given a list of ids, I can query all relevant rows by:
context.Table.Where(q => listOfIds.Contains(q.Id));
But how do you achieve the same functionality when the Table has a composite key?
This is a nasty problem for which I don't know any elegant solution.
Suppose you have these key combinations, and you only want to select the marked ones (*).
Id1 Id2
--- ---
1 2 *
1 3
1 6
2 2 *
2 3 *
... (many more)
How to do this is a way that Entity Framework is happy? Let's look at some possible solutions and see if they're any good.
Solution 1: Join (or Contains) with pairs
The best solution would be to create a list of the pairs you want, for instance Tuples, (List<Tuple<int,int>>) and join the database data with this list:
from entity in db.Table // db is a DbContext
join pair in Tuples on new { entity.Id1, entity.Id2 }
equals new { Id1 = pair.Item1, Id2 = pair.Item2 }
select entity
In LINQ to objects this would be perfect, but, too bad, EF will throw an exception like
Unable to create a constant value of type 'System.Tuple`2 (...) Only primitive types or enumeration types are supported in this context.
which is a rather clumsy way to tell you that it can't translate this statement into SQL, because Tuples is not a list of primitive values (like int or string). For the same reason a similar statement using Contains (or any other LINQ statement) would fail.
Solution 2: In-memory
Of course we could turn the problem into simple LINQ to objects like so:
from entity in db.Table.AsEnumerable() // fetch db.Table into memory first
join pair Tuples on new { entity.Id1, entity.Id2 }
equals new { Id1 = pair.Item1, Id2 = pair.Item2 }
select entity
Needless to say that this is not a good solution. db.Table could contain millions of records.
Solution 3: Two Contains statements (incorrect)
So let's offer EF two lists of primitive values, [1,2] for Id1 and [2,3] for Id2. We don't want to use join, so let's use Contains:
from entity in db.Table
where ids1.Contains(entity.Id1) && ids2.Contains(entity.Id2)
select entity
But now the results also contains entity {1,3}! Well, of course, this entity perfectly matches the two predicates. But let's keep in mind that we're getting closer. In stead of pulling millions of entities into memory, we now only get four of them.
Solution 4: One Contains with computed values
Solution 3 failed because the two separate Contains statements don't only filter the combinations of their values. What if we create a list of combinations first and try to match these combinations? We know from solution 1 that this list should contain primitive values. For instance:
var computed = ids1.Zip(ids2, (i1,i2) => i1 * i2); // [2,6]
and the LINQ statement:
from entity in db.Table
where computed.Contains(entity.Id1 * entity.Id2)
select entity
There are some problems with this approach. First, you'll see that this also returns entity {1,6}. The combination function (a*b) does not produce values that uniquely identify a pair in the database. Now we could create a list of strings like ["Id1=1,Id2=2","Id1=2,Id2=3]" and do
from entity in db.Table
where computed.Contains("Id1=" + entity.Id1 + "," + "Id2=" + entity.Id2)
select entity
(This would work in EF6, not in earlier versions).
This is getting pretty messy. But a more important problem is that this solution is not sargable, which means: it bypasses any database indexes on Id1 and Id2 that could have been used otherwise. This will perform very very poorly.
Solution 5: Best of 2 and 3
So the most viable solution I can think of is a combination of Contains and a join in memory: First do the contains statement as in solution 3. Remember, it got us very close to what we wanted. Then refine the query result by joining the result as an in-memory list:
var rawSelection = from entity in db.Table
where ids1.Contains(entity.Id1) && ids2.Contains(entity.Id2)
select entity;
var refined = from entity in rawSelection.AsEnumerable()
join pair in Tuples on new { entity.Id1, entity.Id2 }
equals new { Id1 = pair.Item1, Id2 = pair.Item2 }
select entity;
It's not elegant, messy all the same maybe, but so far it's the only scalable1 solution to this problem I found, and applied in my own code.
Solution 6: Build a query with OR clauses
Using a Predicate builder like Linqkit or alternatives, you can build a query that contains an OR clause for each element in the list of combinations. This could be a viable option for really short lists. With a couple of hundreds of elements, the query will start performing very poorly. So I don't consider this a good solution unless you can be 100% sure that there will always be a small number of elements. One elaboration of this option can be found here.
