for some of the methods in the List class that takes a delegate (optionally in the form of a lambda expression), that you can use to specify what field to look for.
For example, to look for the item where the Id property is 42:
ReferenceTableEntry found = theList.Find(r => r.Id == 42);
The found variable will have a reference to the first item that matches, or null if no item matched.
There are also some LINQ extensions that takes a delegate or an expression. This will do the same as the Find method:
ReferenceTableEntry found = theList.FirstOrDefault(r => r.Id == 42);
Ok, so if I'm reading this correctly you want to use the contains method. When using this with collections of objects (such as ReferenceTableEntry) you need to be careful because what you're saying is you're checking to see if the collection contains an object that IS the same as the object you're comparing against.
If you use the .Find() or .FindAll() method you can specify the criteria that you want to match on using an anonymous method.
So for example if you want to find all ReferenceTableEntry records in your list that have an Id greater than 1 you could do something like this
List<ReferenceTableEntry> listToSearch = //populate list here
var matches = listToSearch.FindAll(x => x.Id > 1);
matches will be a list of ReferenceTableEntry records that have an ID greater than 1.
having said all that, it's not completely clear that this is what you're trying to do.
Here is the LINQ query involved that creates the object I am talking about, and the problem line is:
.Where (searchWord => queryTerms.Contains(searchWord.Word))
List<queryTerm> queryTerms = MakeQueryTermList();
public static List<RelevanceTableEntry> CreateRelevanceTable(List<queryTerm> queryTerms)
{
SearchDataContext myContext = new SearchDataContext();
var productRelevance = (from pwords in myContext.SearchWordOccuranceProducts
where (myContext.SearchUniqueWords
.Where (searchWord => queryTerms.Contains(searchWord.Word))
.Select (searchWord => searchWord.Id)).Contains(pwords.WordId)
orderby pwords.WordId
select new {pwords.WordId, pwords.Weight, pwords.Position, pwords.ProductId});
}
This query returns a list of WordId's that match the submitted search string (when it was List and it was just the word, that works fine, because as an answerer mentioned before, they were the same type of objects). My custom type here is queryTerms, a List that contains WordId, ProductId, Position, and Weight. From there I go about calculating the relevance by doing various operations on the created object. Sum "Weight" by product, use position matches to bump up Weights, etc. My point for keeping this separate was that the rules for doing those operations will change, but the basic factors involved will not. I would have even rather it be MORE separate (I'm still learning, I don't want to get fancy) but the rules for local and interpreted LINQ queries seems to trip me up when I do.
Since CF has supported queries of queries forever, that's how I tend to lean. Pull the data you need from the db, then do your operations (which includes queries with Aggregate functions) on the in-memory table.
I hope that makes it more clear.
What would be the best approach to allow users to define a WHERE-like constraints on objects which are defined like this:
Collection<object[]> data
Collection<string> columnNames
where object[] is a single row.
I was thinking about dynamically creating a strong-typed wrapper and just using Dynamic LINQ but maybe there is a simpler solution?
DataSet's are not really an option since the collections are rather huge (40,000+ records) and I don't want to create DataTable and populate it every time I run a query.
What kind of queries do you need to run? If it's just equality, that's relatively easy:
public static IEnumerable<object[]> WhereEqual(
this IEnumerable<object[]> source,
Collection<string> columnNames,
string column,
object value)
{
int columnIndex = columnNames.IndexOf(column);
if (columnIndex == -1)
{
throw new ArgumentException();
}
return source.Where(row => Object.Equals(row[columnIndex], value);
}
If you need something more complicated, please give us an example of what you'd like to be able to write.
If I get your point : you'd like to support users writting the where clause externally - I mean users are real users and not developers so you seek solution for the uicontrol, code where condition bridge. I just though this because you mentioned dlinq.
So if I'm correct what you want to do is really :
give the user the ability to use column names
give the ability to describe a bool function (which will serve as where criteria)
compose the query dynamically and run
For this task let me propose : Rules from the System.Workflow.Activities.Rules namespace. For rules there're several designers available not to mention the ones shipped with Visual Studio (for the web that's another question, but there're several ones for that too).I'd start with Rules without workflow then examine examples from msdn. It's a very flexible and customizable engine.
One other thing: LINQ has connection to this problem as a function returning IQueryable can defer query execution, you can previously define a query and in another part of the code one can extend the returned queryable based on the user's condition (which then can be sticked with extension methods).
When just using object, LINQ isn't really going to help you very much... is it worth the pain? And Dynamic LINQ is certainly overkill. What is the expected way of using this? I can think of a few ways of adding basic Where operations.... but I'm not sure how helpful it would be.
How about embedding something like IronPython in your project? We use that to allow users to define their own expressions (filters and otherwise) inside a sandbox.
I'm thinking about something like this:
((col1 = "abc") or (col2 = "xyz")) and (col3 = "123")
Ultimately it would be nice to have support for LIKE operator with % wildcard.
Thank you all guys - I've finally found it. It's called NQuery and it's available from CodePlex. In its documentation there is even an example which contains a binding to my very structure - list of column names + list of object[]. Plus fully functional SQL query engine.
Just perfect.