Building Dynamic Expressions in C#

Building Dynamic Expressions in C# - c#

I'm trying to build a dynamic query in an application which sends from the front to the back some parameters, and then a expression is built with that parameters.
I basically have two main classes that do all the work. A generic class that concatenate expressions (called PredicateBuilder) and a class that uses it to build the expression itself.
PredicateBuilder
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }
public static Expression<Func<T, bool>> Or<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.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
}
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);
}
}
CustomPredicateBuilder
public class CustomPredicateBuilder
{
public Expression<Func<Ticket, bool>> Build(DtoTicketFilter filter)
{
var expression = PredicateBuilder.True<Ticket>();
// APPROACH 1 - Get an exception
expression = expression.And(a => filter.IncludeWords.Select(b => a.Content.Contains(b)).Aggregate((c, d) => c && d));
// APPROACH 2 - It works
foreach (var word in filter.IncludeWords)
expression = expression.And(x => x.Content.Contains(word.ToLower()));
return expression;
}
}
As I showed I tried 2 approaches, and when I use the first one in a query I get and exception. variable 'b' of type 'System.String' referenced from scope '', but it is not defined.
I just want to know why I get that exception and how to solve it.

Related

Concatenating lambda expressions [duplicate]

I have two expressions of type Expression<Func<T, bool>> and I want to take to OR, AND or NOT of these and get a new expression of the same type
Expression<Func<T, bool>> expr1;
Expression<Func<T, bool>> expr2;
...
//how to do this (the code below will obviously not work)
Expression<Func<T, bool>> andExpression = expr AND expr2

Well, you can use Expression.AndAlso / OrElse etc to combine logical expressions, but the problem is the parameters; are you working with the same ParameterExpression in expr1 and expr2? If so, it is easier:
var body = Expression.AndAlso(expr1.Body, expr2.Body);
var lambda = Expression.Lambda<Func<T,bool>>(body, expr1.Parameters[0]);
This also works well to negate a single operation:
static Expression<Func<T, bool>> Not<T>(
this Expression<Func<T, bool>> expr)
{
return Expression.Lambda<Func<T, bool>>(
Expression.Not(expr.Body), expr.Parameters[0]);
}
Otherwise, depending on the LINQ provider, you might be able to combine them with Invoke:
// OrElse is very similar...
static Expression<Func<T, bool>> AndAlso<T>(
this Expression<Func<T, bool>> left,
Expression<Func<T, bool>> right)
{
var param = Expression.Parameter(typeof(T), "x");
var body = Expression.AndAlso(
Expression.Invoke(left, param),
Expression.Invoke(right, param)
);
var lambda = Expression.Lambda<Func<T, bool>>(body, param);
return lambda;
}
Somewhere, I have got some code that re-writes an expression-tree replacing nodes to remove the need for Invoke, but it is quite lengthy (and I can't remember where I left it...)
Generalized version that picks the simplest route:
static Expression<Func<T, bool>> AndAlso<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
// need to detect whether they use the same
// parameter instance; if not, they need fixing
ParameterExpression param = expr1.Parameters[0];
if (ReferenceEquals(param, expr2.Parameters[0]))
{
// simple version
return Expression.Lambda<Func<T, bool>>(
Expression.AndAlso(expr1.Body, expr2.Body), param);
}
// otherwise, keep expr1 "as is" and invoke expr2
return Expression.Lambda<Func<T, bool>>(
Expression.AndAlso(
expr1.Body,
Expression.Invoke(expr2, param)), param);
}
Starting from .NET 4.0, there is the ExpressionVisitor class which allows you to build expressions that are EF safe.
public static Expression<Func<T, bool>> AndAlso<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var parameter = Expression.Parameter(typeof (T));
var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
var left = leftVisitor.Visit(expr1.Body);
var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
var right = rightVisitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(
Expression.AndAlso(left, right), parameter);
}
private class ReplaceExpressionVisitor
: ExpressionVisitor
{
private readonly Expression _oldValue;
private readonly Expression _newValue;
public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
{
_oldValue = oldValue;
_newValue = newValue;
}
public override Expression Visit(Expression node)
{
if (node == _oldValue)
return _newValue;
return base.Visit(node);
}
}