Solution 7: Unions
There's also a solution using UNIONs that I posted later here.
1As far as the Contains statement is scalable: Scalable Contains method for LINQ against a SQL backend
Solution for Entity Framework Core with SQL Server
🎉 NEW! QueryableValues EF6 Edition has arrived!
The following solution makes use of QueryableValues. This is a library that I wrote to primarily solve the problem of query plan cache pollution in SQL Server caused by queries that compose local values using the Contains LINQ method. It also allows you to compose values of complex types in your queries in a performant way, which will achieve what's being asked in this question.
First you will need to install and set up the library, after doing that you can use any of the following patterns that will allow you to query your entities using a composite key:
// Required to make the AsQueryableValues method available on the DbContext.
using BlazarTech.QueryableValues;
// Local data that will be used to query by the composite key
// of the fictitious OrderProduct table.
var values = new[]
{
new { OrderId = 1, ProductId = 10 },
new { OrderId = 2, ProductId = 20 },
new { OrderId = 3, ProductId = 30 }
};
// Optional helper variable (needed by the second example due to CS0854)
var queryableValues = dbContext.AsQueryableValues(values);
// Example 1 - Using a Join (preferred).
var example1Results = dbContext
.OrderProduct
.Join(
queryableValues,
e => new { e.OrderId, e.ProductId },
v => new { v.OrderId, v.ProductId },
(e, v) => e
)
.ToList();
// Example 2 - Using Any (similar behavior as Contains).
var example2Results = dbContext
.OrderProduct
.Where(e => queryableValues
.Where(v =>
v.OrderId == e.OrderId &&
v.ProductId == e.ProductId
)
.Any()
)
.ToList();
Useful Links
Nuget Package
GitHub Repository
Benchmarks
QueryableValues is distributed under the MIT license.
You can use Union for each composite primary key:
var compositeKeys = new List<CK>
{
new CK { id1 = 1, id2 = 2 },
new CK { id1 = 1, id2 = 3 },
new CK { id1 = 2, id2 = 4 }
};
IQuerable<CK> query = null;
foreach(var ck in compositeKeys)
{
var temp = context.Table.Where(x => x.id1 == ck.id1 && x.id2 == ck.id2);
query = query == null ? temp : query.Union(temp);
}
var result = query.ToList();
You can create a collection of strings with both keys like this (I am assuming that your keys are int type):
var id1id2Strings = listOfIds.Select(p => p.Id1+ "-" + p.Id2);
Then you can just use "Contains" on your db:
using (dbEntities context = new dbEntities())
{
var rec = await context.Table1.Where(entity => id1id2Strings .Contains(entity.Id1+ "-" + entity.Id2));
return rec.ToList();
}
You need a set of objects representing the keys you want to query.
class Key
{
int Id1 {get;set;}
int Id2 {get;set;}
If you have two lists and you simply check that each value appears in their respective list then you are getting the cartesian product of the lists - which is likely not what you want. Instead you need to query the specific combinations required
List<Key> keys = // get keys;
context.Table.Where(q => keys.Any(k => k.Id1 == q.Id1 && k.Id2 == q.Id2));
I'm not completely sure that this is valid use of Entity Framework; you may have issues with sending the Key type to the database. If that happens then you can be creative:
var composites = keys.Select(k => p1 * k.Id1 + p2 * k.Id2).ToList();
context.Table.Where(q => composites.Contains(p1 * q.Id1 + p2 * q.Id2));
You can create an isomorphic function (prime numbers are good for this), something like a hashcode, which you can use to compare the pair of values. As long as the multiplicative factors are co-prime this pattern will be isomorphic (one-to-one) - i.e. the result of p1*Id1 + p2*Id2 will uniquely identify the values of Id1 and Id2 as long as the prime numbers are correctly chosen.
But then you end up in a situation where you're implementing complex concepts and someone is going to have to support this. Probably better to write a stored procedure which takes the valid key objects.
