I have two classes - MyBaseClass and BaseClassContainer - that are declared like such:
public class MyBaseClass
{
private Guid id;
public Guid ID
{
if (id == Guid.Empty)
{
id = Guid.NewGuid();
}
return id;
}
//...Other Properties omitted for brevity
}
and
public class BaseClassContainer : INotifyPropertyChanged
{
private ObservableCollection<MyBaseClass> baseClasses;
public ObservableCollection<MyBaseClass> BaseClasses
{
//...Omitted for brevity...
}
}
Then in my code I have an ObservableCollection of type BaseClassContainer (BaseClassContainerCollection). What I'm trying to figure out is how can I use LINQ to select a single BaseClassContainer from the ObservableCollection where one of its MyBaseClass.ID matches a specific Guid. The reason I'm using the Single() method is because I know they're all going to be unique.
I've tried the following but it doesn't work:
var result = BaseClassContainerCollection.Single(container => container.BaseClasses.Single(baseClass => baseClass.ID == specificGuid));
I get an error saying: Cannot implicitly convert type 'MyBaseClass' to 'bool'. What am I missing?
Lets break apart your query:
BaseClassContainerCollection.Single(yourPredicate);
Single, as it is used here, basically says "filter BaseClassContainerCollection on this predicate" (a "filter" function that evaluates to true or false for whether or not to include it in the results). Instead of a function that returns true/false, you're saying you want it to evaluate to a MyBaseClass, which doesn't make sense. Your inner call to Single makes sense, because x => x.Id == guid is a function that returns true/false and filters to only those elements that meet the criteria (then states that you know there will only be one of them in the results or else throw an exception).
What you want to do is Select the single MyBaseClass result from the inner query, then call Single on the result (without a predicate) since you know the result should only have one item returned. I believe you're looking for:
BaseClassContainerCollection.Select(container => container.BaseClasses.Single(baseClass => baseClass.ID == specificGuid)).Single();
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();
I am in the middle of a refactoring cycle where I converted some extension methods that used to look like this:
public static IQueryable<Family> FilterOnRoute(this IQueryable<Family> families, WicRoute route)
{
return families.Where(fam => fam.PODs
.Any(pod => pod.Route.RouteID == route.RouteID));
}
to a more fluent implementation like this:
public class SimplifiedFamilyLinqBuilder
{
private IQueryable<Family> _families;
public SimplifiedFamilyLinqBuilder Load(IQueryable<Family> families)
{
_families = families;
return this;
}
public SimplifiedFamilyLinqBuilder OnRoute(WicRoute route)
{
_families = _families.Where(fam => fam.PODs
.Any(pod => pod.Route.RouteID == route.RouteID));
return this;
}
public IQueryable<Family> AsQueryable()
{
return _families;
}
}
which I can call like this: (note this is using Linq-to-Nhibernate)
var families =
new SimplifiedFamilyLinqBuilder()
.Load(session.Query<Family>())
.OnRoute(new WicRoute() {RouteID = 1})
.AsQueryable()
.ToList();
this produces the following SQL which is fine with me at the moment: (of note is that the above Linq is being translated to a SQL Query)
select ... from "Family" family0_
where exists (select pods1_.PODID from "POD" pods1_
inner join Route wicroute2_ on pods1_.RouteID=wicroute2_.RouteID
where family0_.FamilyID=pods1_.FamilyID
and wicroute2_.RouteID=#p0);
#p0 = 1
my next effort in refactoring is to move the query part that deals with the child to another class like this:
public class SimplifiedPODLinqBuilder
{
private IQueryable<POD> _pods;
public SimplifiedPODLinqBuilder Load(IQueryable<POD> pods)
{
_pods = pods;
return this;
}
public SimplifiedPODLinqBuilder OnRoute(WicRoute route)
{
_pods = _pods.Where(pod => pod.Route.RouteID == route.RouteID);
return this;
}
public IQueryable<POD> AsQueryable()
{
return _pods;
}
}
with SimplifiedFamilyLinqBuilder changing to this:
public SimplifiedFamilyLinqBuilder OnRoute(WicRoute route)
{
_families = _families.Where(fam =>
_podLinqBuilder.Load(fam.PODs.AsQueryable())
.OnRoute(route)
.AsQueryable()
.Any()
);
return this;
}
only I now get this error:
Remotion.Linq.Parsing.ParserException : Cannot parse expression 'value(Wic.DataTests.LinqBuilders.SimplifiedPODLinqBuilder)' as it has an unsupported type. Only query sources (that is, expressions that implement IEnumerable) and query operators can be parsed.
