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Lambda .Where limitation to string

I have this method:
public List<object> GetThings(List<Guid> listOfGuids)
{
var query = serviceContext.Xrm.crmEntity;
bool anyTypeOfSearch = false; // use this to know if we have actually applied any search criteria.
if(listOfGuids != null && listOfGuids.Count > 0)
{
query = query.Where(x => listOfGuids.Contains(x.lgc_muncipalityid.Id));
anyTypeOfSearch = true;
}
var result = new List<object>();
if(anyTypeOfSearch) // instead of a variable here, can i check if there are any whereconditions applied to the query?
result = query
.Select(x => new SupplierSearchResultModel()
{
Id = x.Id,
Name = x.lgc_name,
})
.ToList();
LogMessage("GetThings.Query", <insert code to get query.Where condition tostring()>);
return result;
}
In the real code there are several different if structures with .Where conditions in them and sometimes a call can reach this code without any parameters. In this case I don't want to run the query as the result set would be huge. So I only want to run the query if at least once the .Where() condition has been applied.
Now my question is, can I check a lambda query variable for if it has any .Where() conditions applied without using an external bool like I am?
An alternate interesting usage point would be if there is some way to get some sort of query.Where().ToString() method that would show what conditions will be applied which could be logged in case of errors...

Quick & dirty, if you don't care about having a pretty result:
LogMessage(query.Expression.ToString());
But it will not show you the content of your array parameter, though.
edit Better solutions:
1) What you are looking for is an expression visitor. A template for what you want to do here, which should then be used like:
LogMessage(query.ToPrettyString());
2) Think about an expression query.Where(x=>x.member == GetSomething()) do you want it to be printed like that ? Or do you want GetSomething() result to appear as a string result ? If the second solution, then that's something you can do with this

You can create your own implementation of the ExpressionVisitor to traverse the nodes of the expression. You can do something like this:
public class WhereVisitor : ExpressionVisitor
{
private static bool _filter;
private static WhereVisitor _visitor = new WhereVisitor();
private WhereVisitor() { }
public new static bool Visit(Expression expression)
{
_filter = false;
//Cast to ExpressionVisitor to use the default Visit and not our new one
((ExpressionVisitor)_visitor).Visit(expression);
return _filter;
}
protected override Expression VisitMethodCall(MethodCallExpression node)
{
if (node.Method.Name == "Where")
_filter = true;
return base.VisitMethodCall(node);
}
}
And use it like this:
bool containsWhere = WhereVisitor.Visit(query.Expression);
If you want you can of course expand the visitor to save the expressions that contain a Where clause, but this one will just tell you if there are is Where or not.

Good way to handle NullReferenceException before C# 6.0

My code below gives me a NullReferenceException and the stack trace tells me the problem is in the Count method, so I'm pretty sure at some point foo, bar or baz is null.
My code:
IQueryable<IGrouping<string, Referral>> queryable= ...;
var dict = queryable.ToDictionary(g => g.Key.ToString(),
g => g.Count(r => r.foo.bar.baz.dummy == "Success"));
I'm wondering what's a concise way to handle null cases.
I learn that in C# 6.0 I can just do foo?.bar?.baz?.dummy, however the project I'm working on isn't C# 6.0

A solution for <6.0 would be:
.Count(r => r.foo != null &&
r.foo.bar != null &&
r.foo.bar.baz != null &&
r.foo.bar.baz.dummy == "Success")
Exactly for complex constructs like the one above the null propagation operator was introduced.
Furthermore you could also refactor the expression into a private method:
private Expression<Func<Referral, bool>> Filter(string value)
{
return r => r.foo != null &&
r.foo.bar != null &&
r.foo.bar.baz != null &&
r.foo.bar.baz.dummy == value;
}
and use it as follows:
g => g.Count(Filter("Success"))

You can use the following extension methods.
public static TResult With<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator)
where TResult : class
where TInput : class
{
return o == null ? null : evaluator(o);
}
public static TResult Return<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator, TResult failureValue)
where TInput : class
{
return o == null ? failureValue : evaluator(o);
}
Their combination gives you a nice, readable API for handling nulls:
return foo
.With(o => o.bar)
.With(o => o.baz)
.Return(o => o.dummy, null);