Ran into this issue as well and needed a solution that both did not perform a table scan and also provided exact matches.
This can be achieved by combining Solution 3 and Solution 4 from Gert Arnold's Answer
var firstIds = results.Select(r => r.FirstId);
var secondIds = results.Select(r => r.SecondId);
var compositeIds = results.Select(r => $"{r.FirstId}:{r.SecondId}");
var query = from e in dbContext.Table
//first check the indexes to avoid a table scan
where firstIds.Contains(e.FirstId) && secondIds.Contains(e.SecondId))
//then compare the compositeId for an exact match
//ToString() must be called unless using EF Core 5+
where compositeIds.Contains(e.FirstId.ToString() + ":" + e.SecondId.ToString()))
select e;
var entities = await query.ToListAsync();
For EF Core I use a slightly modified version of the bucketized IN method by EricEJ to map composite keys as tuples. It performs pretty well for small sets of data.
Sample usage
List<(int Id, int Id2)> listOfIds = ...
context.Table.In(listOfIds, q => q.Id, q => q.Id2);
Implementation
public static IQueryable<TQuery> In<TKey1, TKey2, TQuery>(
this IQueryable<TQuery> queryable,
IEnumerable<(TKey1, TKey2)> values,
Expression<Func<TQuery, TKey1>> key1Selector,
Expression<Func<TQuery, TKey2>> key2Selector)
{
if (values is null)
{
throw new ArgumentNullException(nameof(values));
}
if (key1Selector is null)
{
throw new ArgumentNullException(nameof(key1Selector));
}
if (key2Selector is null)
{
throw new ArgumentNullException(nameof(key2Selector));
}
if (!values.Any())
{
return queryable.Take(0);
}
var distinctValues = Bucketize(values);
if (distinctValues.Length > 1024)
{
throw new ArgumentException("Too many parameters for SQL Server, reduce the number of parameters", nameof(values));
}
var predicates = distinctValues
.Select(v =>
{
// Create an expression that captures the variable so EF can turn this into a parameterized SQL query
Expression<Func<TKey1>> value1AsExpression = () => v.Item1;
Expression<Func<TKey2>> value2AsExpression = () => v.Item2;
var firstEqual = Expression.Equal(key1Selector.Body, value1AsExpression.Body);
var visitor = new ReplaceParameterVisitor(key2Selector.Parameters[0], key1Selector.Parameters[0]);
var secondEqual = Expression.Equal(visitor.Visit(key2Selector.Body), value2AsExpression.Body);
return Expression.AndAlso(firstEqual, secondEqual);
})
.ToList();
while (predicates.Count > 1)
{
predicates = PairWise(predicates).Select(p => Expression.OrElse(p.Item1, p.Item2)).ToList();
}
var body = predicates.Single();
var clause = Expression.Lambda<Func<TQuery, bool>>(body, key1Selector.Parameters[0]);
return queryable.Where(clause);
}
class ReplaceParameterVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
/// <summary>
/// Break a list of items tuples of pairs.
/// </summary>
private static IEnumerable<(T, T)> PairWise<T>(this IEnumerable<T> source)
{
var sourceEnumerator = source.GetEnumerator();
while (sourceEnumerator.MoveNext())
{
var a = sourceEnumerator.Current;
sourceEnumerator.MoveNext();
var b = sourceEnumerator.Current;
yield return (a, b);
}
}
private static TKey[] Bucketize<TKey>(IEnumerable<TKey> values)
{
var distinctValueList = values.Distinct().ToList();
// Calculate bucket size as 1,2,4,8,16,32,64,...
var bucket = 1;
while (distinctValueList.Count > bucket)
{
bucket *= 2;
}
// Fill all slots.
var lastValue = distinctValueList.Last();
for (var index = distinctValueList.Count; index < bucket; index++)
{
distinctValueList.Add(lastValue);
}
var distinctValues = distinctValueList.ToArray();
return distinctValues;
}
In the absence of a general solution, I think there are two things to consider:
Avoid multi-column primary keys (will make unit testing easier too).