You can use Expression.AndAlso / OrElse to combine logical expressions, but you have to make sure the ParameterExpressions are the same.
I was having trouble with EF and the PredicateBuilder so I made my own without resorting to Invoke, that I could use like this:
var filterC = filterA.And(filterb);
Source code for my PredicateBuilder:
public static class PredicateBuilder {
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> a, Expression<Func<T, bool>> b) {
ParameterExpression p = a.Parameters[0];
SubstExpressionVisitor visitor = new SubstExpressionVisitor();
visitor.subst[b.Parameters[0]] = p;
Expression body = Expression.AndAlso(a.Body, visitor.Visit(b.Body));
return Expression.Lambda<Func<T, bool>>(body, p);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> a, Expression<Func<T, bool>> b) {
ParameterExpression p = a.Parameters[0];
SubstExpressionVisitor visitor = new SubstExpressionVisitor();
visitor.subst[b.Parameters[0]] = p;
Expression body = Expression.OrElse(a.Body, visitor.Visit(b.Body));
return Expression.Lambda<Func<T, bool>>(body, p);
}
}
And the utility class to substitute the parameters in a lambda:
internal class SubstExpressionVisitor : System.Linq.Expressions.ExpressionVisitor {
public Dictionary<Expression, Expression> subst = new Dictionary<Expression, Expression>();
protected override Expression VisitParameter(ParameterExpression node) {
Expression newValue;
if (subst.TryGetValue(node, out newValue)) {
return newValue;
}
return node;
}
}

If you provider does not support Invoke and you need to combine two expression, you can use an ExpressionVisitor to replace the parameter in the second expression by the parameter in the first expression.
class ParameterUpdateVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ParameterUpdateVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (object.ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
static Expression<Func<T, bool>> UpdateParameter<T>(
Expression<Func<T, bool>> expr,
ParameterExpression newParameter)
{
var visitor = new ParameterUpdateVisitor(expr.Parameters[0], newParameter);
var body = visitor.Visit(expr.Body);
return Expression.Lambda<Func<T, bool>>(body, newParameter);
}
[TestMethod]
public void ExpressionText()
{
string text = "test";
Expression<Func<Coco, bool>> expr1 = p => p.Item1.Contains(text);
Expression<Func<Coco, bool>> expr2 = q => q.Item2.Contains(text);
Expression<Func<Coco, bool>> expr3 = UpdateParameter(expr2, expr1.Parameters[0]);
var expr4 = Expression.Lambda<Func<Recording, bool>>(
Expression.OrElse(expr1.Body, expr3.Body), expr1.Parameters[0]);
var func = expr4.Compile();
Assert.IsTrue(func(new Coco { Item1 = "caca", Item2 = "test pipi" }));
}

Nothing new here but married this answer with this answer and slightly refactored it so that even I understand what's going on:
public static class ExpressionExtensions
{
public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
ParameterExpression parameter1 = expr1.Parameters[0];
var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
var body2WithParam1 = visitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, body2WithParam1), parameter1);
}
private class ReplaceParameterVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
}

I combined some beautiful answers here to make it possible to easily support more Expression operators.
This is based on the answer of #Dejan but now it's quite easy to add the OR as well. I chose not to make the Combine function public, but you could do that to be even more flexible.
public static class ExpressionExtensions
{
public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> leftExpression,
Expression<Func<T, bool>> rightExpression) =>
Combine(leftExpression, rightExpression, Expression.AndAlso);
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> leftExpression,
Expression<Func<T, bool>> rightExpression) =>
Combine(leftExpression, rightExpression, Expression.Or);
public static Expression<Func<T, bool>> Combine<T>(Expression<Func<T, bool>> leftExpression, Expression<Func<T, bool>> rightExpression, Func<Expression, Expression, BinaryExpression> combineOperator)
{
var leftParameter = leftExpression.Parameters[0];
var rightParameter = rightExpression.Parameters[0];
var visitor = new ReplaceParameterVisitor(rightParameter, leftParameter);
var leftBody = leftExpression.Body;
var rightBody = visitor.Visit(rightExpression.Body);
return Expression.Lambda<Func<T, bool>>(combineOperator(leftBody, rightBody), leftParameter);
}
private class ReplaceParameterVisitor : ExpressionVisitor
{
private readonly ParameterExpression _oldParameter;
private readonly ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
return ReferenceEquals(node, _oldParameter) ? _newParameter : base.VisitParameter(node);
}
}
}
Usage is not changed and still like this:
Expression<Func<Result, bool>> noFilterExpression = item => filters == null;
Expression<Func<Result, bool>> laptopFilterExpression = item => item.x == ...
Expression<Func<Result, bool>> dateFilterExpression = item => item.y == ...
var combinedFilterExpression = noFilterExpression.Or(laptopFilterExpression.AndAlso(dateFilterExpression));
efQuery.Where(combinedFilterExpression);
(This is an example based on my actual code, but read it as pseudo-code)

