I am trying to convert a Linq query that I have working in Linq to be able to work in dynamic linq (using System.Linq.Dynamic) because I want a user to be able to form their own queries and this query string would be added onto other query strings at runtime.
I have a query:
db.incidents.Where(a => a.incidentLocations.Single().location.street.Contains(location);
and I have tried to convert it to the following dynamic linq string:
query =
string.Concat("incidentLocations.Single().location.street.Contains(\"", location, "\")");
db.incidents.Where(query);
Where location is a string that includes search text.
I have managed to convert all my other queries to dynamic linq but this one i am struggling with the exception error:
"No applicable aggregate method 'Single' exists"
I understand that dynamic linq does not support all extension methods, could someone possibly tell me how I could get round this problem.
Get the source of Linq.Dynamic, copy paste the Where method, change the signature and the string with the function name inside the method and you're good to go. I did it to add Single First etc, I can't copy it here because I'm not on my dev machine but I'll do it later if necessary ;)
EDIT: here's the Single method if you decide to use it:
public static object Single(this IQueryable source)
{
if (source == null) throw new ArgumentNullException("source");
return source.Provider.Execute(
Expression.Call(
typeof(Queryable), "Single",
new Type[] { source.ElementType },
source.Expression));
}
Sorry to digg up a very old thread, but I thought I could add somevaluable information!
I had to do this for First()/FirstOrDefault() with Linq to Entities instead of your Linq to SQL and I can confirm #Guillaume86 's solution sure works!
Here's how I modified MicrosoftDynamic.sql:
I added this inside the static DynamicQueryable class:
public static object FirstOrDefault(this IQueryable source)
{
if (source == null) throw new ArgumentNullException("source");
return source.Provider.Execute(
Expression.Call(
typeof(Queryable), "FirstOrDefault ",
new Type[] { source.ElementType },
source.Expression));
}
I also modified interface IEnumerableSignatures as
void FirstOrDefault();
(I used FirstOrDefault because First() isn't supported when it's not the last call in linq to entities)
You can repeat this for any supported function :)
Related
More a general question, but how can I write LINQ Lambda expressions such that they will return a default string or simply an empty string if the LINQ expression fails or returns nothing. In XSLT XPath if a match fails then one just got nothing, and the application did not crash whereas in LINQ one seems to get exceptions.
I use First() and have tried FirstOrDefault().
So example queries may be:
Customers.First(c=>c.id==CustId).Tasks.ToList();
or
Customers.Where(c=>c.id==CustId).ToList();
or
Model.myCustomers.Where(c=>c.id==CustId);
etc.
Whatever the query, if it returns no records or null, then is there a general approach to ensure the query fails gracefully?
Thanks.
There isn't anything elegant built into C# for propagating nulls when you access properties. You could create your own extension methods:
public static class Extensions
{
public static TValue SafeGet<TObject, TValue>(
this TObject obj,
Func<TObject, TValue> propertyAccessor)
{
return obj == null ? default(TValue) : propertyAccessor(obj);
}
public static IEnumerable<T> OrEmpty<T>(this IEnumerable<T> collection)
{
return collection ?? Enumerable.Empty<T>();
}
}
Used like this:
Customers.FirstOrDefault(c => c.id==CustId).SafeGet(c => c.Tasks).OrEmpty().ToList();
Customers.First(c=>c.id==CustId) will crash if there is no matching record.
There are few ways you can try to find it, if you use FirstOrDefault that'll return NULL if no match is found and you can check for NULL.
Or, you can use the .Any syntax which checks if you have any record and returns boolean.
The only query I would expect to throw an exception would be the first one (assuming that Customers is a valid collection and not null itself):
Customers.First(c=>c.id==CustId).Tasks.ToList();
This will throw an exception if there is no customer with an id of CustId (you have some casing issues with your property and variable names).
