here is my choco solver code:
Model model = new Model();
IntVar p1=model.intVar("type",new int[]{45,46,47});
I want to access IntVar p1 by it's Name,which is the String type,not JAVA variable name p1.
Is there any thing like:
type.eq(number).post()
Thanks~
Related
I am doing some coding in dapper and I get the error No mapping exists from object type to a known managed provider native type this error occurs on the myfriends var for dapper . I am using dapper to get a list of INT values from a table then comparing them against another.. this is the code that gives me that error
int myid = Convert.ToInt32(User.Identity.Name);
// The var myfriend is giving me that error above
var myfriends = sqlConnection.Query<friend>("Select otherfriendsID from friends where profileID=#myidd", new { myidd = myid }).ToList();
var profiles = sqlConnection.Query<profile>("Select top 40 * from profiles where photo is not null AND profileID=#friendship order by profileID desc", new {friendship=myfriends}).ToList();
however if I use entity everything works fine for instance this code below works..
var myfriends = (from s in db.friends where s.profileID == myid select s.otherfriendsID).ToList();
What could be going on here..
myfriends is a List<friend>. You then pass that in as a query parameter, i.e.
new {friendship=myfriends}
with:
AND profileID=#friendship
Now... what is #friendship ? How should it pass in a List<friend> here? What does that even mean to pass in a list of objects (each of which could have multiple properties) as a single parameter? (note: I'm ignoring table-valued-parameters for the purposes of this question)
So: how many myfriends do you expect? 0? 1? any number? This could be, for example:
var profileIds = myfriends.Select(x => x.ProfileId);
...
new {profileIds}
...
AND profileID in #profileIds
or maybe:
new {profileId = myfriends.Single().ProfileId}
...
AND profileID = #profileId
I have List where MyType : DynamicObject. The reason for MyType inheriting from DynamicObject, is that I need a type that can contain unknown number of properties.
It all works fine until I need to filter List. Is there a way I can do a linq that will do something like this:
return all items where any of the properties is empty string or white space?
(from the comment) can I do above linq query with List?
Yes, here is how you can do it with ExpandoObject:
var list = new List<ExpandoObject>();
dynamic e1 = new ExpandoObject();
e1.a = null;
e1.b = "";
dynamic e2 = new ExpandoObject();
e2.x = "xxx";
e2.y = 123;
list.Add(e1);
list.Add(e2);
var res = list.Where(
item => item.Any(p => p.Value == null || (p.Value is string && string.IsNullOrEmpty((string)p.Value)))
);
The ExpandoObject presents an interface that lets you enumerate its property-value pairs as if they were in a dictionary, making the process of checking them a lot simpler.
Well as long as each object's properties are not unknown internally to themselves you could do it.
There isn't a great generic way to test all the properties of a dynamic object, if you don't have control over the DynamicObject you hope the implementer implemented GetDynamicMemberNames() and you can use the nuget package ImpromptuInterface's methods for getting the property names and dynamically invoking those names.
return allItems.Where(x=> Impromptu.GetMemberNames(x, dynamicOnly:true)
.Any(y=>String.IsNullOrWhiteSpace(Impromptu.InvokeGet(x,y));
Otherwise, since it's your own type MyType you can add your own method that can see internal accounting for those member values.
return allItems.Where(x => x.MyValues.Any(y=>String.IsNullOrWhitespace(x));
When is it required to use new in the select clause as in the following example? Thanks.
var results = from person in people
select new { FN = person.FirstName, LN = person.LastName };
I've searched and partially understand this: "In the select new, we're creating a new anonymous type with only the properties you need." But does it mean I can omit new and not get an anonymous type?
To restate my question in another way: is 'new' optional? Is 'new' here similar to C# new where it is not required with value type?
Can I do this?
var results = from person in people
select { FN = person.FirstName, LN = person.LastName };
No, new is not optional if you're selecting "new" objects. (which is what you are doing. You are creating new objects of an anonymous type with 2 properties FN and LN)
Simply omitting new is a syntax error.
If you have an existing type you would like to use, (say you've defined a class Person with 2 properties FN and LN) then you can use that type. E.g.
var results = from person in people
select new Person { FN = person.FirstName, LN = person.LastName };
(assuming Person has a parameterless constructor and FN and LN are properties with setters)
The only time you can omit new is if you are not creating a new object, for instance:
var results = from person in people
select LN = person.LastName;
There you are selecting just the LastName and the result is an IEnumerable<string>
If you are projecting to a new type (anonymous class or known class/struct) it is not optional but required.