I needed to achieve the same results, but using something more generic (as the type was not known). Thanks to marc's answer I finally figured out what I was trying to achieve:
public static LambdaExpression CombineOr(Type sourceType, LambdaExpression exp, LambdaExpression newExp)
{
var parameter = Expression.Parameter(sourceType);
var leftVisitor = new ReplaceExpressionVisitor(exp.Parameters[0], parameter);
var left = leftVisitor.Visit(exp.Body);
var rightVisitor = new ReplaceExpressionVisitor(newExp.Parameters[0], parameter);
var right = rightVisitor.Visit(newExp.Body);
var delegateType = typeof(Func<,>).MakeGenericType(sourceType, typeof(bool));
return Expression.Lambda(delegateType, Expression.Or(left, right), parameter);
}

I suggest one more improvement to PredicateBuilder and ExpressionVisitor solutions. I called it UnifyParametersByName and you can find it in MIT library of mine: LinqExprHelper. It allows for combining arbitary lambda expressions. Usually the questions are asked about predicate expression, but this idea extends to projection expressions as well.
The following code employs a method ExprAdres which creates a complicated parametrized expression, using inline lambda. This complicated expression is coded only once, and then reused, thanks to the LinqExprHelper mini-library.
public IQueryable<UbezpExt> UbezpFull
{
get
{
System.Linq.Expressions.Expression<
Func<UBEZPIECZONY, UBEZP_ADRES, UBEZP_ADRES, UbezpExt>> expr =
(u, parAdrM, parAdrZ) => new UbezpExt
{
Ub = u,
AdrM = parAdrM,
AdrZ = parAdrZ,
};
// From here an expression builder ExprAdres is called.
var expr2 = expr
.ReplacePar("parAdrM", ExprAdres("M").Body)
.ReplacePar("parAdrZ", ExprAdres("Z").Body);
return UBEZPIECZONY.Select((Expression<Func<UBEZPIECZONY, UbezpExt>>)expr2);
}
}
And this is the subexpression building code:
public static Expression<Func<UBEZPIECZONY, UBEZP_ADRES>> ExprAdres(string sTyp)
{
return u => u.UBEZP_ADRES.Where(a => a.TYP_ADRESU == sTyp)
.OrderByDescending(a => a.DATAOD).FirstOrDefault();
}
What I tried to achieve was to perform parametrized queries without need to copy-paste and with ability to use inline lambdas, which are so pretty. Without all these helper-expression stuff, I would be forced to create whole query in one go.

using System;
using System.Linq.Expressions;
namespace Extensions
{
public class Example
{
//How to use it
public static void Main()
{
Expression<Func<string, bool>> expression1 = exp => true;
Expression<Func<string, bool>> expression2 = exp => false;
Expression<Func<string, bool>> expression3 = ExpressionExtensions.AndAlso(expression1, expression2);
Expression<Func<string, bool>> expression4 = ExpressionExtensions.OrElse(expression1, expression2);
Expression<Func<string, bool>> expression = ExpressionExtensions.AndAlso(expression3, expression4);
}
}
public static class ExpressionExtensions
{
public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
ParameterExpression parameter1 = expr1.Parameters[0];
var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
var body2WithParam1 = visitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, body2WithParam1), parameter1);
}
public static Expression<Func<T, bool>> OrElse<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
ParameterExpression parameter1 = expr1.Parameters[0];
var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
var body2WithParam1 = visitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(Expression.OrElse(expr1.Body, body2WithParam1), parameter1);
}
private class ReplaceParameterVisitor : ExpressionVisitor
{
private readonly ParameterExpression _oldParameter;
private readonly ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
}
}

