In a project I'm currently working on, we have many static Expressions that we have to bring in local scope with a variable when we call the Invoke method on them and pass our lambda expressions' arguments to.
Today, we declared a static method whose parameter is exactly the type that the query is expecting. So, my coworker and I were messing around to see if we could get this method to do the project in the Select statement of our query, instead of invoking it on the whole object, without bringing it into local scope.
And it worked! But we do not understand why.
Imagine code like this
// old way
public static class ManyExpressions {
public static Expression<Func<SomeDataType, bool> UsefulExpression {
get {
// TODO implement more believable lies and logic here
return (sdt) => sdt.someCondition == true && false || true;
}
}
}
public class ARealController : BaseController {
/* many declarations of important things */
public ARealClass( /* many ninjected in things */) {
/* many assignments */
}
public JsonNet<ImportantDataResult> getSomeInfo(/* many useful parameter */) {
var usefulExpression = ManyExpressions.UsefulExpression;
// the db context is all taken care of in BaseController
var result = db.SomeDataType
.Where(sdt => usefulExpression.Invoke(sdt))
.Select(sdt => new { /* grab important things*/ })
.ToList();
return JsonNet(result);
}
}
And then you get to do this!
// new way
public class SomeModelClass {
/* many properties, no constructor, and very few useful methods */
// TODO come up with better fake names
public static SomeModelClass FromDbEntity(DbEntity dbEntity) {
return new SomeModelClass { /* init all properties here*/ };
}
}
public class ARealController : BaseController {
/* many declarations of important things */
public ARealClass( /* many ninjected in things */) {
/* many assignments */
}
public JsonNet<SomeModelClass> getSomeInfo(/* many useful parameter */) {
// the db context is all taken care of in BaseController
var result = db.SomeDataType
.Select(SomeModelClass.FromDbEntity) // TODO; explain this magic
.ToList();
return JsonNet(result);
}
}
So when ReSharper prompts me to do this (which is not often, as this condition of matching the type that is expected by the Delegate isn't often satisfied), it says convert to a Method Group. I kind of vaguely understand that a Method Group is a set of methods, and the C# compiler can take care of converting the method group to an explicitly typed and appropriate overload for the LINQ provider and what not... but I'm fuzzy on why this works exactly.
What's going on here?
It's great to ask a question when you don't understand something, but the problem is that it can be hard to know which bit someone doesn't understand. I hope I help here, rather than tell you a bunch of stuff you know, and not actually answer your question.
Let's go back to the days before Linq, before expressions, before lambda, and before even anonymous delegates.
In .NET 1.0 we didn't have any of those. We didn't even have generics. We did though have delegates. And a delegate is related to a function pointer (if you know C, C++ or languages with such) or function as argument/variable (if you know Javascript or languages with such).
We could define a delegate:
public delegate int MyDelegate(double someValue, double someOtherValue);
And then use it as a type for a field, property, variable, method argument or as the basis of an event.
But at the time the only way to actually give a value for a delegate was to refer to an actual method.
public int CompareDoubles(double x, double y)
{
if (x < y) return -1;
return y < x ? 1 : 0;
}
MyDelegate dele = CompareDoubles;
We can invoke that with dele.Invoke(1.0, 2.0) or the shorthand dele(1.0, 2.0).
Now, because we have overloading in .NET, we can have more than one thing that CompareDoubles refers to. That isn't a problem, because if we also had e.g. public int CompareDoubles(double x, double y, double z){…} the compiler could know that you could only possibly have meant to assign the other CompareDoubles to dele so it's unambiguous. Still, while in the context CompareDoubles means a method that takes two double arguments and returns an int, outside of that context CompareDoubles means the group of all the methods with that name.
Hence, Method Group which is what we call that.
Now, with .NET 2.0 we got generics, which is useful with delegates, and at the same time in C#2 we got anonymous methods, which is also useful. As of 2.0 we could now do:
MyDelegate dele = delegate (double x, double y)
{
if (x < y) return -1;
return y < x ? 1 : 0;
};
This part was just syntactic sugar from C#2, and behind the scenes there's still a method there, though it has an "unspeakable name" (a name that is valid as a .NET name but not valid as a C# name, so C# names can't clash with it). It was handy if, as was often the case, one was creating methods just to have them used once with a particular delegate though.
Move forward a bit further, and at .NET 3.5 have covariance and contravariance (great with delegates) the Func and Action delegates (great for reusing the same name based on type, rather than having a bunch of different delegates which were often very similar) and along with it came C#3 which had lambda expressions.
Now, these are a bit like anonymous methods in one use, but not in another.
That's why we can't do:
var func = (int i) => i * 2;
var works out what it means from what's been assigned to it, but lamdas work out what they are from what they've been assigned to, so this is ambiguous.
