C# in a Nutshell has a free class called PredicateBuilder which constructs LINQ predicates piece by piece available here. Here's an extract of the method which adds a new expression to the predicate. Could someone explain it? (I have seen this question, I don't want a general answer like there. I am looking for a specific explanation of how Expression.Invoke and Expression.Lambda build the new expression).
public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
return Expression.Lambda<Func<T, bool>>
(Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters);
}
Let's say you have:
Expression<Func<Person, bool>> isAdult = p1 => p1.Age >= 18;
// I've given the parameter a different name to allow you to differentiate.
Expression<Func<Person, bool>> isMale = p2 => p2.Gender == "Male";
And then combine them with PredicateBuilder
var isAdultMale = isAdult.And(isMale);
What PredicateBuilder produces is an expression that looks like this:
// Invoke has no direct equivalent in C# lambda expressions.
p1 => p1.Age >= 18 && Invoke(p2 => p2.Gender == "Male", p1)
As you can see:
The resulting lambda reuses the parameters of the first expression.
Has a body that invokes the second expression by passing the parameters of the first expression as a replacement for the second expression's parameters. The resulting InvocationExpression is sort of like the expression-equivalent of a method-call (calling a routine by passing in arguments for parameters).
Ands the first expression's body and this InvocationExpression together to produce the body of the resulting lambda.
The idea is that the LINQ provider should be able to understand the semantics of this operation and take a sensible course of action (e.g. generate SQL like WHERE age >= 18 AND gender = 'Male').
Often though, providers have problems with InvocationExpressions, because of the obvious complications of processing a 'nested expression-call inside an expression.'
To get around this, LINQKit also provides the Expand helper. This essentially 'inlines' the invocation call smartly by replacing the call with the body of the nested expression, substituting uses of the nested expression's parameters appropriately (in this case, replacing p2 with p1). This should produce something like:
p1 => p1.Age >= 18 && p1.Gender == "Male"
Note that this how you would have manually combined those predicates if you'd done it yourself in a lambda. But with LINQKit around, you can get these predicates from independent sources and easily combine them:
Without writing "by hand" expression code.
Optionally, in a way that is transparent to consumers of the resulting lambda.
Related
I'm building a SQL "WHERE" clause dynamically using the System.Linq.Expressions.Expression class. It works well for simple clauses, e.g. to add "PhaseCode = X" clause, I do the following:
var equalTarget = Expression.Constant(phaseCode, typeof(int?));
var phaseEquals = Expression.Equal(Expression.PropertyOrField(projParam, "PhaseCode"), equalTarget);
However, now I'm trying to build an expression that will return the record if a project has been assigned to a particular group. Project and Group has many-to-many relationship.
Without the expression trees, I would do it as follows:
db.Projects.Where(p => .... && p.GroupsAssigned.Any(g => g.ID == groupId))
However, I can't seem to find a way to express that with the Expression class.
There are actually two things I can't figure out:
How to traverse the relationships between tables
How to do x.Any()
Any help is greatly appreciated.
Calling an extension method, like Enumerable.Any or Queryable.Any, is simply a static method call on the sequence and the lambda expression you created for the WHERE clause. You can use Expression.Call to do this:
// for Enumerable.Any<T>(IEnumerable<T>,Predicate<T>)
var overload = typeof(Enumerable).GetMethods("Any")
.Single(mi => mi.GetParameters().Count() == 2);
var call = Expression.Call(
overload,
Expression.PropertyOrField(projParam, "GroupsAssigned"),
anyLambda);
For Queryable.Any<T>, you'll need to roll this up into a method:
static Expression BuildAny<TSource>(Expression<Func<TSource, bool>> predicate)
{
var overload = typeof(Queryable).GetMethods("Any")
.Single(mi => mi.GetParameters().Count() == 2);
var call = Expression.Call(
overload,
Expression.PropertyOrField(projParam, "GroupsAssigned"),
predicate);
return call;
}
Although this seems odd that you're unable to do this through a normal query.
How can I create a property selector for entity framework like this?
public static List<T> StandardSearchAlgorithm<T>(this IQueryable<T> queryable, Func<T, string> property, string query)
{
return queryable.Where(e => property(e).ToLower().IndexOf(query) > -1).ToList();
}
I want the calling code to be able to be clean and simple like this:
var usernameResults = _db.Users.StandardSearchAlgorithm(u => u.Username, query);
I get a "The LINQ expression node type 'Invoke' is not supported in LINQ to Entities." error. I cannot work out how to get the expression built.
UPDATE:
Based on the answer by MBoros here is the code I ended up with. It works great.
The key to expression trees is to understand expression trees are all about breaking up what you normally write in code (like "e => e.Username.IndexOf(query)") into a series of objects: "e" gets its own object, "Username" its own object, "IndexOf()" its own object, the "query" constant its own object, and so on. The second key is to know that you can use a series of static methods on the Expression class to create various kinds of these objects, as shown below.