I started to implement IQueryable on SimplifiedPODLinqBuilder(as that seemed more logical than implementing IEnumberable) and thought I would be clever by doing this:
public class SimplifiedPODLinqBuilder : IQueryable
{
private IQueryable<POD> _pods;
...
public IEnumerator GetEnumerator()
{
return _pods.GetEnumerator();
}
public Expression Expression
{
get { return _pods.Expression; }
}
public Type ElementType
{
get { return _pods.ElementType; }
}
public IQueryProvider Provider
{
get { return _pods.Provider; }
}
}
only to get this exception (apparently Load is not being called and _pods is null):
System.NullReferenceException : Object reference not set to an instance of an object.
is there a way for me to refactor this code out that will parse properly into an expression that will go to SQL?
The part fam => _podLinqBuilder.Load(fam.PODs.AsQueryable() is never going to work, because the linq provider will try to parse this into SQL and for that it needs mapped members of Family after the =>, or maybe a mapped user-defined function but I don't know if Linq-to-Nhibernate supports that (I never really worked with it, because I still doubt if it is production-ready).
So, what can you do?
To be honest, I like the extension methods much better. You switched to a stateful approach, which doesn't mix well with the stateless paradigm of linq. So you may consider to retrace your steps.
Another option: the expression in .Any(pod => pod.Route.RouteID == route.RouteID)); could be paremeterized (.Any(podExpression), with
OnRoute(WicRoute route, Expression<Func<POD,bool>> podExpression)
(pseudocode).
Hope this makes any sense.
You need to separate methods you intend to call from expressions you intend to translate.
This is great, you want each of those methods to run. They return an instance that implements IQueryable<Family> and operate on that instance.
var families = new SimplifiedFamilyLinqBuilder()
.Load(session.Query<Family>())
.OnRoute(new WicRoute() {RouteID = 1})
.AsQueryable()
.ToList();
This is no good. you don't want Queryable.Where to get called, you want it to be an expression tree which can be translated to SQL. But PodLinqBuilder.Load is a node in that expression tree which can't be translated to SQL!
families = _families
.Where(fam => _podLinqBuilder.Load(fam.PODs.AsQueryable())
.OnRoute(route)
.AsQueryable()
.Any();
You can't call .Load inside the Where expression (it won't translate to sql).
You can't call .Load outside the Where expression (you don't have the fam parameter).
In the name of "separation of concerns", you are mixing query construction methods with query definition expressions. LINQ, by its Integrated nature, encourages you to attempt this thing which will not work.
Consider making expression construction methods instead of query construction methods.
public static Expression<Func<Pod, bool>> GetOnRouteExpr(WicRoute route)
{
int routeId = route.RouteID;
Expression<Func<Pod, bool>> result = pod => pod.Route.RouteID == route.RouteID;
return result;
}
called by:
Expression<Func<Pod, bool>> onRoute = GetOnRouteExpr(route);
families = _families.Where(fam => fam.PODs.Any(onRoute));
With this approach, the question is now - how do I fluidly hang my ornaments from the expression tree?
I have a function inside a class that will run a Linq to Entities query (or any type of Linq query actually), and it's gonna return 2 columns in the resultset. I would like to return an object to whoever is calling my function that will allow Intellisense to know what I have returned.
Let me explain. If I have a function like this:
public static IQueryable GetInfo(MyEntityModel oEntityModel)
{
var query =
(from t in oEntityModel.Table1
from u in t.Table2
where t.Status == true &&
u.Status == true
select new
{
t.Column1,
u.Column2
})
return query;
}
What can (should) I put instead of IQueryable so that whoever calls my GetInfo function, will get Intellisense from the resultset, and show that it has a Column1 and Column2?
var linqresult = ClsLinqTeste.GetInfo(oEntityModel);
if (linqresult.Column1 == 1)
{
foreach (var oItem in linqresult)
{
.. do stuff...
}
}
Tks
You cannot return an anonymous type from a function, they are strictly "inline" classes. When you return it, the foreach loop will only be able to interpret the result as an plain object. I guess you could use reflection to query the property names and values, however it seems much more straight forward to define a data transfer type to hold the results.
See this question, and this blog post.