It could mean:
Func<int, int> func = i => i * 2;
In which case it's shorthand for:
Func<int, int> func = delegate(int i){return i * 2;};
Which in turn is shorthand something like for:
int <>SomeNameImpossibleInC# (int i)
{
return i * 2;
}
Func<int, int> func = <>SomeNameImpossibleInC#;
But it can also be used as:
Expression<Func<int, int>> func = i => i * 2;
Which is shorthand for:
Expression<Func<int, int>> func = Expression.Lambda<Func<int, int>>(
Expression.Multiply(
param,
Expression.Constant(2)
),
param
);
And we also with .NET 3.5 have Linq which makes heavy use of both of these. Indeed, Expressions is considered part of Linq and is in the System.Linq.Expressions namespace. Note that the object we get here is a description of what we want done (take the parameter, multiply it by two, give us the result) not of how to do it.
Now, Linq operates in two main ways. On IQueryable and IQueryable<T> and on IEnumerable and IEnumerable<T>. The former defines operations to be used on "a provider" with just what "a provider does" being up to that provider, and the latter defines the same operations on in-memory sequences of values.
We can move from one to the other. We can turn an IEnumerable<T> into an IQueryable<T> with AsQueryable which will give us a wrapper on that enumerable, and we can turn the IQueryable<T> into an IEnumerable<T> just by treating it as one, because IQueryable<T> derives from IEnumerable<T>.
The enumerable form uses the delegates. A simplified version of how Select works (there are many optimisations this version leaves out, and I'm skipping error checking and in indirection to ensure that error checking happens immediately) would be:
public static IEnumerable<TResult> Select(this IEnumerable<TSource> source, Func<TSource, TResult> selector)
{
foreach(TSource item in source) yield return selector(item);
}
The queryable version on the other hand works by taking the expression tree from the Expression<TSource, TResult> making it part of an expression that includes the call to Select, and the source queryable, and returns an object wrapping that expression. So in other words a call to queryable's Select returns an object that represents a call to queryable's Select!
Just what is done with that depends on the provider. Database providers turn them into SQL, enumerables call Compile() on the expression to create a delegate and then we're back at the first version of Select above, and so on.
But that history considered, let's go backwards through the history again. A lambda can represent either an expression or a delegate (and if an expression, we can Compile() it to get the same delegate). A delegate is a way of pointing to a method through a variable, and a method is part of a method group. All of this is built on technology which in the first version could only be called by creating a method and then passing that.
Now, lets say we have a method that takes a single argument and has a result.
public string IntString(int num) { return num.ToString(); }
Now lets say we referenced it in a lambda selector:
Enumerable.Range(0, 10).Select(i => IntString(i));
We have a lambda creating an anonymous method for a delegate, and that anonymous method in turn calls a method with the same argument and return types. In a way that's a bit like if we had:
public string MyAnonymousMethod(int i){return IntString(i);}
MyAnonymousMethod is a bit pointless here; all it does is call IntString(i) and return the result, so why not just call IntString in the first place and cut out going through that method:
Enumerable.Range(0, 10).Select(IntString);
We've cut out a needless (though see note below about delegate caching) level of indirection by taking the lambda-based delegate and converting it to a method group. Hence ReSharper's advice "Convert to Method Group" or however it's worded (I don't use ReSharper myself).
There is though something to be careful of here. IQueryable<T>'s Select only takes expressions, so the provider can try to work out how to convert it to its way of doing stuff (e.g. SQL against a database). IEnumerable<T>'s Select only takes delegates so they can be executed in the .NET application itself. We can go from the former to the latter (when the queryable is really a wrapped enumerable) with Compile(), but we can't go from the latter to the former: We don't have a way of taking a delegate and turning it into an expression that means anything other than "call this delegate" which isn't something that can be turned into SQL.
Now when we use a lambda expression like i => i * 2 it will be an expression when used with IQueryable<T> and a delegate when used with IEnumerable<T> due to overload resolution rules favouring the expression with queryable (as a type it can handle both, but the expression form works with the most derived type). If though we explicitly give it a delegate, whether because we typed it somewhere as Func<> or it comes from a method group, then the overloads taking expressions aren't available and those taking delegates are used. This means it doesn't get passed to the database but rather the linq expression up to that point becomes the "database part" and it gets called and the rest of the work done in memory.
95% of the time that's best avoided. So 95% of the time if you get advice of "convert to method group" with a database-backed query you should think "uh oh! that's actually a delegate. Why is that a delegate? Can I change it to be an expression?". Only the remaining 5% of the time should you think "that'll be slightly shorter if I just pass in the method name". (Also, using a method group instead of a delegate prevents caching of delegates the compiler can do otherwise, so it might be less efficient).
There, I hope I covered the bit that you didn't understand in the course of all that, or at least there's a bit here you can point to and say "that bit there, that's the bit I don't grok".