The problem is that the ToDictionary method isn't actually done on the queryable - instead, you get the whole collection, and do the aggregation in your application, rather than on the DB server.
So instead of using ToDictionary directly, use Select first:
IQueryable<IGrouping<string, Referral>> queryable= ...;
var dict = queryable.Select(g => new { Key = g.Key.ToString(),
Count = g.Count(r => r.foo.bar.baz.dummy == "Success") })
.ToDictionary(i => i.Key, i => i.Count);
This will make sure the aggregation is done in the database (where you don't care about those nulls) and not in the C# code (where you get a NullReferenceException).
Of course, this assumes that the queryable you're using is a DB query (or, to be more precise, a queryable that supports aggregation and has ANSI SQL-like NULL semantics). If you have a different custom queryable, it's not going to help (unless you explicitly add those NULL semantics yourself).

// if null, use null
if(objectvariable == null)
{
// throw exception
}
// if not null
if(objectvariable != null)
{
// continue
}

Entityframework - Adding Expression to where clause

Let's say I have this method to seach my DB for products that fit a certain keyword:
public List<Product> GetByKeyword(string keyword)
{
using(var db = new DataEntities())
{
var query = db.Products.Where(x => x.Description.Contains(keyword);
return query.ToList();
}
}
This works fine, but somewhere else in my project, I want to get the active products only, still by keyword. I would like to do something like :
...
var result = ProductStore.GetByKeyword("Apple", x => x.isActive == 1);
Therefore, I created this method:
public List<Product> GetByKeyword(string keyword, Func<Product, bool> predicate = null)
{
using(var db = new DataEntities())
{
var query = db.Products.Where(x => x.Description.Contains(keyword);
if(predicate != null)
query = query.Where(x => predicate(x));
return query.ToList();
}
}
While this compiles well, the ToList() call generates a NotSupportedException because LINQ does not support the Invoke method.
Of course, I could to it with another method
i.e. GetActiveByKeyword(string keyword) but then I would have to do one for every possible variation, including the ones I didn't think of...
How do I get this to work? Thanks!

Isn't it just this:
if(predicate != null)
query = query.Where(predicate);

it's just as AD.Net says the reason why it works with Expression before is because if you say that the compiler knows it would be a lambda expression

Alternative for multiple IF/CASE statements

Is there a way to avoid multiple IF/CASE statements in C#?
In my app I will end up with 8+ fields used to create a Linq query where every expression can be null or != null so it will give me 64 scenarios.
I'm not providing any code samples because I can do it using IF/CASE and simplify it as much as I can.
If You are familiar with some useful approaches to that problem I will appreciate any advice.
Code Sample (it only includes two delegates but I'll have to add more to filter data)
Repository
public virtual IEnumerable<T> Get(Expression<Func<T, bool>> filter = null, Expression<Func<T, bool>> filter1 = null)
{
IQueryable<T> query = dbSet;
if (filter != null)
{
query = query.Where(filter);
return query.ToList();
}
if (filter1 != null)
{
query = query.Where(filter1);
return query.ToList();
}
else
{
return query.ToList();
}
}
Controller
public ActionResult Index(string searchFullName, string searchExtension)
{
var extensionList = new List<string>();
var extensions = from n in unitofwork.DomainRepository.Get()
select n.Extension;
extensionList.AddRange(extensions.Distinct());
ViewBag.searchExtension = new SelectList(extensionList);
if (!String.IsNullOrEmpty(searchFullName) && !String.IsNullOrEmpty(searchExtension))
{
return View(unitofwork.DomainRepository.Get(n => n.Extension == searchExtension && n.Name.Contains(searchFullName)));
}
else if (!String.IsNullOrEmpty(searchExtension))
{
return View(unitofwork.DomainRepository.Get(n => n.Extension == searchExtension));
}
else if (String.IsNullOrEmpty(searchFullName) && String.IsNullOrEmpty(searchExtension))
{
return View(unitofwork.DomainRepository.Get());
}
else if (!String.IsNullOrEmpty(searchFullName) && String.IsNullOrEmpty(searchExtension))
{
return View(unitofwork.DomainRepository.Get(n => n.Name.Contains(searchFullName)));
}
else
{
return View(unitofwork.DomainRepository.Get());
}
}