PropertyInfo pinfo = (PropertyInfo)((MemberExpression)property.Body).Member;
ParameterExpression parameter = Expression.Parameter(typeof(T), "e");
MemberExpression accessor = Expression.Property(parameter, pinfo);
ConstantExpression queryString = Expression.Constant(query, typeof(string));
ConstantExpression minusOne = Expression.Constant(-1, typeof(int));
MethodInfo indexOfInfo = typeof(string).GetMethod("IndexOf", new[] { typeof(string) }); // easiest way to do this
Expression indexOf = Expression.Call(accessor, indexOfInfo, queryString);
Expression expression = Expression.GreaterThan(indexOf, minusOne);
Expression<Func<T, bool>> predicate = Expression.Lambda<Func<T, bool>>(expression, parameter);
//return predicate.Body.ToString(); // returns "e => e.Username.IndexOf(query) > -1" which is exactly what we want.
var results = queryable.Where(predicate).ToList();
return results;
Now I have a real problem, but I will ask it in a separate question. My real query looks like this:
public static List<T> StandardSearchAlgorithm<T>(this IQueryable<T> queryable, Func<T, string> property, string query)
{
return queryable.Where(e => property(e).IndexOf(query) > -1).Select(e=> new { Priority = property(e).IndexOf(query), Entity = e } ).ToList();
}
So I need to build an expression that returns an Anonymous Type!! Or even if I create a class to help, I need to write an expression that returns a new object. But I will include this in a separate question.
You cannot invoke CLR delegates so simply in sql. But you can pass in the property selector as an Expression tree., so your signature would be:
public static List<T> StandardSearchAlgorithm<T>(this IQueryable<T> queryable, Expression<Func<T, string>> property, string query)
Calling would look the same. But now that you have an expression in your hand, you can have a look at this answer:
Pass expression parameter as argument to another expression
It gives you the tools to simply put an expression tree inside another one. In your case it would look like:
Expression<Func<T, bool>> predicate = e => property.AsQuote()(e).Contains(query);
predicate = predicate.ResolveQuotes();
return queryable.Where(predicate).ToList();
Once you are there, you still have the .ToLower().Contains() calls (use .Contains instead of the .IndexOf()> 1). This is actually tricky. Normally the db uses its default collation, so if it set to CI (case insensitive), then it will do the compare that way. If you don't have any constraints, and can adjust the db collation, I would go for that. In this case you can omit the .ToLower() call.
Otherwise check out this anser: https://stackoverflow.com/a/2433217/280562
I have been playing with expression trees this week and I am wondering why this expression produces error when ran.
var pe = Expression.Parameter(typeof(Nullable<DateTime>));
var ex = Expression.Lambda<Func<DateTime?, bool>>(
(Expression<Func<DateTime?, bool>>) (x => x.HasValue), pe);
The idea behind this is to write expression trees with a mix of expression tree api and linq expressions. It would make things easier to write for example instead of calling Expression.Property(...,..) I would just have x => x.Prop, right?
In my example instead of this Expression.Property(..hasvalue..) I would have this: x.HasValue. It would save me time on writing and it would look shorter, right?
The question is, is this possible?
I guess I might be missing something about
Expression<Func<DateTime?, bool>> foo = x => x.HasValue (this works)
and
Func<DateTime?, bool> bar = x => x.HasValue (this works too)
What is happening behind those two? Are they the same?
Can linq expression be mixed with standard expression tree api???
Please enlighten me on this, I feel lost. :)
This is a good question. Your two quotations
Expression<Func<DateTime?, bool>> foo = x => x.HasValue
and
Func<DateTime?, bool> bar = x => x.HasValue
are examples of homoiconicity: the same symbol (in your case x => x.HasValue) stands for two very different objects. In the first case, it indicates an expression tree; in the second, a function. The former can be compiled down to the latter, but they are different types with different purposes. It is the declaration in your case that tells the compiler which version to go for. In the absence of this context, the compiler cannot read your mind and decides to bail out instead. That's why this won't compile:
var bat = x => x.HasValue;
And that is why your statement won't compile.
Homoiconicity is what makes IQueryable and IEnumerable look so similar. When you invoke
var filteredCollection = myCollection.Where(e => e.IsActive);
you are actually calling methods with a different signature depending on the type of filteredCollection (It's Func<MyClass, bool> for IEnumerable and Expression<Func<MyClass, bool>> for IQueryable).
Regarding your specific situation, you can't achieve what you want to do directly, but if you write a sneaky extension method:
public static class ExpressionExtensions
{
public static Expression<Func<T, TProperty>> Lambda<T, TProperty>(this ParameterExpression pe, Expression<Func<T, TProperty>> property)
{
return Expression.Lambda<Func<T, TProperty>>(property, pe);
}
}
then you can do this:
var pe = Expression.Parameter(typeof(DateTime?));
var ex = pe.Lambda<DateTime?, bool>(x => x.HasValue);
Expression<Func<MyObject, string>> fn1 = x => x.PossibleSubPath.MyStringProperty;
Expression<Func<string, bool>> fn2 = x => x.Contains("some literal");
Is there a way to create a new lambda expression which basically uses the output of fn1 and uses it as input for fn2?