One more solution (with no ExpressionVisitor) via one method and small enum to identity expression operation type
private static Expression<Func<T, bool>> BindExpressions<T>(ExpressionOperationType operationType, Expression<Func<T, bool>>[] expressionPredicates)
{
var filterExpressionPredicate = expressionPredicates.FirstOrDefault() ?? (x => false);
if (expressionPredicates.Length > 1)
for (int i = 1; i < expressionPredicates.Length; i++)
{
var expressionBody = Expression.Invoke(expressionPredicates[i], filterExpressionPredicate?.Parameters);
var handledExpressionUnits = operationType switch
{
ExpressionOperationType.AndAlso => Expression.AndAlso(filterExpressionPredicate.Body, expressionBody),
_ => Expression.OrElse(filterExpressionPredicate.Body, expressionBody),
};
filterExpressionPredicate = Expression.Lambda<Func<T, bool>>(handledExpressionUnits, filterExpressionPredicate.Parameters);
}
return filterExpressionPredicate;
}
enum ExpressionOperationType
{
AndAlso = 0,
OrElse = 1
}
For instance:
We have a model AuditLog
public class AuditLog
{
public Guid Id { get; set; }
public string OldValues { get; set; }
public string NewValues { get; set; }
public DateTime Timestamp { get; set; }
}
And we want to build specific query: search all audit records with key words "oranges", "cars", "birds" within the boundaries of date (Timestamp)
public IQueryable<AuditLog> BuildQuery()
{
var query = _context.AuditLogs.AsNoTracking();
var commonFilterList = new List<Expression<Func<AuditLog, bool>>>();
commonFilterList.Add(x => x.Timestamp >= DateTime.Now);
commonFilterList.Add(x => x.Timestamp <= DateTime.Now.AddDays(1));
//real world such simple filter case I would use way like:
//query = query
// .Where(x => x.Timestamp >= DateTime.Now)
// .Where(x => x.Timestamp <= DateTime.Now.AddDays(1));
//but this point we keep the example
//using AndAlso
query = query.Where(BindExpressions(ExpressionOperationType.AndAlso, commonFilterList.ToArray()));
//at this point we look at more useful example of using BindExpressions implementation via OrElse expression operation type
var specificFilterList = new List<Expression<Func<AuditLog, bool>>>();
var keyWordsToSearch = new List<string>() { "oranges", "cars", "birds" };
foreach (var keyWord in keyWordsToSearch)
{
//real world we would to use EF.Functions.Contains / EF.Functions.FreeText statements <- but it is question another scope
specificFilterList.Add(x => EF.Functions.Like(x.NewValues, $"%{keyWord}%"));
specificFilterList.Add(x => EF.Functions.Like(x.OldValues, $"%{keyWord}%"));
}
//using OrElse
query = query.Where(BindExpressions(ExpressionOperationType.OrElse, specificFilterList.ToArray()));
//as result we get commonFilterList AND specificFilterList
return query;
}

I think this works fine, isn't it ?
Func<T, bool> expr1 = (x => x.Att1 == "a");
Func<T, bool> expr2 = (x => x.Att2 == "b");
Func<T, bool> expr1ANDexpr2 = (x => expr1(x) && expr2(x));
Func<T, bool> expr1ORexpr2 = (x => expr1(x) || expr2(x));
Func<T, bool> NOTexpr1 = (x => !expr1(x));

'The parameter 'f' was not bound in the specified LINQ to Entities query expression'