But if you have to, chances are that one of them will reduce the
query result size to O(n) where n is the size of the ideal query
result. From here, its Solution 5 from Gerd Arnold above.
For example, the problem leading me to this question was querying order lines, where the key is order id + order line number + order type, and the source had the order type being implicit. That is, the order type was a constant, order ID would reduce the query set to order lines of relevant orders, and there would usually be 5 or less of these per order.
To rephrase: If you have a composite key, changes are that one of them have very few duplicates. Apply Solution 5 from above with that.
I tried this solution and it worked with me and the output query was perfect without any parameters
using LinqKit; // nuget
var customField_Ids = customFields?.Select(t => new CustomFieldKey { Id = t.Id, TicketId = t.TicketId }).ToList();
var uniqueIds1 = customField_Ids.Select(cf => cf.Id).Distinct().ToList();
var uniqueIds2 = customField_Ids.Select(cf => cf.TicketId).Distinct().ToList();
var predicate = PredicateBuilder.New<CustomFieldKey>(false); //LinqKit
var lambdas = new List<Expression<Func<CustomFieldKey, bool>>>();
foreach (var cfKey in customField_Ids)
{
var id = uniqueIds1.Where(uid => uid == cfKey.Id).Take(1).ToList();
var ticketId = uniqueIds2.Where(uid => uid == cfKey.TicketId).Take(1).ToList();
lambdas.Add(t => id.Contains(t.Id) && ticketId.Contains(t.TicketId));
}
predicate = AggregateExtensions.AggregateBalanced(lambdas.ToArray(), (expr1, expr2) =>
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<CustomFieldKey, bool>>
(Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
});
var modifiedCustomField_Ids = repository.GetTable<CustomFieldLocal>()
.Select(cf => new CustomFieldKey() { Id = cf.Id, TicketId = cf.TicketId }).Where(predicate).ToArray();
I ended up writing a helper for this problem that relies on System.Linq.Dynamic.Core;
Its a lot of code and don't have time to refactor at the moment but input / suggestions appreciated.
public static IQueryable<TEntity> WhereIsOneOf<TEntity, TSource>(this IQueryable<TEntity> dbSet,
IEnumerable<TSource> source,
Expression<Func<TEntity, TSource,bool>> predicate) where TEntity : class
{
var (where, pDict) = GetEntityPredicate(predicate, source);
return dbSet.Where(where, pDict);
(string WhereStr, IDictionary<string, object> paramDict) GetEntityPredicate(Expression<Func<TEntity, TSource, bool>> func, IEnumerable<TSource> source)
{
var firstP = func.Parameters[0];
var binaryExpressions = RecurseBinaryExpressions((BinaryExpression)func.Body);
var i = 0;
var paramDict = new Dictionary<string, object>();
var res = new List<string>();
foreach (var sourceItem in source)
{
var innerRes = new List<string>();
foreach (var bExp in binaryExpressions)
{
var emp = ToEMemberPredicate(firstP, bExp);
var val = emp.GetKeyValue(sourceItem);
var pName = $"#{i++}";
paramDict.Add(pName, val);
var str = $"{emp.EntityMemberName} {emp.SQLOperator} {pName}";
innerRes.Add(str);
}
res.Add( "(" + string.Join(" and ", innerRes) + ")");
}
var sRes = string.Join(" || ", res);
return (sRes, paramDict);
}
EMemberPredicate ToEMemberPredicate(ParameterExpression firstP, BinaryExpression bExp)
{
var lMember = (MemberExpression)bExp.Left;
var rMember = (MemberExpression)bExp.Right;
var entityMember = lMember.Expression == firstP ? lMember : rMember;
var keyMember = entityMember == lMember ? rMember : lMember;
return new EMemberPredicate(entityMember, keyMember, bExp.NodeType);
}
List<BinaryExpression> RecurseBinaryExpressions(BinaryExpression e, List<BinaryExpression> runningList = null)
{
if (runningList == null) runningList = new List<BinaryExpression>();
if (e.Left is BinaryExpression lbe)
{
var additions = RecurseBinaryExpressions(lbe);
runningList.AddRange(additions);
}
