Develop Reference

Merge two expressions of same type (without bool)

I have a method which takes expression Expression<Func<TFoo, string>> exp.
I can pass single expression like this
MyMethod(o => o.SomeStringProperty);
But now i want to combine expressions (from two string properties) and pass in this method
Every other example I found is of Expression<Func<Foo, bool>>.
I tried
Expression<Func<TFoo, string>> fn1 = x => x.SomeStringProperty1;
Expression<Func<TFoo, string>> fn2 = x => x.SomeStringProperty2;
var body = Expression.Coalesce(fn1.Body, fn2.Body);
var lambda = Expression.Lambda<Func<TFoo, string>>(body, fn1.Parameters[0]);
but almost every function of Expression throws exception. How to do this combine?

When combining lambda expressions, you should make sure they are bound to the one and the same parameter instances used in the resulting expression.
It can be achieved in two ways.
The first is to use Expression.Invoke method:
var body = Expression.Coalesce(fn1.Body, Expression.Invoke(fn2, fn1.Parameters[0]));
var lambda = Expression.Lambda<Func<TFoo, string>>(body, fn1.Parameters[0]);
This is the simplest method, but creates unsupported expression for Entity Framework and similar which do not support invocation expressions.
The second approach uses a simple parameter replacer helper to rebind the second lambda expression body to the first lambda expression parameter:
public static class ExpressionUtils
{
public static Expression ReplaceParameter(this Expression expression, ParameterExpression source, Expression target)
{
return new ParameterReplacer { Source = source, Target = target }.Visit(expression);
}
class ParameterReplacer : ExpressionVisitor
{
public ParameterExpression Source;
public Expression Target;
protected override Expression VisitParameter(ParameterExpression node)
{
return node == Source ? Target : base.VisitParameter(node);
}
}
}
like this
var body = Expression.Coalesce(fn1.Body,
fn2.Body.ReplaceParameter(fn2.Parameters[0], fn1.Parameters[0]));
var lambda = Expression.Lambda<Func<TFoo, string>>(body, fn1.Parameters[0]);

Add And statement to Expression<Func<T, bool>>

I have this function:
public List<T> Find(Expression<Func<T, bool>> query)
{
}
Find(x => x.Id == 4);
Inside the method Find I want to chain And Condition.
something like:
query.And(x => x.Secured == false);//Secured is a memeber inside T like Id.

Your problem is that you want to access a member of T within the generic method. T could be anything at this point so the compiler will not let you access Secured since T may not have a Secured member.
You could cast T to dynamic, but this just changes a compile time error to a runtime error (plus it's horrible).
The best way would be to ensure T implements some known interface that has a Secured member.
public List<T> Find(Expression<Func<T, bool>> query) where T : ISecured

The expression must be "opened" and rebuilt, like this:
public List<T> Find<T>(Expression<Func<T, bool>> query)
{
ParameterExpression parameter = query.Parameters[0];
Expression body = query.Body;
MemberExpression property = Expression.Property(parameter, "Secured");
body = Expression.AndAlso(body, Expression.Not(property));
Expression<Func<T, bool>> query2 = Expression.Lambda<Func<T, bool>>(body, parameter);
// Now you can use query2
return null;
}
Note that I'm considering this x.Secured == false to be equivalent to !x.Secured. Clearly Secured could be a strange class that overloads the == operator, but I'll ignore that case.
As suggested by #Ralf, you could even simply do two .Where. like:
public List<T> Find<T>(Expression<Func<T, bool>> query)
{
ParameterExpression parameter = query.Parameters[0];
MemberExpression property = Expression.Property(parameter, "Secured");
Expression<Func<T, bool>> query2 = Expression.Lambda<Func<T, bool>>(Expression.Not(property), parameter);
return context.Set<T>
.Where(query)
.Where(query2)
.ToList();
}
(I'm using as an example context.Set<T>, that is very similar to what you would do if you are using Entity Framework, but in general nearly all the IQuerable<>/IEnumerable<> treat two .Where() like a single .Where() with an && condition)

Something like
Find(x => x.Id == 4 && x.Secured == false);