If you don't wish to throw an exception on no match, then use FirstOrDefault as you mention, and do a null check, e.g:
var customer = Customers.FirstOrDefault(c => c.id == CustId);
if (customer == null)
{
// deal with no match
return;
}
var taskList = customer.Tasks.ToList();
It seems as if only DataSet, DataTable and DataView can be used as sources when you want to use BindingSource.Find. I'm binding IQueryable and IEnumerable to my binding source, but would very much like to enjoy the 'convenience' of the BindingSource.Find method, without writing reams of code myself, as time is of the essence.
Does anyone know of an existing implementation, or at least detailed 'how to' article that can help me achieve this?
You can use the Type.GetProperty and Array.FindIndex methods to create a concise extension method.
public static int Find<T>(this IEnumerable<T> items, string propertyName, Object key)
{
PropertyInfo property = typeof(T).GetProperty(propertyName);
if(property == null)
{
throw new ArgumentException(String.Format("Type {0} contains no property named \"{1}\". ",
typeof(T).Name, propertyName), "propertyName");
}
return Array.FindIndex(items.ToArray(), i => Object.Equals(property.GetValue(i, null), key));
}
I'm trying to use the IQueryOver interface of a NHibernate session object together with a LINQ expression as a criteria for selecting records in a static class. The LINQ expressions are defined in a mapping class as Expression<Func<T, object>> to get a value for an object T:
public void SearchParameter(Expression<Func<T, object>>)
These parameters get added by extending the mapping class:
public MyMapping : FindMap<MyNHibernateMappedObject>
{
public MyMapping()
{
this.SearchParameter(x => x.SomeColumn);
}
}
My Find class defines static methods for getting the previous and next record of the same type on the time axis. Each search parameter has to be identical in both records.
The Find class gets the search parameters from the mapping configuration and compiles the expressions with .Compile(). So I have the GetQueryWithSearchParameters method:
private static Func<T, object> searchParameter;
...
public static IQueryOver<T, T> GetQueryWithSearchParameters(ISession session, T current)
{
var query = session.QueryOver<T>()
.Where(x => searchParameter(x) == searchParameter(current));
return query;
}
However when building the query, I get the following exception:
System.InvalidOperationException: variable 'x' of type MyNHibernateMappedObject' referenced from scope '', but it is not defined
I don't know exactly what is going on here, but I suspect that x is not available in the delegate somehow. What am I doing wrong here?
session.QueryOver().Where(...) takes an expression, so it's going to try evaluate your expression and translate it to a query - ie. it will try to convert searchParameter(x) == searchParameter(current) into a SQL query, which it won't know how to do.
To get this to work you will need to construct the expression in code (not using a lambda expression). However I think that this is going to be a painful exercise and I think you will find it much easier to build a Criterion and add that to the QueryOver.
I know there is a way to use Expressions and Lambdas to accomplish this but I having a hard time piecing it all together. All I need is a method that will dynamically query an Entity Framework DBSet object to find the row where the propery with the given name matches the value.
My context:
public class MyContext : DbContext
{
public IDbSet<Account> Accoounts{ get { return Set<Account>(); } }
}
The method that I'm looking to write:
public T Get<T>(string property, object value) : where T is Account
{...}
I would rather not have to use Dynamic SQL to accomplish this so no need to suggest it because I already know it's possible. What I'm really looking for is some help to accomplish this using Expressions and Lambdas
Thanks in advance, I know it's brief but it should be pretty self-explanatory. Comment if more info is needed
I'm trying to avoid dynamic linq as much as possible because the main point of linq is strongly typed access. Using dynamic linq is a solution but it is exactly the oppose of the linq purpose and it is quite close to using ESQL and building the query from sting concatenation. Anyway dynamic linq is sometimes real time saver (especially when it comes to complex dynamic ordering) and I successfully use it in a large project with Linq-to-Sql.
