First, I have a variable like below:
List<string> values;
Now I need build query condition like below:
Expression<Func<docinstance, bool>> filter = d=>d.values.any(o=>o.value==values[0]||o.value==value[1]||.....)
Because I don't know how many items in variable values, so how can I build the query condition
You can use Any again inside
d => d.values.Any(o => values.Any(x => x == o.value))
You can use Contains:
d => d.values.Any(o => values.Contains(o.value))
See Documentation
The DbContext class of EF has a method Set<TEntity>() which return type is DbSet<TEntity> which implements Queryable interface which has all the query methods as extension methods.
At the end your query expression in translated into SQL and then executed. Which gives you the desired capability so that you can combine query methods and construct dynamic expressions.
Related
I have the following Linq statement, which works totally fine:
query = query.OrderBy(m => m.MATERIAL_TXT.Where(mt => mt.LANG == "EN").FirstOrDefault().LTEXT);
Now I'm trying to make it dynamic by using the string based syntax from Linq.Dynamic:
query = query.OrderBy("MATERIAL_TXT.Where(LANG==\"EN\").FirstOrDefault().LTEXT");
But it throws the exception :
"No applicable aggregate method 'FirstOrDefault' exists"
It has to bedynamic so that it accepts other names instead of "MATERIAL_TXT".
What am I missing?
According to the documentation:
A subset of the Standard Query Operators is supported for objects that implement IEnumerable. Specifically, the following constructs are permitted, where seq is an IEnumerable instance, predicate is a boolean expression, and selector is an expression of any type:
seq.Where(predicate)
seq.Any()
seq.Any(predicate)
seq.All(predicate)
seq.Count()
seq.Count(predicate)
seq.Min(selector)
seq.Max(selector)
seq.Sum(selector)
seq.Average(selector)
FirstOrDefault isn't on the list, so it's reasonably safe to assume it isn't supported.
You can't use FirstOrDefault as string like that.
if you want to create dynamic orderBy try this :
Func<IQueryable<YourEntityType>, IOrderedQueryable<YourEntityType>> orderBy;
orderBy = x => x.OrderBy(m => m.MATERIAL_TXT.Where(mt => mt.LANG == "EN").FirstOrDefault().LTEXT);
Then you can use it like this :
orderBy(query);
for example you can use it in another method :
public List<YourEntityType> YourMethodName(Func<IQueryable<YourEntityType>, IOrderedQueryable<YourEntityType>> orderBy,IQueryable<YourEntityType> query=null)
{
query=query ?? GetYourEntityTypeList().AsQueryable();
return orderBy(query).ToList();
}
I Hope it will be useful .
I have the following expression:
public Expression<Func<T, bool>> UserAccessCheckExpression<T>(int userId) where T : class
{
return x => (IsAdmin || userId == CurrentUserId || userId == 0);
}
Then I want to apply this filter to several collections (IQueryable) like this one:
return tasks
.Where(t => t.TaskUsers
.Any(x => UserAccessCheckExpression<TaskUser>(x.User) && x.SomeBool == true));
I'm getting the following error while doing so:
Error 40 Cannot implicitly convert type System.Linq.Expressions.Expression<System.Func<TaskUser,bool>> to bool
I can't use workaround with interface inheritance (like TaskUser inherits interface with int UserId property (where T : IHasUserId)) since I want to combine logic.
The problem is that your UserAccessCheckExpression() method is returning an Expression while the Any() method is expecting a boolean.
Now, you can get your code to compile by compiling the Expression and invoking the method (using UserAccessCheckExpression<TaskUser>(x.User).Compile().Invoke(x.User)) but that would obviously fail on runtime because Linq-to-Entities wouldn't be able to translate your Any() to a store query as it no longer contains an Expression.
LinqKit is aiming to solve this problem using its own Invoke extension method that while letting your code compile, will make sure your Expression will get replaced back to its original form using another extension method named AsExpandable() that is extending the entity set.