Select(SomeModelClass.FromDbEntity)
This uses Enumerable.Select which is not what you want. This transitions out of "queryable-LINQ" into LINQ to objects. This means the database cannot execute this code.
.Where(sdt => usefulExpression.Invoke(sdt))
Here, I assume you meant .Where(usefulExpression). This passes the expression into the expression tree underlying the query. The LINQ provider can translate this expression.
When you perform experiments like this use SQL Profiler to see what SQL goes over the wire. Make sure all relevant parts of the query are translatable.
I don't want to disappoint you, but there is no magic at all. And I would suggest you to be very careful with this "new way".
Always check the result of a function by hovering it in VS. Remember that IQueryable<T> "inherits" IEnumerable<T> and also Queryable contains the extension methods with the same names as the Enumerable, and the only difference is that the former works with Expression<Func<...>> while the later just with Func<..>.
So anytime you use Func or method group over IQueryable<T>, the compiler will pick the Enumerable overload, thus silently switching from LINQ to Entities to LINQ to Objects context. But there is a huge difference between the two - this former is executed in database while the later in memory.
The key point is to stay as long as possible in the IQueryable<T> context, so the "old way" should be preferred. E.g. from your examples
.Where(sdt => sdt.someCondition == true && false || true)
or
.Where(ManyExpressions.UsefulExpression)
or
.Where(usefulExpression)
but not
.Where(sdt => usefulExpression.Invoke(sdt))
And never
.Select(SomeModelClass.FromDbEntity)
This solution threw up some red flags for me. Key among them was:
var result = db.SomeDataType
.Select(SomeModelClass.FromDbEntity) // TODO; explain this magic
.ToList(); // <<!!!!!!!!!!!!!
Whenever you're dealing with Entity Framework, you can read "ToList()" as "Copy the whole thing into memory." So "ToList()" should only be done at the last possible second.
Consider: there are lots of useful object you can pass around when dealing with EF:
The database context
The specific dataset you're targeting (e.g. context.Orders)
Queries against a context:
.
var query = context.Where(o => o.Customer.Name == "John")
.Where(o => o.TxNumber > 100000)
.OrderBy(o => o.TxDate);
//I've pulled NO data so far! "var query" is just an object I can pass around
//and even add on to! For example, I can now do this:
query = query.ThenBy(o => o.Items.Description); //and now I've appended that to my query
The real magic is that those lambdas can be thrown in to a variable too. Here's a method I use in one of my projects to do that:
/// <summary>
/// Generates the Lambda "TIn => TIn.memberName [comparison] value"
/// </summary>
static Expression<Func<TIn, bool>> MakeSimplePredicate<TIn>(string memberName, ExpressionType comparison, object value)
{
var parameter = Expression.Parameter(typeof(TIn), "t");
Expression left = Expression.PropertyOrField(parameter, memberName);
return (Expression<Func<TIn, bool>>)Expression.Lambda(Expression.MakeBinary(comparison, left, Expression.Constant(value)), parameter);
}
With this code, you can write something like the following:
public GetQuery(string field, string value)
{
var query = context.Orders;
var condition = MakeSimplePredicate<Order>(field, ExpressionType.Equal, value);
return query.Where(condition);
}
The best thing is that at this time, no data call has been. You can continue to add conditions as you wish. When you're ready to fetch the data, simply iterate through it or call ToList().
Enjoy!
Oh, and check this out if you'd like to see a more thoroughly-developed solution, albeit from a different context.
My Post on Linq Expression Trees
I have read that is recomended to use functions instead of predicates in the extension methods, so I am trying to do it.
public static void Insert<T>(this ObservableCollection<T> paramOC, T NewElement, Func<T, T, bool> condition)
{
//code
}
I am trying to use in this way:
myOC.Insert(myNewElement, e=>e.Name.CompareTo(myNewElement.Name) > 0));
But I get an error that says that the delete System.Func does not take 1 argument.
However, if I use a predicate intead of the function, it works.
What am I missing?
thank so much.
You need Func<T,bool> (which takes one argument and returns bool), not Func<T,T,bool>
Predicate<T> works because it takes one argument and returns bool, so it matches with the lambda expression.Func<T,T,bool> expects two arguments and returns bool which doesn't match with your expression hence the error.
Your Func<T,T,bool> takes 2 parameters and returns bool
You have two choise
as Selman22 says change it to Func<T,bool>
call it as (e1,e2) => ..
I am looking at code containing:
public virtual ICollection<T> GetPk(string pk)
{
Expression<Func<T, bool>> predicate = c => c.PartitionKey == pk;
return this.GetAll(predicate);
}
Can someone explain the syntax of <Func<T, bool>> ?