What I usually do is defining some SearchCriteria class like:
public class SearchCriteria
{
public string Property1 { get; set; }
public int? Property2 { get; set; }
}
And helper query extension method like:
public static IQueryable<SomeClass> Filter(this IQueryable<SomeClass> query, SearchCriteria filter)
{
if (filter.Property1 != null) query = query.Where(s => s.Property1 == filter.Property1);
if (filter.Property2 != null) query = query.Where(s => s.Property2 == filter.Property2);
return query;
}
It is not generic solution. Again generic solution is for some strongly typed processing of classes sharing some behavior.
The more complex solution would be using predicate builder and build expression tree yourselves but again building expression tree is only more complex way to build ESQL query by concatenating strings.
Here's my implementation:
public T Get<T>(string property, object value) : where T is Account
{
//p
var p = Expression.Parameter(typeof(T));
//p.Property
var propertyExpression = Expression.Property(p, property);
//p.Property == value
var equalsExpression = Expression.Equal(propertyExpression, Expression.Constant(value));
//p => p.Property == value
var lambda = Expression.Lambda<Func<T,bool>>(equalsExpression, p);
return context.Set<T>().SingleOrDefault(lambda);
}
It uses EF 5's Set<T>() method. If you are using a lower version, you'll need to implement a way of getting the DbSet based on the <T> type.
Hope it helps.
Dynamic Linq may be an option. Specify your criteria as a string and it will get built as an expression and ran against your data;
An example from something I have done;
var context = new DataContext(ConfigurationManager.ConnectionStrings["c"].ConnectionString);
var statusConditions = "Status = 1";
var results = (IQueryable)context.Contacts.Where(statusConditions);
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
I am using Entity Framework 4.0 and trying to use the "Contains" function of one the object sets in my context object. to do so i coded a Comparer class:
public class RatingInfoComparer : IEqualityComparer<RatingInfo>
{
public bool Equals(RatingInfo x, RatingInfo y)
{
var a = new {x.PlugInID,x.RatingInfoUserIP};
var b = new {y.PlugInID,y.RatingInfoUserIP};
if(a.PlugInID == b.PlugInID && a.RatingInfoUserIP.Equals(b.RatingInfoUserIP))
return true;
else
return false;
}
public int GetHashCode(RatingInfo obj)
{
var a = new { obj.PlugInID, obj.RatingInfoUserIP };
if (Object.ReferenceEquals(obj, null))
return 0;
return a.GetHashCode();
}
}
when i try to use the comparer with this code:
public void SaveRatingInfo2(int plugInId, string userInfo)
{
RatingInfo ri = new RatingInfo()
{
PlugInID = plugInId,
RatingInfoUser = userInfo,
RatingInfoUserIP = "192.168.1.100"
};
//This is where i get the execption
if (!context.RatingInfoes.Contains<RatingInfo>(ri, new RatingInfoComparer()))
{
//my Entity Framework context object
context.RatingInfoes.AddObject(ri);
context.SaveChanges();
}
}
i get an execption:
"LINQ to Entities does not recognize the method 'Boolean Contains[RatingInfo](System.Linq.IQueryable1[OlafCMSLibrary.Models.RatingInfo], OlafCMSLibrary.Models.RatingInfo,
System.Collections.Generic.IEqualityComparer1[OlafCMSLibrary.Models.RatingInfo])' method, and his method cannot be translated into a store expression."
Since i am not proficient with linQ and Entity Framework i might be making a mistake with my use of the "var" either in the "GetHashCode" function or in general.
If my mistake is clear to you do tell me :) it does not stop my project! but it is essential for me to understand why a simple comparer doesnt work.
Thanks
Aaron
LINQ to Entities works by converting an expression tree into queries against an object model through the IQueryable interface. This means than you can only put things into the expression tree which LINQ to Entities understands.
It doesn't understand the Contains method you are using, so it throws the exception you see. Here is a list of methods which it understands.
Under the Set Methods section header, it lists Contains using an item as supported, but it lists Contains with an IEqualityComparer as not supported. This is presumably because it would have to be able to work out how to convert your IEqualityComparer into a query against the object model, which would be difficult. You might be able to do what you want using multiple Where clauses, see which ones are supported further up the document.