Try this:
using LinqKit.Extensions;
return tasks
.AsExpandable()
.Where(t => t.TaskUsers.Any(
x => UserAccessCheckExpression<TaskUser>(x.User).Invoke(x)
&& x.SomeBool == true));
More on LinqKit
Yeah, so, you can't do that. There's a difference between an Expression<> and a Func<>. You're trying to use the UserAccessCheckExpression as a func. I'm not sure what you're trying to do, but you can compile it to a func and then use it sorta like you are:
var expr = UserAccessCheckExpression<TaskUser>(x.User);
var func = expr.Compile();
// Later use it like ...
var result = func();
But I expect you're using this with EF or Linq2Sql? That being the case you'll need to rewrite the expression. It can be done by hand (not easy) or, better, use a tool like PredicateBuilder.
This is not about the reuse of a result but more the statement itself.
Nor is it about an error when using var as mentioned in: LINQ to SQL: Reuse lambda expression
Out of sheer curiosity I was wondering if it is possible to reuse a single LINQ statement.
Lets say I have the following LINQ statement:
.Where(x => x.Contains(""));
Is it possible to extract the statement x => x.Contains("") and use some kind of reference to this for later usage in, lets say, another class?
So I can call it like: .Where(previouslySavedStatement);
You can store it in a variable. If you are working with IQueryable then use:
System.Linq.Expressions.Expression<Func<Foo, bool>> selector = x => x.Contains("");
If you are using IEnumerable then use:
Func<Foo, bool> selector = x => x.Contains("");
And use it in your query:
query.Where(selector);
Yes, you can write a function containing the query you want to reuse, which takes and returns an IQueryable<T>
public IQueryable<T> ContainsEmpty(IQueryable<T> query)
{
return query.Where(x => x.Contains(""));
}
Now you can reuse it:
query1 = ContainsEmpty(query1);
query2 = ContainsEmpty(another);
It depends. There's two Where methods, Enumerable.Where and Queryable.Where. If you're applying the .Where to an IEnumerable than the first one is called, if you're applying it to an IQueryable the second one is called.
Since Enumerable.Where takes in a Func, it isn't reusable. Since Queryable.Where takes in an expression, it is reusable. You can do so as follows:
var x = new List<string>().AsQueryable();
var query = x.Where (n => n.Contains("some string"));
//Extract the lambda clause
var expr = query.Expression;
var methodExpr = (MethodCallExpression)expr;
var quoteExpr = (UnaryExpression)methodExpr.Arguments[1];
var funcExpr = (Expression<Func<string, bool>>)quoteExpr.Operand;
You can then later re-apply the where expression:
var query2 = x.Where(funcExpr);
I wrote a library to address exactly this concern, it's called CLinq and you can find an implementation for the EntityFramework here: https://www.nuget.org/packages/CLinq.EntityFramework
It allows to create query snippets and use them everywhere you in a linq query. Following the example of Hamid, create the following expression:
System.Linq.Expressions.Expression<Func<Foo, bool>> selector = x => x.Contains("");
You can now use this query everywhere in your linq queries like this:
query.AsComposable().Where(o => selector.Pass(o));
Additionally to this simple example you're also able to combine your query snippets:
query.AsComposable().Where(o => selector.Pass(o) || anotherSelector.Pass(o));
or even merge them together:
query.AsComposable().Where(o => anotherSelector.Pass(selector.Pass(o)));
There's some more features, but I think it's really helpful, so check it out :)
I have users searching records of type Record. They type a search term in a textbox and then I search records by matching several fields with the search term.
My query looks like:
var results = from record in DataContext.Records
where
record.Field1.ToLower().Contains(term) ||
record.Field2.ToLower().Contains(term) ||
record.Field3.ToLower().Contains(term)
select record;
I have a number of queries that all use the same filter and thus I would like to extract the filtering so it can be reused. Something like:
var filter = new Func<Record, string, bool>(
(record, term) =>
record.Field1.ToLower().Contains(term) ||
record.Field2.ToLower().Contains(term) ||
record.Field3.ToLower().Contains(term)
);
var results = from record in DataContext.Records
where filter(record, term)
select record;
However, it does not work because:
Method 'System.Object DynamicInvoke(System.Object[])' has no supported translation to SQL.
How can I reuse my where condition across queries?
You need to build an expression instead of a function:
Expression<Func<Record, bool>> filter =
record => record.Field1.ToLower().Contains(term); // rest omitted
The lambda expression remains the same, but you need to return it into a variable of type Expression<Func<Record, bool>> -- that will make the C# compiler compile it as an expression instead of a delegate, allowing it to be passed to LINQ to SQL.