Yes it is possible, using Linq's powerful Aggregate method (a version of the fold function from functional programming).
public virtual IEnumerable<T> FilterOnAll(params Expression<Predicate<T> filters)
{
return filters.Aggregate(dbSet, (acc, element) => acc.Where(element));
}
public virtual IEnumerable<T> FilterOnAny(params Expression<Predicate<T> filters)
{
Expression<Predicate<T>> alwaysTrue = _ => true;
var compositeFilter = filters.Aggregate(alwaysTrue, (acc, element) => acc.Or(element));
return dbSet.Where(compositeFilter);
}
You can then compose these two builders to create pretty much any logical condition you want from within your controller.
Good luck.

What you can do is an or statement in your where clause
where (variable1 == null || b.data = variable1)
and just do that for all 8 if im understanding your problem if its null it passes over it as true else it checks against the data.

Your entire if .. elseif .. else block can be replaced by:
return View(unitofwork.DomainRepository.Get(n =>
(string.IsNullOrEmpty(searchFullName) || n.Name.Contains(searchFullName))
&& (string.IsNullOrEmpty(searchExtension) || n.Extension == searchExtension)));

How can I conditionally apply a Linq operator?

We're working on a Log Viewer. The use will have the option to filter by user, severity, etc. In the Sql days I'd add to the query string, but I want to do it with Linq. How can I conditionally add where-clauses?

if you want to only filter if certain criteria is passed, do something like this
var logs = from log in context.Logs
select log;
if (filterBySeverity)
logs = logs.Where(p => p.Severity == severity);
if (filterByUser)
logs = logs.Where(p => p.User == user);
Doing so this way will allow your Expression tree to be exactly what you want. That way the SQL created will be exactly what you need and nothing less.

If you need to filter base on a List / Array use the following:
public List<Data> GetData(List<string> Numbers, List<string> Letters)
{
if (Numbers == null)
Numbers = new List<string>();
if (Letters == null)
Letters = new List<string>();
var q = from d in database.table
where (Numbers.Count == 0 || Numbers.Contains(d.Number))
where (Letters.Count == 0 || Letters.Contains(d.Letter))
select new Data
{
Number = d.Number,
Letter = d.Letter,
};
return q.ToList();
}

I ended using an answer similar to Daren's, but with an IQueryable interface:
IQueryable<Log> matches = m_Locator.Logs;
// Users filter
if (usersFilter)
matches = matches.Where(l => l.UserName == comboBoxUsers.Text);
// Severity filter
if (severityFilter)
matches = matches.Where(l => l.Severity == comboBoxSeverity.Text);
Logs = (from log in matches
orderby log.EventTime descending
select log).ToList();
That builds up the query before hitting the database. The command won't run until .ToList() at the end.

I solved this with an extension method to allow LINQ to be conditionally enabled in the middle of a fluent expression. This removes the need to break up the expression with if statements.
.If() extension method:
public static IQueryable<TSource> If<TSource>(
this IQueryable<TSource> source,
bool condition,
Func<IQueryable<TSource>, IQueryable<TSource>> branch)
{
return condition ? branch(source) : source;
}
This allows you to do this:
return context.Logs
.If(filterBySeverity, q => q.Where(p => p.Severity == severity))
.If(filterByUser, q => q.Where(p => p.User == user))
.ToList();
Here's also an IEnumerable<T> version which will handle most other LINQ expressions:
public static IEnumerable<TSource> If<TSource>(
this IEnumerable<TSource> source,
bool condition,
Func<IEnumerable<TSource>, IEnumerable<TSource>> branch)
{
return condition ? branch(source) : source;
}

