I'm trying to pass a Predicate to a function but it has no input. I'm actually trying to delay the computation until the call is made.
Is there a way to use c# Predicate type for that? If not why.
I know a way to do this with Func
Func<bool> predicate = () => someConditionBool;
But I want to do this:
Predicate<> predicate = () => someConditionBool;
If it's just a readability problem, then you can create your own delegate which returns boolean and don't have any parameters:
public delegate bool Predicate();
And use it this way
Predicate predicate = () => someConditionBool;
NOTE: You should keep in mind that everyone is familiar with default Action and Func delegates which are used in .NET, but Predicate is your custom delegate and it will be less clear at first time for average programmer.
Is there a way to use c# Predicate type for that? If not why?
Looking at the signature of Predicate<T> is shows that it requires an argument or parameter:
public delegate bool Predicate<in T>(T obj)
So the quick answer here would be no. Because you have no parameters in your anonymous function.
But if you play a little around you can give the predicate some irrelevant object which it will not use anyway, like an alibi parameter:
Func<bool> predicate = () => 1 == 1;
Predicate<object> predicate2 = (x) => 1 == 1;
and the call would look like this:
Console.WriteLine(predicate());
Console.WriteLine(predicate2(null));
And this compiles and returns the correct result
Related
Is there a way to find parameters which is passed in predicate variable. Lets say, I have this method;
List<User> GetUsers(Predicate<UserModel> userPredicate)
{
// how to find what values are passed in userPredicate
}
Function Call:
GetUsers(_ => _.Name == "abc");
How can I find that in predicate, Name property has been set to "abc" within GetUsers function?
public static void GetUsers(Expression<Func<UserModel, bool>> predicate)
{
var expr = predicate.Body as BinaryExpression;
var value = expr.Right as ConstantExpression;
// this will be the value of that predicate Func
var yourvalue = value.Value;
}
This is one way to do it, and you have to be careful about casting those expressions because it might not work if you change the body of expression.
FYI, I wouldn't recommend this solution at all to achieve what you need to do, there are better ways to design it but it gives you are looking for.
I would like to create a lambda expression and invoke it immediately and I would like to avoid creating a delegate; a trivial example1:
int i = (() => 42)();
This produces the error:
CS0149 Method name expected
There are two workarounds:
Declare a (local) method:
int Return42() => 42;
int i = Return42();
Create a delegate:
int i = ((Func<int>)(() => 42))();
It is possible to create and immediately invoke a lambda expression without creating a delegate and without naming it? And if possible, how can one create such a lambda?
1. In reality it is an async Task that I would like to use instead of Task.ContinueWith (I tried to follow what Stephen Cleary said: you should strive to replace ContinueWith with await); e.g:
Task<int> future = (async () =>
(await RetrieveLookupFromFarAway())["42"].First())();
With RetrieveLookupFromFarAway something like:
async Task<ILookup<string, int>> RetrieveLookupFromFarAway()
{
await Task.Delay(1000);
return Enumerable.Empty<int>().ToLookup((x) => x.ToString());
}
The concept of a lambda expression only exists as source code. It doesn't even have a type in itself (just like the null literal doesn't have a type). It has to be converted to either an expression tree or a delegate: that's what exists as far as the IL and the CLR are concerned. The compiler has to emit code to create something, and you need to tell it which type you want that to be.
The compiler doesn't play favourites in terms of delegate types: while it could "know" about Func<T> and use that as a default delegate type for a lambda expression with no parameters, it doesn't.
The closest you'll get to what you want is to have a convenience method that you can call accepting a Func<T>, which could either just return the function, or execute it and return the result. For example:
public static Func<T> CreateFunc<T>(Func<T> func) => func;
public static T ExecuteFunc<T>(Func<T> func) => func();
Then you can call it as:
CreateFunc(() => 42)();
or
ExecuteFunc(() => 42);
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.
I have a feeling I already know the answer to this question.
I have an expression that common but complex, and I would like to re-use the code in multiple locations. I would like to use a function that returns a Func with some code:
public static Func<MyClass, bool> GetCheck()
{
return (x) => x.Value > 10;
}
Seems easy. The problem that I'm having is when I apply it to a LINQ expression that's used in LINQ To Entities. I get an error that says Invoke is not supported. And I understand why. What I don't know is if there's a way that I can get around this. I would like to say...
var check = GetCheck();
IQueryable<MyClass> results = MyClasses.Where(y => check(y));
... and have the expression tree inspector realize that everything that happens in the Func is perfectly legal both in LINQ To Entities and on the DB. It seems like it should be inline'd.
Is there anything I can do to make this happen? Any form of declaration for the delegate that will allow this?
Since you are querying an IQueryable<> then you should try this:
public static Expression<Func<MyClass, bool>> GetCheck()
{
return (x) => x.Value > 10;
}
I need to pass a parameter to a method that requires an Expression<Func<T, bool>>.
How to do I pass an expression that would always return true?
Using obj => true doesn't work because the framework complains at runtime that it cannot determine the memeber type from the True constant.
If you have a function like this
void TakeExpression<T>(Expression<Func<T, bool>> expr)
You should call it this way, specifying the T type :
TakeExpression<int>(_ => true)
It should work.
You need to define the parameter type you are passing:
(object o) => true
Or
(int a) => true
We can achieve the result as follows.
Consider context as your DbContext instance and Entity as your entity class name.
context.Entity.Where(t=> t.EntityID == t.EntityID);
By doing this the where clause will always return true and all the data will be shown.
There are two problems here:
1) If you're passing a predicate such that you always want to return true, then it's not much of a predicate. You may be able to omit whatever call you are trying to make.
2) If you want to just return true, you can simple use a more verbose lambda syntax to get what you want:
sample.AsQueryable().Where((x) => { return true; });
The more verbose syntax allows you to specify closer to an anonymous function while still being an expression.