Using EF 6. Can someone tell me what am I doing wrong?
public List<SearchResult> SearchDocuments(List<SearchCriteria> searchCriterias)
{
List<SearchResult> results = new List<SearchResult>();
var fieldSettings = LoadCoordinateSystemFieldSettings(searchCriterias);
using (var context = CreateContext())
{
var tractsQuery = PredicateBuilder.False<v_UploadByTract_ActiveUploads>();
foreach (var searchCriteria in searchCriterias)
{
var queryBuilder = PredicateBuilder.True<v_UploadByTract_ActiveUploads>();
// tractsQuery
if (searchCriteria.StateAPI.HasValue)
queryBuilder = queryBuilder.And(a => a.StateAPI == searchCriteria.StateAPI.Value);
if (searchCriteria.CountyAPI.HasValue)
queryBuilder = queryBuilder.And(a => a.CountyAPI == searchCriteria.CountyAPI.Value);
// ...
// ... many more similar IF-ANDs
// ...
tractsQuery = tractsQuery.Or(queryBuilder);
}
var searchQuery = context.v_UploadByTract_ActiveUploads.AsExpandable().Where(tractsQuery).ToList();
//.Select(a => SearchResult.Create(a));
//results.AddRange(searchQuery.ToList());
}
return results;
}
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }
public static Expression<Func<T, bool>> Or<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.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
}
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);
}
}

quetzalcoatl thanks for the answer. The other post did it. From else where I had come across AsExpandable() and overlooked this one thinking it's talking about the same. After reading your post and re-reading the discussion, this worked. Since your post shows up as "comment", I am unable to mark that as the answer.
Any suggestions on how to consolidate those nasty IFs.
C# PredicateBuilder Entities: The parameter 'f' was not bound in the specified LINQ to Entities query expression

For anyone else googling into here, my solution was to Visit a modified expression before returning it into the expression tree. This bound the lambda parameter.

Merge two linq expressions

I have two expressions that are built out at separate times, but need to be merged in order to get an accurate 'grouping' of a where clause. I did try this option, but I am using Entity Framework and it doesn't understand the Invoke function. I have seen some of the ExpressionVisitor alternative but I do not think I have a good enough understanding of what I need to do.
If anyone could please point me in the right direction I would much appreciate it, it feels like it is close to there.
Where Clause 1A (object type Expression<Func<T, bool>>)
{parm => parm.Name.Contains("hat")}
Where Clause 1B (object type LambdaExpression, but can use (Expression<Func<T, bool>>)LambdaExpression )
{parm => parm.Attributes.Any(parm => ((parm.Name == "test") AndAlso (parm.Value == "21")))}
Needed Where Clause
{parm =>
parm.Name.Contains("hat") (&&/||)
parm.Attributes.Any(parm => ((parm.Name == "test") AndAlso (parm.Value == "21")))
}
If someone could please help me merge Where Clause 1A and Where Clause 1B, I would be very thankful..
Just an FYI Where() and Any() clause are generic methods obtained from IQueryable, not sure if that matters.
Func<MethodInfo, bool> methodLambda = m => m.Name == "Any" && m.GetParameters().Length == 2;
MethodInfo method = typeof(Queryable).GetMethods().Where(methodLambda).Single().MakeGenericMethod(ActualQueryableType);
ParameterExpression parentMember = Expression.Parameter(typeof(T), "parentParm");
// Create Any() Expression tree and convert it to lambda
MethodCallExpression callAny = Expression.Call(method, member, exp);
LambdaExpression lambdaAny = Expression.Lambda(callAny, param);
var combExp = parentExp.And((Expression<Func<T, bool>>)lambdaAny);
MethodCallExpression whereCall = Expression.Call(typeof(Queryable), "Where", new Type[] { query.ElementType }, new Expression[] {
query.Expression,
Expression.Quote(combExp)
});
query = (IQueryable<T>)query.Provider.CreateQuery(whereCall);
The error I get when using Invoke is:
NotSupportedException
The LINQ expression node type 'Invoke' is not supported in LINQ to Entities.

Here's an implementation of PredicateBuilder that doesn't use Invoke:
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }
public static Expression<Func<T, bool>> Or<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var secondBody = expr2.Body.Replace(expr2.Parameters[0], expr1.Parameters[0]);
return Expression.Lambda<Func<T, bool>>
(Expression.OrElse(expr1.Body, secondBody), expr1.Parameters);
}
public static Expression<Func<T, bool>> And<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var secondBody = expr2.Body.Replace(expr2.Parameters[0], expr1.Parameters[0]);
return Expression.Lambda<Func<T, bool>>
(Expression.AndAlso(expr1.Body, secondBody), expr1.Parameters);
}
}
It instead uses a Replace method (implementation below) that replaces all instances of one expression with another.
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);
}
}
Using this you can now use And to AND together two predicate expressions that take the same input.