if (e.Right is BinaryExpression rbe)
{
var additions = RecurseBinaryExpressions(rbe);
runningList.AddRange(additions);
}
if (e.Left is MemberExpression && e.Right is MemberExpression)
{
runningList.Add(e);
}
return runningList;
}
}
Helper class:
public class EMemberPredicate
{
public readonly MemberExpression EntityMember;
public readonly MemberExpression KeyMember;
public readonly PropertyInfo KeyMemberPropInfo;
public readonly string EntityMemberName;
public readonly string SQLOperator;
public EMemberPredicate(MemberExpression entityMember, MemberExpression keyMember, ExpressionType eType)
{
EntityMember = entityMember;
KeyMember = keyMember;
KeyMemberPropInfo = (PropertyInfo)keyMember.Member;
EntityMemberName = entityMember.Member.Name;
SQLOperator = BinaryExpressionToMSSQLOperator(eType);
}
public object GetKeyValue(object o)
{
return KeyMemberPropInfo.GetValue(o, null);
}
private string BinaryExpressionToMSSQLOperator(ExpressionType eType)
{
switch (eType)
{
case ExpressionType.Equal:
return "==";
case ExpressionType.GreaterThan:
return ">";
case ExpressionType.GreaterThanOrEqual:
return ">=";
case ExpressionType.LessThan:
return "<";
case ExpressionType.LessThanOrEqual:
return "<=";
case ExpressionType.NotEqual:
return "<>";
default:
throw new ArgumentException($"{eType} is not a handled Expression Type.");
}
}
}
Use Like so:
// This can be a Tuple or whatever.. If Tuple, then y below would be .Item1, etc.
// This data structure is up to you but is what I use.
[FromBody] List<CustomerAddressPk> cKeys
var res = await dbCtx.CustomerAddress
.WhereIsOneOf(cKeys, (x, y) => y.CustomerId == x.CustomerId
&& x.AddressId == y.AddressId)
.ToListAsync();
Hope this helps others.
in Case of composite key you can use another idlist and add a condition for that in your code
context.Table.Where(q => listOfIds.Contains(q.Id) && listOfIds2.Contains(q.Id2));
or you can use one another trick create a list of your keys by adding them
listofid.add(id+id1+......)
context.Table.Where(q => listOfIds.Contains(q.Id+q.id1+.......));
I tried this on EF Core 5.0.3 with the Postgres provider.
context.Table
.Select(entity => new
{
Entity = entity,
CompositeKey = entity.Id1 + entity.Id2,
})
.Where(x => compositeKeys.Contains(x.CompositeKey))
.Select(x => x.Entity);
This produced SQL like:
SELECT *
FROM table AS t
WHERE t.Id1 + t.Id2 IN (#__compositeKeys_0)),
Caveats
this should only be used where the combination of Id1 and Id2 will always produce a unique result (e.g., they're both UUIDs)
this cannot use indexes, though you could save the composite key to the db with an index
I have been referring this post to group by using expression tree. Here is my code:
String[] fields = { "DepartmentID", "SkillID" };
var groupLambda = GroupByExpression<Person>(fields);
var query = dbContext.People.GroupBy(groupLambda.Compile());
var queryResult = query.ToList();
Here is the method GroupByExpression which uses solution given in aforesaid post (Thanks Daniel!):
public static Expression<Func<TItem, object>> GroupByExpression<TItem>(string[] propertyNames)
{
var properties = propertyNames.Select(name => typeof(TItem).GetProperty(name)).ToArray();
var propertyTypes = properties.Select(p => p.PropertyType).ToArray();
var tupleTypeDefinition = typeof(Tuple).Assembly.GetType("System.Tuple`" + properties.Length);
var tupleType = tupleTypeDefinition.MakeGenericType(propertyTypes);
var constructor = tupleType.GetConstructor(propertyTypes);
var param = Expression.Parameter(typeof(TItem), "x");
var body = Expression.New(constructor, properties.Select(p => Expression.Property(param, p)));
var expr = Expression.Lambda<Func<TItem, object>>(body, param);
return expr;
}
I want to be able to identify fields in the group by keys with strong names in select part like query.Select(x => new { x.Key.DepartmentID, x.Key.SkillID });
How do I do this?