If you are projecting to an existing instance though of course you don't need the new, i.e. in your example if you just wanted to project to the first name of each person (to get a string enumeration you could just do:
var results = from person in people select person.FirstName
So rule of thumb if you are constructing a new type you need new in the projection (Select), if you are just selecting something that already exists you don't.
Instead of creating an anonymous type you can instantiate an instance of a named type that takes a Person as an argument or assign values using the object initializer syntax.
Example:
var results = from person in people
select new Report(person);
var results = from person in people
select new Report() { Customer = person };
Edit: Or of course just select a single property from the person object as BrokenGlass pointed out!
Edit: I just re-read your question and wanted to point out that anonymous types in C# are just normal types with the type name generated by the compiler. So yes, you still need to use the new operator because you're creating a new instance of a type. Anonymous Types (C# Programming Guide)
I have a simlar requirement where in i have 3 main entities in ADO.NET entity model... I am building a framework wherein basedon incoming XML root element i have to create an instance of specific entity using reflection and set its properties..
But when it comes to child entities.. I am not able to use LINQ queries on it as the type is not known at design time. PropertyInfo.GetValue gives me an object on which i can not run LINQ queries (even when i typecast it to IQueryable or IEnumerable). I am not even able to typecast it at design time as that would be kind of hardcoding and will fail my generic framework purpose.
I tried to use dynamic keyword.. but on that too i can not write LINQ queries.. it gives a message that LINQ queries are not supported on dynamic dispatch model.!!!
Can someone help..
Regards Badal
Like #Yuriy Faktorovich recommended, dynamic LINQ may be your best shot.
Another option you have is to dynamically build expression trees and execute your expression via reflection. Keep in mind that this is not easy and may take a little time to wrap your head around the expression API, but for simple expression it's not too bad.
Example: Say you want to replicate this expression:
p => p.FirstName == firstName
You can build the expression tree as follows:
var myType = Type.GetType("Person"); // <-- Find your type based on XML
var firstName = "John";
var param = Expression.Parameter(myType, "p");
var firstNameProperty = Expression.Property(param, "FirstName");
var constantExpression = Expression.Constant(firstName);
var equalsExpression = Expression.Equal(firstNameProperty, constantExpression);
var lambda = Expression.Lambda(equalsExpression, param);
The lambda will have a runtime type of Expression<Func<Person,bool>> which you can then pass to any IQueryable implementation.
UPDATE
Then to dynamcially call some method on your IQueryable you can do something similar to the following:
var queryableType = typeof(Queryable);
var whereMethod = queryableType.GetMethod("Where", BindingFlags.Public | BindingFlags.Static);
var parameters = new object[] { query, lambda }; // query is your IQueryable object
var list = whereMethod.Invoke(null, parameters);
What does var really do in the following case?
var productInfos =
from p in products
select new { p.ProductName, p.Category, Price = p.UnitPrice };
The two lines:
var productInfos = from p in products
select new { p.ProductName, p.Category, Price = p.UnitPrice };
and
IEnumerable<CompilerGeneratedType> productInfos = from p in products
select new { p.ProductName, p.Category, Price = p.UnitPrice };
are equivalent. CompilerGeneratedType is a type that will be created by the compiler and has the three public properties ProductName, Price, and Category. var is helpful for two reasons:
CompilerGeneratedType will be generated by the compiler so it's impossible for you to use the type name in the declaration.
You don't have to think too hard about the collection type of the result. Linq can do its thing and you don't have to worry about it.
var is a placeholder for a compiler-created ("anonymous") type that has three properties, ProductName, Category and Price.
It is NOT a variant (e.g. as in Visual Basic). It is a concrete type and can be used as such in other places in the code.
In this particular case, the type of productInfos is a compiler-generated Anonymous Type with 3 properties, ProductName, Category and Price.
variables with var are implicitly typed local variable which are strongly typed just as if you had declared the type yourself, but the compiler determines the type. it gets the type of the result.
and here a nice read C# Debate: When Should You Use var?
and here another C# 3.0 Tutorial
var = programmer friendly = less typing = makes you lazy(another way of looking at it) = brings obscurity to code if new to 3.5 FW
It eases you from the pain of having to declare the exact type of your query result manually. But I have to empathize, this is not dynamic typing: the productInfos variable will have a static type, but it is created by the compiler instead of you.