So you could create a simple struct or class:
public class MyDataResult
{
public object Column1 { get; set; }
public object Column2 { get; set; }
}
Then modify your query in the function:
public static IQueryable<MyDataResult> GetInfo(MyEntityModel oEntityModel)
{
var query =
(from t in oEntityModel.Table1
from u in t.Table2
where t.Status == true &&
u.Status == true
select new MyDataResult
{
Column1 = t.Column1,
Column2 = u.Column2
})
return query;
}
Something like that should work. Note that I used "object" for the properties in MyDataResult. I don't know the types of the columns you are returning, you should use the actual types in order to get full intellisense.
You are returning a collection of anonymous types, they will be casted to objects, so when you try to iterate over them, altough they will be your objects (and they will contain your properties) at compile time they will be casted to objects:
foreach (var x in ClsLinqTeste.GetInfo(oEntityModel))
{
//x is an Object
}
You can read more about it here.
If you want to have intellisense, I suggest you create a custom class they will hold your properties and return not an anonymous type (using new {}) but object of your class (new MyClass(prop1, prop2)). You also need to change signature of your method, so it returns IQueryable<YourClass> and not just plain non-generic IQueryable.
As others have said, creating a new type to hold the two columns is usually the best option.
But if, for some reason, you don't want to do that and you are using .Net 4.0, you can use Tuple:
public static IQueryable<Tuple<Column1Type, Column2Type>>
GetInfo(MyEntityModel oEntityModel)
{
return from …
select Tuple.Create(t.Column1, u.Column2);
}
var linqresult = ClsLinqTeste.GetInfo(oEntityModel);
foreach (var oItem in linqresult)
Console.WriteLIne(oItem.Item1, oItem.Item2);
When you return your resultset AsQueryable, the app is already able to give you intellisense, however in your example, you must specify either .FirstOrDefault if you know your collection will only have a single row, or iterate over your collection to get the items from it, like so:
This is what you're doing:
var linqresult = ClsLinqTeste.GetInfo(oEntityModel);
if (linqresult.Column1 == 1)
{
..do stuff...
}
This is how you should do it:
var linqresult = ClsLinqTeste.GetInfo(oEntityModel);
foreach(var item in linqresult)
{
if (item.Column1 == 1)
{
..do stuff...
}
}
You must iterate over linqresult because when you query with link, it returns a result set, even if it just has one column. As with any collection, your data columns aren't available on the whole result set, only with individual items.
If you want to strongly typed enumerate a non-generic IEnumerable (IEnumerable.GetEnumerator() instead of IEnumerable<T>.GetEnumerator<T>()) you can use the Cast<>() extension, like so
var myquery = GetQueryable();
for (var item in myquery.Cast<MyDataType>())
{
// use item.Column1 directly and strongly typed with intellisense
}
I have IQueryable object and I need to take the data inside the IQueryable to put it into Textboxs controls. Is this possible?
I try something like:
public void setdata (IQueryable mydata)
{
textbox1.text = mydata.????
}
Update:
I'm doing this:
public IQueryable getData(String tableName, Hashtable myparams)
{
decimal id = 0;
if (myparams.ContainsKey("id") == true)
id = (decimal)myparams["id"];
Type myType= Type.GetType("ORM_Linq." + tableName + ", ORM_Linq");
return this.GetTable(tableName , "select * from Articu where id_tipo_p = '" + id + "'");
}
public IQueryable<T> GetTable<T>(System.Linq.Expressions.Expression<Func<T, bool>> predicate) where T : class
{
return _datacontext.GetTable<T>().Where(predicate);
}
This returns a {System.Data.Linq.SqlClient.SqlProvider+OneTimeEnumerable1[ORM_Linq.Articu]}`
I don't see any method like you tell me. I see Cast<>, Expression, ToString...
EDIT: Updated based on additional info from your other posts...
Your getData method is returning IQueryable instead of a strongly typed result, which is why you end up casting it. Try changing it to:
public IQueryable<ORM_Linq.Articu> getData(...)
Are you trying to query for "Articu" from different tables?
With the above change in place, your code can be rewritten as follows:
ORM_Linq.Articu result = mydata.SingleOrDefault();
if (result != null)
{
TextBoxCode.Text = result.id.ToString();
TextBoxName.Text = result.descrip;
}
If you have a single result use SingleOrDefault which will return a default value if no results are returned:
var result = mydata.SingleOrDefault();
if (result != null)
{
textbox1.text = result.ProductName; // use the column name
}
else
{
// do something
}
If you have multiple results then loop over them:
foreach (var item in mydata)
{
string name = item.ProductName;
int id = item.ProductId;
// etc..