Now... I won't give you the solution to the question you asked, but I'll try to help you :-)
If you want to do dynamic queries, you should probably use DynamicLinq
With DynamicLinq you can do things like:
IQueryable query = context.YourTable;
var groups = query.GroupBy("new (Field1, Field2)");
I'm rereading your question...
I want to be able to identify fields in the group by keys with strong names in select part like query.Select(x => new { x.Key.DepartmentID, x.Key.SkillID });
You can't. GroupBy in general will return a IGrouping<TKey, TSource>. TKey is dynamic (because you build it based on strings), so you can't "extract" it and pass it to the compiler, so you can't do the select with strong names.
There is a single exception: if you know the types and numbers of the GroupBy TKey then something can be done. So, you gave us:
String[] fields = { "DepartmentID", "SkillID" };
If you always have two int then you can cast your query with:
.Cast<IGrouping<Tuple<int, int>, Person>>()
.Select(x => new { x.Key.DepartmentID, x.Key.SkillID });
Note that, as I've written in a comment, your GroupBy will be executed client-side, and everything after the GroupBy will be executed client-side (where client-side == where your program is vs sql-side == where your sql server is)!
DynamicLinq will solve the problem of executing the query sql-side instead of client-side, but won't solve the problem of strong vs weak naming (after a DynamicLinq you can: A) use .Cast<>() method or B) return a dynamic object/IEnumerable<dynamic>)
The syntax you're using new { x.Key.DepartmentID, x.Key.SkillID } constructs an anonymous class at compile time. If you want to create an anonymous class at runtime, see here. However, that won't allow you to "identify fields in the group by keys with strong names". If you want to construct an anonymous class at runtime, but be able to use those names at compile time, I'm afraid that's impossible.
Trying to work out this lambda query without doing a foreach etc. I currently have a database table with a column which contains a comma separated list of strings. It's basically a filter (it could for example be 'pants,tops,t-shirts,gloves'). In the function that queries the database is basically has a parameter that accepts a similar string.
I don't know if I'm just too tired at the moment and can't work it out but struggling. I know it will be Intersect but can't figure out the syntax.
Currently I have...
public static List<ItemListItem> GetItems(string filter = "")
{
var db = new dbConnection();
var results = (from i in db.Items
select i);
if (!string.IsNullOrEmpty (filter))
results = results.Where(x => x.Filters.Split(',').Intersect(filter.Split(',')) )
}
You need Enumerable.Any at the end of your Intersect like:
x.Filters.Split(',').Intersect(filter.Split(',')).Any()
So your query would be:
results = results.Where(x => x.Filters.Split(',')
.Intersect(filter.Split(','))
.Any());
Enumerable.Where would require an expression returning bool. Intersect would return an IEnumerable<T> and Enumerable.Where would not accept it. Adding Enumerable.Any would means return those rows where intersection resulted in Any row.
I have a table in my database called Citites. I want to retrieve all cities whose name contain any of the values from the strings list.
List<string> strings = new List<string>(new string[] {"burg", "wood", "town"} );
I tried this but it will only match the exact value from the strings list. I need to find values that contain e.g town, like cape town and townsend
List<City> cities = db.Cities.Where(c => strings.Contains(c.name));
EDIT
I'm using LINQ to SQL and Any() doesn't seem to be supported here:
Local sequence cannot be used in LINQ to SQL implementations of query
operators except the Contains operator.
This will do what you need, assuming your LINQ provider supports it - since you did not mention what are you using, we can't test it.
List<City> cities = db.Cities.Where(c => strings.Any(s => c.name.Contains(s)));
In detail: for a single value (like Capetown) you would write
strings.Any(s => "Capetown".Contains(s))
Then you just apply this expression inside your current Where condition as shown in the initial code example.