Expression<Func<MyObject, bool>> fnCombined = ...
I know that I can create the function at once, but the problem is that I'm making some generic code and therefore really need to be able to create these two functions separately, then combine them in such a way that Linq can use them on my database objects (Entity Framework).
So logically what we want to be able to do is create a new lambda in which it has a parameter of the input to the first function, and a body that calls the first function with that parameter and then passes the result as the parameter to the second function, and then returns that.
We can replicate that easily enough using Expression objects:
public static Expression<Func<T1, T3>> Combine<T1, T2, T3>(
Expression<Func<T1, T2>> first,
Expression<Func<T2, T3>> second)
{
var param = Expression.Parameter(typeof(T1), "param");
var body = Expression.Invoke(second, Expression.Invoke(first, param));
return Expression.Lambda<Func<T1, T3>>(body, param);
}
Sadly, EF and most other query providers won't really know what to do with that and won't function properly. Whenever they hit an Invoke expression they generally just throw an exception of some sort. Some can handle it though. In theory all the information they need is there, if they're written with the robustness to get at it.
What we can do however is, from a conceptual standpoint, replace every instance of the first lambda's parameter in that lambda's body with the parameter of a new lambda we're creating, and then replace all instances of the second lambda's parameter in the second lambda with the new body of the first lambda. Technically, if these expressions have side effects, and these parameters are used more than once, they wouldn't be the same, but as these are going to be parsed by an EF query provider they really shouldn't ever have side effects.
Thanks to David B for providing a link to this related question which provides a ReplaceVisitor implementation. We can use that ReplaceVisitor to go through the entire tree of an expression and replace one expression with another. The implementation of that type is:
class ReplaceVisitor : ExpressionVisitor
{
private readonly Expression from, to;
public ReplaceVisitor(Expression from, Expression to)
{
this.from = from;
this.to = to;
}
public override Expression Visit(Expression node)
{
return node == from ? to : base.Visit(node);
}
}
And now we can write our proper Combine method:
public static Expression<Func<T1, T3>> Combine<T1, T2, T3>(
this Expression<Func<T1, T2>> first,
Expression<Func<T2, T3>> second)
{
var param = Expression.Parameter(typeof(T1), "param");
var newFirst = new ReplaceVisitor(first.Parameters.First(), param)
.Visit(first.Body);
var newSecond = new ReplaceVisitor(second.Parameters.First(), newFirst)
.Visit(second.Body);
return Expression.Lambda<Func<T1, T3>>(newSecond, param);
}
and a simple test case, to just demonstrate what's going on:
Expression<Func<MyObject, string>> fn1 = x => x.PossibleSubPath.MyStringProperty;
Expression<Func<string, bool>> fn2 = x => x.Contains("some literal");
var composite = fn1.Combine(fn2);
Console.WriteLine(composite);
Which will print out:
param => param.PossibleSubPath.MyStringProperty.Contains("some literal")
Which is exactly what we want; a query provider will know how to parse something like that.
I have a query that needs to be reused all over the place and I need to vary which property/column gets used for a join.
What I'd like to be able to do is something like:
query = RestrictByProp(query, x=>x.ID);
An extremely simplified RestrictByProp() could be*:
private static IQueryable<Role> RestrictByProp(IQueryable<Role> query,
Func<Role, int> selector)
{
return query.Where(x => selector(x) == 1);
}
The problem is that even this simple implementation causes a runtime exception:
Method 'System.Object DynamicInvoke(System.Object[])' has no
supported translation to SQL.
**(Here I'm just adding a simple 'where' clause - in my real code I'd be using the lambda to pick which property to use for a join).*
I find this strange because if the member access lambda is done inline it is fine:
private static IQueryable<Role> RestrictByID(IQueryable<Role> query)
{
return query.Where(x=> x.ID == 1);
}
LINQ to SQL is also happy if you pass in an Expression<Func<Role, bool>> (i.e. when the parameter is x=>x.ID == 1) but that defeats the object because I need the value of the right-hand operand to be determined within the query.
Is there a way to somehow munge the lambda expression in RestrictByProp() so that LINQ to SQL knows how to generate the SQL?
First, you need to change your method signature:
private static IQueryable<Role> RestrictByProp(IQueryable<Role> query,
Expression<Func<Role, int>> selector)
That will mean your lambda expression is converted into an expression tree instead of a delegate.
You'll then need to build an Expression<Func<Role, bool>> from the existing expression tree.
It will look something like this:
LambdaExpression lambda = (LambdaExpression) selector;
var predicate = Expression.Equal(selector, Expression.Constant(1));
var lambdaPredicate = Expression.Lambda<Func<Role, bool>>(predicate,
lambda.Parameters);
return query.Where(lambdaPredicate);