Simply Func<T, bool> is the anonymous method signature. The first type T is the input parameter type and the second type is the return type. This is more like a method when you consider your representation:
bool AnonMethod(T arg0)
{
return arg0.PartitionKey == pk;
}
One of the best explanation can be found at MSDN
You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. The encapsulated method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have one parameter that is passed to it by value, and that it must return a value.
As for argument in your example T is the type of input parameter and bool is the return type of exacted method.
A Func<T, bool> represents a function that takes an object of type T and returns a bool. It's commonly referred to as a "predicate", and is used to verify a condition on an object.
An Expression<Func<T, bool>> represents the abstract syntax tree of the function, i.e. its syntactic structure. It can be used to analyse the code of the function for various purposes, such as transforming it to SQL to be executed against a database.
I always find MSDN to be worth checking on things like this first,
http://msdn.microsoft.com/en-us/library/bb549151.aspx
Beaten by Maheep, didn't see the post message pop-in.
Basically, you're declaring a method that matches a signature, that can then be passed in to the call to get the data.
It is confusing at first but Func<T, bool> describes a function that returns a bool and accepts a parameter as type T.
In this case, T is an object that has a PartitionKey property and this GetPk method is using the Func<T, bool> to match all the T items in the instance object which have a PartitionKey that matches the string pk.
For some background; prior to Func<T, TResult> (and the rest of this family) being part of the framework, you either had to explicitly define delegates or use anonymous methods.
Func and Action were added as part of the addition of lambda expressions to the language. They are the framework-defined delegates which lambda expressions are typed as, but which you as a developer can also use in place of your own custom delegate definitions.
You can get a nice history here;
http://blogs.msdn.com/b/ericwhite/archive/2006/10/03/lambda-expressions.aspx
It's additional syntax so you know what goes in and out of the function.
Func<T, bool> means:
function has 1 input T and 1 output that's bool.
This is other variations of the function
I'm trying to determine the equality of two predicates:
public T FirstOrDefault(Func<T, bool> predicate)
{
if (EntityCache.ContainsKey(predicate.GetHashCode()))
return EntityCache[predicate.GetHashCode()];
else
{
var entity = _objectSet.FirstOrDefault<T>(predicate);
EntityCache.Add(predicate.GetHashCode(), entity);
return entity;
}
}
The issue I'm having is the hash code of the predicate doesn't account for the values used inside it, and I'm not sure how to go about retrieving them.
If for instance the predicate passed to our method above is: (r => r.Id == id) how would I go about finding the value of 'id' inside my FirstOrDefault method?
You'll need to use Expression's. They contains func's, but they also contain the syntax tree and you can examine their 'source code' at runtime.
If I understand you correctly, you're trying to determine the value of a captured variable inside a delegate. I'm pretty sure there's no easy way to do this... perhaps if you used an expression tree instead of a delegate it would be simpler ?
Another option (probably much simpler) is keeping the values passed to the delegates alongside them in EntityCache
Also note that instead of
if (EntityCache.ContainsKey(predicate.GetHashCode()))
return EntityCache[predicate.GetHashCode()];
It would be better to use TryGetValue
I have a method with the following signature:
private PropertyInfo getPropertyForDBField(string dbField, out string prettyName)
In it, I find the associated value prettyName based on the given dbField. I then want to find all properties, if any, that have the name prettyName, so I'm trying to do the following:
IEnumerable<PropertyInfo> matchingProperties =
getLocalProperties().Where(prop =>
prop.Name.Equals(prettyName)
);
However, this gives the following error:
Cannot use ref or out parameter 'prettyName' inside an anonymous method, lambda expression, or query expression
By the point in the method where I'm trying to use prettyName in the Where lambda parameter, prettyName is definitely initialized. I return if prettyName cannot be initialized to a valid value. Is there some trick I could do here to let me use prettyName in the lambda expression?
Edit: I'm using .NET 3.5 if it matters.
Just to clarify. It's possible to use ref/out arguments from a called method in a lambda.
You can also use a ref or out if you specify type of the parameter. Which means sending prettyName as a parameter to the lambda.
(prop, ref string prettyName) => prop.Name.Equals(prettyName);
Where clause takes in only one argument, which is the property element in the list. This is what prevents you from adding an argument to the lambda.
Didn't want to leave people the false impression that you cannot use these arguments in a lambda. You just can't use them by capture.
As the compiler error indicates, it isn't allowed to use out or ref parameters inside lambda expressions.
Why not just use a copy? It's not like the lambda wants to mutate the variable anyway, so I don't see a downside.
string prettyNameCopy = prettyName;
var matchingProperties = getLocalProperties()
.Where(prop => prop.Name == prettyNameCopy);
Alternatively, you can use a local throughout (to evaluate the appropriate name etc.), and assign the outparameter prettyName just before returning from the method. This will probably be more readable if there isn't significant branching within the method.