IQueryable Expression Translation

I am creating an IQueryable that I want to use for a query passed to entity framework. My repository does not expose queryable.
var query = new List<Entity>().AsQueryable().Where(x => x.Property == "argument");
I have a method on my repository that will take in an IQueryable.
How do I query my DbSet with the same queryable? I am trying to extract the expression from the queryable to build a new expression for the dbset. Here is what I have so far but it does not work:
public IDbSet<TEntity> DbSet { get; set; }
public IEnumerable<TEntity> Find(IQueryable<TEntity> queryable)
{
var parameter = Expression.Parameter(typeof (TEntity));
var body = queryable.Expression;
var lambda = Expression.Lambda<Func<TEntity, bool>>(body, parameter);
var result = DbSet.Where(lambda);
return null;
}
The code fails when I try and create the lambda with the following error:
Expression of type 'System.Linq.IQueryable`1[MyTEntity]' cannot be used for return type 'System.Boolean'
I'm clearly not building the expression correctly, what am I missing? Is there an easier way to do what I'm trying to accomplish?
Also I've seen some examples that show an Expression should have a parameters property. But no matter what type of expression type I cast to, and this one is ConstantExpression, I don't see a parameters property from the IQueryable.Expression.

In your case, queryable.Expression represents the whole expression Queryable.Where(constantList, x => x.Property == "argument"). If you want just the Where() lambda, you need to extract it. To do that, you could use code like this:
public IEnumerable<TEntity> Find<TEntity>(IQueryable<TEntity> queryable)
{
var methodCall = queryable.Expression as MethodCallExpression;
Func<IQueryable<TEntity>, Expression<Func<TEntity,Boolean>>, IQueryable<TEntity>> whereDelegate = Queryable.Where;
if (methodCall.Method == whereDelegate.Method
&& methodCall.Arguments[0] is ConstantExpression)
{
var whereLambdaQuote = (UnaryExpression)methodCall.Arguments[1];
var whereLambda = (Expression<Func<TEntity, bool>>)whereLambdaQuote.Operand;
var result = DbSet.Where(whereLambda);
}
return null;
}

Trying to use parent property as parameter in child collection expression; LinqKit throws "Unable to cast MethodCallExpressionN to LambdaExpression"