LINQ to SQL complex query

I have the following object
public Cell {
public string CategoryId {get; set;}
public DateTime FromTime {get; set}
public DateTime ToTime {get; set}
}
I have a DB table that is called Item that looks like this:
Item
-------------------------
CategoryId LastUpdated
Now in the code I have a list of Cell List<Cell> ToBeFetchedFromDB that contains more than one Cell, suppose the list contains Foo and Bar, I want to dynamically build a query like this BY INTERATING THROUGH THE COLLECTION ToBeFetchedFromDB WITHIN MY LINQ TO SQL QUERY instead of statically constructing the query:
from x in Item
where x.CategoryId == Foo.CategoryId && Foo.FromTime < x.LastUpdated < Foo.ToTime
|| x.CategoryId == Bar.CategoryId && Bar.FromTime < x.LastUpdated < Bar.ToTime
select x
I have been trying but can't figure it out :(
Thanks guys!

Using PredicateBuilder you can go through each item in your list using a foreach and add a new option for your filter:
var filter = PredicateBuilder.False<Item>();
foreach (var cell in ToBeFetchedFromDB)
{
filter = PredicateBuilder.Or(filter, item =>
item.CategoryId == cell.CategoryId &&
cell.FromTime < item.LastUpdated &&
item.LastUpdated < cell.ToTime);
}
var query = Item.Where(filter);
A copy of PredicateBuilder from the link:
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }
public static Expression<Func<T, bool>> Or<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.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
}
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);
}
}

Combining two expressions (Expression<Func<T, bool>>)

I have two expressions of type Expression<Func<T, bool>> and I want to take to OR, AND or NOT of these and get a new expression of the same type
Expression<Func<T, bool>> expr1;
Expression<Func<T, bool>> expr2;
...
//how to do this (the code below will obviously not work)
Expression<Func<T, bool>> andExpression = expr AND expr2

Well, you can use Expression.AndAlso / OrElse etc to combine logical expressions, but the problem is the parameters; are you working with the same ParameterExpression in expr1 and expr2? If so, it is easier:
var body = Expression.AndAlso(expr1.Body, expr2.Body);
var lambda = Expression.Lambda<Func<T,bool>>(body, expr1.Parameters[0]);
This also works well to negate a single operation:
static Expression<Func<T, bool>> Not<T>(
this Expression<Func<T, bool>> expr)
{
return Expression.Lambda<Func<T, bool>>(
Expression.Not(expr.Body), expr.Parameters[0]);
}
Otherwise, depending on the LINQ provider, you might be able to combine them with Invoke:
// OrElse is very similar...
static Expression<Func<T, bool>> AndAlso<T>(
this Expression<Func<T, bool>> left,
Expression<Func<T, bool>> right)
{
var param = Expression.Parameter(typeof(T), "x");
var body = Expression.AndAlso(
Expression.Invoke(left, param),
Expression.Invoke(right, param)
);
var lambda = Expression.Lambda<Func<T, bool>>(body, param);
return lambda;
}
Somewhere, I have got some code that re-writes an expression-tree replacing nodes to remove the need for Invoke, but it is quite lengthy (and I can't remember where I left it...)
Generalized version that picks the simplest route:
static Expression<Func<T, bool>> AndAlso<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
// need to detect whether they use the same
// parameter instance; if not, they need fixing
ParameterExpression param = expr1.Parameters[0];
if (ReferenceEquals(param, expr2.Parameters[0]))
{
// simple version
return Expression.Lambda<Func<T, bool>>(
Expression.AndAlso(expr1.Body, expr2.Body), param);
}
// otherwise, keep expr1 "as is" and invoke expr2
return Expression.Lambda<Func<T, bool>>(
Expression.AndAlso(
expr1.Body,
Expression.Invoke(expr2, param)), param);
}
Starting from .NET 4.0, there is the ExpressionVisitor class which allows you to build expressions that are EF safe.
public static Expression<Func<T, bool>> AndAlso<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var parameter = Expression.Parameter(typeof (T));
var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
var left = leftVisitor.Visit(expr1.Body);
var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
var right = rightVisitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(
Expression.AndAlso(left, right), parameter);
}
private class ReplaceExpressionVisitor
: ExpressionVisitor
{
private readonly Expression _oldValue;
private readonly Expression _newValue;
public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
{
_oldValue = oldValue;
_newValue = newValue;
}
public override Expression Visit(Expression node)
{
if (node == _oldValue)
return _newValue;
return base.Visit(node);
}
}