However, you won't be able to use an expression variable with a C#-syntax where clause: you'll need to use the Where extension method:
var results = DataContext.Records.Where(filter);
Edited to add: If you want to be able to create filters on different terms, you just need a method to produce an expression from a term:
private static Expression<Func<Record, bool>> Filter(string term)
{
return r => r.Field1.ToLower().Contains(term);
}
var results = DataContext.Records.Where(Filter(term));
If you prefer to keep filter as a lambda as you have at the moment, you can do so, but the generics get a bit nested:
Func<string, Expression<Func<Record, bool>>> filter =
term => (r => r.Field1.ToLower().Contains(term));
var results = DataContext.Records.Where(filter(term));
Regardless, the important thing is that what goes in the Where clause must be an Expression<Func<Record, bool>> -- but as shown above you can make the expression depend on term by building a suitable expression on the fly. Which is exactly what LINQ to SQL would be doing if you spelled out the filter longhand in the Where clause.
Use a CompiledQuery!
var filter = CompiledQuery.Compile(
(DatabaseDataContext dc, Record record, string term) =>
record.Field1.ToLower().Contains(term) ||
record.Field2.ToLower().Contains(term) ||
record.Field3.ToLower().Contains(term)
);
var results = from record in DataContext.Records
where filter(DataContext, record, term)
select record;
For more information, see How to: Store and Reuse Queries.
In addition to the Expression<Func<Record, bool>> issue that others have pointed out, I suggest looking into PredicateBuilder. It's very good for dynamically combining lambda expressions.
I think you need to make it an Expression<Func<Record, bool>>. Otherwise it's trying to translate the actual C# method call to SQL rather than the description of it. This is not a guarantee that this version will work; I'm not sure which string functions are translatable to SQL.
I have the following code:
public OTestTable GetTestCode(Func<TestTable, bool> whereClause)
{
return CoreContext.TestTables.Where(whereClause).Select(TestTableMap.DataToObject).FirstOrDefault();
}
CoreContext is my data context (which is initialized in a base class)
My TestTableMap is as follows:
public class TestTableMap
{
public static readonly Func<TestTable, OTestTable> DataToObject = mapper =>
new OTestTable
{
Code = mapper.mycode
};
}
Then in my business method i have the following:
public OTestTable GetTestCode(string code)
{
return QueryEngine.GetTestCode(id => id.mycode == code);
}
From my main program, i am calling GetTestCode with a string value.
When I watch SQL profiler, I get the following:
SELECT [t0].[mycode]
FROM [dbo].[TestTable] AS [t0]
It does not have the where clause appended to the SQL query. If i add the where clause to the LINQ as var query = from c in DataContext.TestTable where c.mycode == '' select c;
It will add the where clause.
However, when I run my code, it will return the correct record, but it seems like I am pulling back all records from the database and filtering in my code (which should not happen).
Any thoughts with what I am doing wrong?
Thanks
In order to construct SQL statements, LINQ to SQL requires an expression tree. Func<TestTable, bool> does not represent an expression tree, it is a "black box" function pointer. LINQ cannot do anything intelligent with this apart from blindly execute it on an in-memory collection.
You need to do this instead:
public OTestTable GetTestCode(Expression<Func<TestTable, bool>> whereClause) {
return CoreContext.TestTables.Where(whereClause).Select(TestTableMap.DataToObject).FirstOrDefault();
}
This code compiles using the Queryable.Where extension method, which does accept an expression tree, rather than the Enumerable.Where extension method, which only accepts a raw delegate.
Try creating your where clause as:
Expression<Func<T, bool>> whereClause
Where the T parameter is your source type Table<T> source
Also see the PredicateBuilder here: http://www.albahari.com/nutshell/predicatebuilder.aspx
It provides you convenient extension methods to predicate IQueryable<T>. like this:
var predicate = PredicateBuilder.True<Family>();
predicate = predicate.And(o => o.Birthday < new DateTime(1980, 1, 1));
.Or(o => o.Name.Contains("ke"));
var result = Source.Where(predicate).ToList();