I'm trying to dynamically construct an expression similar to the one below, where I can use the same comparison function, but where the values being compared can be passed in, since the value is passed from a property 'higher-up' in the query.
var people = People
.Where(p => p.Cars
.Any(c => c.Colour == p.FavouriteColour));
I believe I've constructed the query correctly, but the ExpressionExpander.VisitMethodCall(..) method throws the following exception when I try to use it:
"Unable to cast object of type 'System.Linq.Expressions.InstanceMethodCallExpressionN' to type 'System.Linq.Expressions.LambdaExpression'"
In real-world code, using Entity Framework and actual IQueryable<T>, I often get:
"Unable to cast object of type 'System.Linq.Expressions.MethodCallExpressionN' to type 'System.Linq.Expressions.LambdaExpression'" as well.
I've constructed a LinqPad-friendly example of my problem, as simple as I could make it.
void Main()
{
var tuples = new List<Tuple<String, int>>() {
new Tuple<String, int>("Hello", 4),
new Tuple<String, int>("World", 2),
new Tuple<String, int>("Cheese", 20)
};
var queryableTuples = tuples.AsQueryable();
// For this example, I want to check which of these strings are longer than their accompanying number.
// The expression I want to build needs to use one of the values of the item (the int) in order to construct the expression.
// Basically just want to construct this:
// .Where (x => x.Item1.Length > x.Item2)
var expressionToCheckTuple = BuildExpressionToCheckTuple();
var result = queryableTuples
.AsExpandable()
.Where (t => expressionToCheckTuple.Invoke(t))
.ToList();
}
public Expression<Func<string, bool>> BuildExpressionToCheckStringLength(int minLength) {
return str => str.Length > minLength;
}
public Expression<Func<Tuple<string, int>, bool>> BuildExpressionToCheckTuple() {
// I'm passed something (eg. Tuple) that contains:
// * a value that I need to construct the expression (eg. the 'min length')
// * the value that I will need to invoke the expression (eg. the string)
return tuple => BuildExpressionToCheckStringLength(tuple.Item2 /* the length */).Invoke(tuple.Item1 /* string */);
}
If I'm doing something obviously wrong, I'd really appreciate a nudge in the right direction! Thanks.
Edit: I know that the following would work:
Expression<Func<Tuple<string, int>, bool>> expr = x => x.Item1.Length > x.Item2;
var result = queryableTuples
.AsExpandable()
.Where (t => expr.Invoke(t))
.ToList();
However, I'm trying to separate the comparison from the location of the parameters, since the comparison could be complex and I would like to re-use it for many different queries (each with different locations for the two parameters). It is also intended that one of the parameters (in the example, the 'min length') would actually be calculated via another expression.
Edit: Sorry, I've just realised that some answers will work when attempted against my example code since my example is merely masquerading as an IQueryable<T> but is still a List<T> underneath. The reason I'm using LinqKit in the first place is because an actual IQueryable<T> from an EntityFramework DbContext will invoke Linq-to-SQL and so must be able to be parsed by Linq-to-SQL itself. LinqKit enables this by expanding everything to expressions.
Solution! Thanks to Jean's answer below, I think I've realised where I'm going wrong.
If a value has come from somewhere in the query (i.e. not a value that is known before-hand.) then you must build the reference/expression/variable to it into the expression.
In my original example, I was trying to pass the 'minLength' value taken from within the expression and pass it to a method. That method call could not be done before-hand, since it used a value from the expression, and it could not be done within the expression, since you can't build an expression within an expression.
So, how to get around this? I chose to write my expressions so that they can be invoked with the additional parameters. Though this has the downside that the parameters are no longer 'named' and I could end up with an Expression<Func<int, int, int, int, bool>> or something down the line.
// New signature.
public Expression<Func<string, int, bool>> BuildExpressionToCheckStringLength() {
// Now takes two parameters.
return (str, minLength) => str.Length > minLength;
}
public Expression<Func<Tuple<string, int>, bool>> BuildExpressionToCheckTuple() {
// Construct the expression before-hand.
var expression = BuildExpressionToCheckStringLength();
// Invoke the expression using both values.
return tuple => expression.Invoke(tuple.Item1 /* string */, tuple.Item2 /* the length */);
}

OK, so what you are trying to do (the transformation from a function that takes a single argument, that returns another function that takes a single argument f(x)(y) into a function that takes two arguments f(x, y)) is known as uncurrying. Look it up! :)
Now, the issue that you have in your code is that, in the expression returned by BuildExpressionToCheckTuple, there is a method call to BuildExpressionToCheckStringLength, which is not resolved. And you cannot resolve it because it takes an argument that is embedded in the tuple parameter.
The solution is, instead of using a method call, to use a lambda expression that will be equivalent to that method call.
That is:
public Expression<Func<int, Func<string, bool>>> ExpressionToCheckStringLengthBuilder() {
return minLength =>
str => str.Length > minLength;
}
public Expression<Func<Tuple<string, int>, bool>> BuildExpressionToCheckTuple() {
// I'm passed something (eg. Tuple) that contains:
// * a value that I need to construct the expression (eg. the 'min length')
// * the value that I will need to invoke the expression (eg. the string)
// Putting builder into a variable so that the resulting expression will be
// visible to tools that analyze the expression.
var builder = ExpressionToCheckStringLengthBuilder();
return tuple => builder.Invoke(tuple.Item2 /* the length */).Invoke(tuple.Item1 /* string */);
}