}
First, you should be using a strongly-typed version of IQueryable. Say that your objects are of type MyObject and that MyObject has a property called Name of type string. Then, first change the parameter mydata to be of type IQueryable<MyObject>:
public void setdata (IQueryable<MyObject> mydata)
Then we can write a body like so to actually get some data out of. Let's say that we just want the first result from the query:
public void setdata (IQueryable<MyObject> mydata) {
MyObject first = mydata.FirstOrDefault();
if(first != null) {
textbox1.Text = first.Name;
}
}
Or, if you want to concatenate all the names:
public void setdata(IQueryable<MyObject> mydata) {
string text = String.Join(", ", mydata.Select(x => x.Name).ToArray());
textbo1.Text = text;
}
Well, as the name suggests, an object implementing IQueryable is... Queryable! You'll need to write a linq query to get at the internal details of your IQueryable object. In your linq query you'll be able to pull out its data and assign bits of it where ever you'd like - like your text box.
Here's a great starting place for learning Linq.
I think you find the same mental struggle when coming from FoxPro and from DataSet. Really nice, powerful string-based capabilities(sql for query, access to tables and columns name) in these worlds are not available, but replaced with a compiled, strongly-typed set of capabilities.
This is very nice if you are statically defining the UI for search and results display against a data source known at compile time. Not so nice if you are trying to build a system which attaches to existing data sources known only at runtime and defined by configuration data.
If you expect only one value just call FirstOrDefault() method.
public void setdata (IQueryable mydata)
{
textbox1.text = mydata.FirstOrDefault().PropertyName;
}
If I have something like:
var query = from children in _data.Children
where children.ChildId == childId
select new CustomModel.MyChild
{
ChildId = children.ChildId,
Name = children.ChildName
};
return query.FirstOrDefault();
Where I want the resultant object to be my custom model.
Can I handle the custom model instantiation in a different method, which could be reused if I had multiple linq queries that all generated a custom child model?
For example,
var query = from children in _data.Children
where children.ChildId == childId
select CreateMyCustomChild([param ??]);
return query.FirstOrDefault();
This may well be impossible, I don't know, but what would the method signature be like if it is possible?
I'm only thinking reuse for when multiple linq queries contain duplicate object initialisation code.
Thanks
It really depends on what version of LINQ you're using. If you're using LINQ to SQL, I don't believe you can call arbitrary methods in the query. The query translator wouldn't know what to do with the method call
If you're using LINQ to Objects, you're absolutely fine to do it, like this:
var query = from children in _data.Children
where children.ChildId == childId
select CreateMyCustomChild(children)
return query.FirstOrDefault();
// Elsewhere
public CustomModel.MyChild CreateMyCustomChild(OtherChild child)
{
return new CustomModel.MyChild
{
ChildId = child.ChildId,
Name = child.ChildName
};
}
(Side note: I'd call the range variable in the query "child" rather than "children" as at any one time it only represents a single child.)
If you wanted you could write "select 1" or in your case "CreateMyCustomChild(children)" since "children" is containing all your info. In your case you aren't adding a lot of info to "children", so why not "select children"?
In other words, just try it out. The return type of your value will determine over which type your LINQ enumerates.
Suppose you had a method that did the transform for you.
public static class Conversions
{
public static CustomModel.MyChild ToCustomModel(this DataModel.MyChild source)
{
return new CustomModel.MyChild()
{
ChildId = source.ChildId,
Name = source.ChildName
}
}
}
You can use such a method to do the conversion of a single item.
DataModel.MyChild myResult = getResult();
CustomModel.MyChild myConvertedResult = myResult.ToCustomModel()
Such a method can also be used in a Enumerable.Select method call.
IEnumerable<DataModel.MyChild> myQueriedResults = getResult();
IEnumerable<CustomModel.MyChild> myConvertedResults =
myQueryiedResults.Select(c => c.ToCustomModel());
While you can do with expressions, I don't think it is worth the hassle. Instead I suggest you define an extension method like:
IQueryable<CustomModel.MyChild> ToModel(this IQueryable<Child> childs)
{
return childs.Select( c=>
select new CustomModel.MyChild
{
ChildId = children.ChildId,
Name = children.ChildName
}
);
}
You can then call:
return _data.Children
.Where(c=>c.ChildId == childId)
.ToModel()
.FirstOrDefault();