Since you mention that your LINQ provider does not support .Any() in this context, here is a much more complicated code that builds the query expression dynamically.
var strings = new [] { "burg", "wood", "town" };
// just some sample data
var cities = new[] { new City("Capetown"), new City("Hamburg"), new City("New York"), new City("Farwood") };
var param = Expression.Parameter(typeof(City));
var cityName = Expression.PropertyOrField(param, "Name"); // change the property name
Expression condition = Expression.Constant(false);
foreach (var s in strings)
{
var expr = Expression.Call(cityName, "Contains", Type.EmptyTypes, Expression.Constant(s));
condition = Expression.OrElse(condition, expr);
}
// you can apply the .Where call to any query. In the debugger view you can see that
// the actual expression applied is just a bunch of OR statements.
var query = cities.AsQueryable().Where(Expression.Lambda<Func<City, bool>>(condition, param));
var results = query.ToList();
// the class used in the test
private class City
{
public City(string name) { this.Name = name; }
public string Name;
}
But note that since you mentioned in other comments that the strings collection is rather large, you should really look into building a stored procedure and pass the values as XML parameter to that procedure (then load the XML as table and join it in the query) because this approach of building the query will probably soon run into some sort of "query has too many operands" exception.
I'm not sure if it is supported by your LINQ-provider, but at least in LINQ-To-Objects this works:
List<City> cities = db.Cities.Where(c => strings.Any(s=> c.Name.Contains(s)));
You need to check if the City name contains any of the string in the list, not the other way around:
protected bool ContainsSubstring(string cityName, List<string> strings)
{
foreach(string subString in strings)
{
if (cityName.Contains(subString)) return true;
}
return false;
}
...
List<City> cities = db.Cities.Where(c => this.ContainsSubstring(c.name, strings));
If you find it with a lot of loops, try using FUNC<> which will be better (in performance). I have a sample for that :
List<string> _lookup = new List<string>() { "dE", "SE","yu" };
IEnumerable<string> _src = new List<string> { "DER","SER","YUR" };
Func<string, List<string>, bool> test = (i,lookup) =>
{
bool ispassed = false;
foreach (string lkstring in lookup)
{
ispassed = i.Contains(lkstring, StringComparison.OrdinalIgnoreCase);
if (ispassed) break;
}
return ispassed;
};
var passedCities = _src.Where(i => test(i, _lookup));
var cities = from c in db.Cities.AsEnumerable()
from s in strings
where c.name.ToLower().Contains(s.ToLower())
select c.name;
Do you have the ability to call a stored proc or sql? - you could use SQL fulltextsearch, especially if you're searching multiple terms. It'd probably be a lot quicker than doing string comparisons in SQL.
http://technet.microsoft.com/en-us/library/ms142583.aspx
You could create your search terms by doing string.Join(" ", strings)
We have a list of strings and we need to filter our results by that list. Example would be find all students who have SSNs that start with 465, 496, or 497 (plus x more)
List<string> list = GetPossibleStartsWithValues();
var qry = from student in entities.Students.WhereStartsWith(x=>x.SSN, list)
select new SearchedStudent
{
Name = student.Name,
SSN = student.SSN,
...
}
The code provided here is close to what we need, but we can't figure out how to impliment the StartsWith that we need using the Expression Class.
Well, you could try this:
public static IQueryable<T> WhereStartsWith<T>(this IQueryable<T> source,
Expression<Func<T, string>> projection,
List<T> list)
{
return source.Where(x => list.Any(y => projection(x).StartsWith(y)));
}
That may well not work, but it would be worth trying before you go into anything more complicated.
EDIT: As you say, the above won't compile - you basically need to build an expression tree representing the bit within the Where clause. Oops. However, before you start doing that, it would be worth seeing whether it'll work in the end. Try this:
List<string> list = GetPossibleStartsWithValues();
var qry = from student in entities.Students
.Where(student => list.Any(y => student.SSN.StartsWith(y)))
select new SearchedStudent
{
Name = student.Name,
SSN = student.SSN,
...
}
If that doesn't work, then making a more general method won't be any use :(
How about using a compound statement such as
var qry = from student in entities.Students.Where(
s => list.Where( x => s.StartsWith(x)).Count() != 0 )