When it comes to conditional linq, I am very fond of the filters and pipes pattern.
http://blog.wekeroad.com/mvc-storefront/mvcstore-part-3/
Basically you create an extension method for each filter case that takes in the IQueryable and a parameter.
public static IQueryable<Type> HasID(this IQueryable<Type> query, long? id)
{
return id.HasValue ? query.Where(o => i.ID.Equals(id.Value)) : query;
}

Doing this:
bool lastNameSearch = true/false; // depending if they want to search by last name,
having this in the where statement:
where (lastNameSearch && name.LastNameSearch == "smith")
means that when the final query is created, if lastNameSearch is false the query will completely omit any SQL for the last name search.

Another option would be to use something like the PredicateBuilder discussed here.
It allows you to write code like the following:
var newKids = Product.ContainsInDescription ("BlackBerry", "iPhone");
var classics = Product.ContainsInDescription ("Nokia", "Ericsson")
.And (Product.IsSelling());
var query = from p in Data.Products.Where (newKids.Or (classics))
select p;
Note that I've only got this to work with Linq 2 SQL. EntityFramework does not implement Expression.Invoke, which is required for this method to work. I have a question regarding this issue here.

It isn't the prettiest thing but you can use a lambda expression and pass your conditions optionally. In TSQL I do a lot of the following to make parameters optional:
WHERE Field = #FieldVar OR #FieldVar IS NULL
You could duplicate the same style with a the following lambda (an example of checking authentication):
MyDataContext db = new MyDataContext();
void RunQuery(string param1, string param2, int? param3){
Func checkUser = user =>
((param1.Length > 0)? user.Param1 == param1 : 1 == 1) &&
((param2.Length > 0)? user.Param2 == param2 : 1 == 1) &&
((param3 != null)? user.Param3 == param3 : 1 == 1);
User foundUser = db.Users.SingleOrDefault(checkUser);
}

I had a similar requirement recently and eventually found this in he MSDN.
CSharp Samples for Visual Studio 2008
The classes included in the DynamicQuery sample of the download allow you to create dynamic queries at runtime in the following format:
var query =
db.Customers.
Where("City = #0 and Orders.Count >= #1", "London", 10).
OrderBy("CompanyName").
Select("new(CompanyName as Name, Phone)");
Using this you can build a query string dynamically at runtime and pass it into the Where() method:
string dynamicQueryString = "City = \"London\" and Order.Count >= 10";
var q = from c in db.Customers.Where(queryString, null)
orderby c.CompanyName
select c;

You can create and use this extension method
public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool isToExecute, Expression<Func<TSource, bool>> predicate)
{
return isToExecute ? source.Where(predicate) : source;
}

Just use C#'s && operator:
var items = dc.Users.Where(l => l.Date == DateTime.Today && l.Severity == "Critical")
Edit: Ah, need to read more carefully. You wanted to know how to conditionally add additional clauses. In that case, I have no idea. :) What I'd probably do is just prepare several queries, and execute the right one, depending on what I ended up needing.

You could use an external method:
var results =
from rec in GetSomeRecs()
where ConditionalCheck(rec)
select rec;
...
bool ConditionalCheck( typeofRec input ) {
...
}
This would work, but can't be broken down into expression trees, which means Linq to SQL would run the check code against every record.
Alternatively:
var results =
from rec in GetSomeRecs()
where
(!filterBySeverity || rec.Severity == severity) &&
(!filterByUser|| rec.User == user)
select rec;
That might work in expression trees, meaning Linq to SQL would be optimised.

Well, what I thought was you could put the filter conditions into a generic list of Predicates:
var list = new List<string> { "me", "you", "meyou", "mow" };
var predicates = new List<Predicate<string>>();
predicates.Add(i => i.Contains("me"));
predicates.Add(i => i.EndsWith("w"));
var results = new List<string>();
foreach (var p in predicates)
results.AddRange(from i in list where p.Invoke(i) select i);
That results in a list containing "me", "meyou", and "mow".
You could optimize that by doing the foreach with the predicates in a totally different function that ORs all the predicates.