So you are looking for something like this:
public static class Program
{
public class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public static IQueryable<T> WherePropertyEquals<T, TProperty>(
this IQueryable<T> src, Expression<Func<T, TProperty>> property, TProperty value)
{
var result = src.Where(e => property.Invoke(e).Equals(value));
return result;
}
public static IQueryable<T> WhereGreater<T, TProperty>(
this IQueryable<T> src, Expression<Func<T, TProperty>> property, TProperty value)
where TProperty : IComparable<TProperty>
{
var result = src.Where(e => property.Invoke(e).CompareTo(value) > 0);
return result;
}
public static IQueryable<T> WhereGreater<T, TProperty>(
this IQueryable<T> src, Expression<Func<T, TProperty>> left, Expression<Func<T, TProperty>> right)
where TProperty : IComparable<TProperty>
{
var result = src.Where(e => left.Invoke(e).CompareTo(right.Invoke(e)) > 0);
return result;
}
public static void Main()
{
var persons = new List<Person>()
{
new Person
{
FirstName = "Jhon",
LastName = "Smith"
},
new Person
{
FirstName = "Chuck",
LastName = "Norris"
},
new Person
{
FirstName = "Ben",
LastName = "Jenkinson"
},
new Person
{
FirstName = "Barack",
LastName = "Obama"
}
}
.AsQueryable()
.AsExpandable();
var chuck = persons.WherePropertyEquals(p => p.FirstName, "Chuck").First();
var ben = persons.WhereGreater(p => p.LastName.Length, 6).First();
var barack = persons.WhereGreater(p => p.FirstName.Length, p => p.LastName.Length).First();
}

Convert Expression<T, string>> to Expression<T, bool>>

so I want to make filtering work automaticly based on some easy settings. The code I have is this:
public ActionResult Index() // here I want to add filtering for Status I only want to show the active ones
{
IQueryable<Ticket> cases = db.Cases().AsQueryable();
cases = cases.EnablePaging().EnableFilterFor(x => x.Status);
return View(cases);
}
EnableFilterFor looks like this:
public static IQueryable<T> EnableFilterFor<T>(this IQueryable<T> queryable, Expression<Func<T, string>> keySelector)
{
string filterValue= "Active";
//Expression<Func<T, bool>> whereexpresion = keySelector.Compile() == "Active"
queryable = queryable.Where(
//here do the magic !! so that the result will be 'x=>x.Status == filterValue');
);
return queryable;
}
I googled a lot, tried many different things but no success. I somehow have to combine the keySelector and the filterValue to work (I need an Expression for the Where method to work). Any help would be greatly appreciated.
EDIT: After testing both solutions (thank you both!) I found out that Poke had the best one. Poke his code is the only code that doesn't change the way that the SQL is generated. When I took a look at Servy his generated SQL it always did an EXTRA Sql select query and an EXTRA and in the WHERE clause... No idea why :)

IQueryable.Where requires an Expression<Func<T, bool>>, so that will be the thing we need to build. As we want to integrate something from another expression (a Expression<Func<T, string>>), we have to build the expression “by hand”.
So in the end, we want to call LambdaExpression.Lambda<Func<T, bool>>(…) to get our expression for Where, but we need to fill in the expression body:
// first, we reuse the parameter from the `keySelector` expression
ParameterExpression param = keySelector.Parameters[0];
// The body is now just an equality comparison of the `keySelector`
// body, and the constant `filterValue`
Expression body = Expression.Equal(keySelector.Body, Expression.Constant(filterValue));
// now we just need to create a lambda expression for that body with the
// saved parameter and it’s all done:
queryable = queryable.Where(Expression.Lambda<Func<T, bool>>(body, param));

What we'll need here is a Compose method, for expressions. It'll take an expression that uses a value, and another expression that conceptually will use the result of the first expression as its input, generating a new output.
public static Expression<Func<TFirstParam, TResult>>
Compose<TFirstParam, TIntermediate, TResult>(
this Expression<Func<TFirstParam, TIntermediate>> first,
Expression<Func<TIntermediate, TResult>> second)
{
var param = Expression.Parameter(typeof(TFirstParam), "param");
var newFirst = first.Body.Replace(first.Parameters[0], param);
var newSecond = second.Body.Replace(second.Parameters[0], newFirst);
return Expression.Lambda<Func<TFirstParam, TResult>>(newSecond, param);
}
It will require the ability to replace one expression with another, which we can do using the following:
public static Expression Replace(this Expression expression,
Expression searchEx, Expression replaceEx)
{
return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
}
internal 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);
}
}
Now we can write:
public static IQueryable<T> EnableFilterFor<T>(
this IQueryable<T> queryable,
Expression<Func<T, string>> keySelector)
{
string filterValue= "Active";
return queryable.Where(keySelector.Compose(status => status == filterValue));
}