You can use Expression.AndAlso / OrElse to combine logical expressions, but you have to make sure the ParameterExpressions are the same.
I was having trouble with EF and the PredicateBuilder so I made my own without resorting to Invoke, that I could use like this:
var filterC = filterA.And(filterb);
Source code for my PredicateBuilder:
public static class PredicateBuilder {
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> a, Expression<Func<T, bool>> b) {
ParameterExpression p = a.Parameters[0];
SubstExpressionVisitor visitor = new SubstExpressionVisitor();
visitor.subst[b.Parameters[0]] = p;
Expression body = Expression.AndAlso(a.Body, visitor.Visit(b.Body));
return Expression.Lambda<Func<T, bool>>(body, p);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> a, Expression<Func<T, bool>> b) {
ParameterExpression p = a.Parameters[0];
SubstExpressionVisitor visitor = new SubstExpressionVisitor();
visitor.subst[b.Parameters[0]] = p;
Expression body = Expression.OrElse(a.Body, visitor.Visit(b.Body));
return Expression.Lambda<Func<T, bool>>(body, p);
}
}
And the utility class to substitute the parameters in a lambda:
internal class SubstExpressionVisitor : System.Linq.Expressions.ExpressionVisitor {
public Dictionary<Expression, Expression> subst = new Dictionary<Expression, Expression>();
protected override Expression VisitParameter(ParameterExpression node) {
Expression newValue;
if (subst.TryGetValue(node, out newValue)) {
return newValue;
}
return node;
}
}

If you provider does not support Invoke and you need to combine two expression, you can use an ExpressionVisitor to replace the parameter in the second expression by the parameter in the first expression.
class ParameterUpdateVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ParameterUpdateVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (object.ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
static Expression<Func<T, bool>> UpdateParameter<T>(
Expression<Func<T, bool>> expr,
ParameterExpression newParameter)
{
var visitor = new ParameterUpdateVisitor(expr.Parameters[0], newParameter);
var body = visitor.Visit(expr.Body);
return Expression.Lambda<Func<T, bool>>(body, newParameter);
}
[TestMethod]
public void ExpressionText()
{
string text = "test";
Expression<Func<Coco, bool>> expr1 = p => p.Item1.Contains(text);
Expression<Func<Coco, bool>> expr2 = q => q.Item2.Contains(text);
Expression<Func<Coco, bool>> expr3 = UpdateParameter(expr2, expr1.Parameters[0]);
var expr4 = Expression.Lambda<Func<Recording, bool>>(
Expression.OrElse(expr1.Body, expr3.Body), expr1.Parameters[0]);
var func = expr4.Compile();
Assert.IsTrue(func(new Coco { Item1 = "caca", Item2 = "test pipi" }));
}

Nothing new here but married this answer with this answer and slightly refactored it so that even I understand what's going on:
public static class ExpressionExtensions
{
public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
ParameterExpression parameter1 = expr1.Parameters[0];
var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
var body2WithParam1 = visitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, body2WithParam1), parameter1);
}
private class ReplaceParameterVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
}

I combined some beautiful answers here to make it possible to easily support more Expression operators.
This is based on the answer of #Dejan but now it's quite easy to add the OR as well. I chose not to make the Combine function public, but you could do that to be even more flexible.
public static class ExpressionExtensions
{
public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> leftExpression,
Expression<Func<T, bool>> rightExpression) =>
Combine(leftExpression, rightExpression, Expression.AndAlso);
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> leftExpression,
Expression<Func<T, bool>> rightExpression) =>
Combine(leftExpression, rightExpression, Expression.Or);
public static Expression<Func<T, bool>> Combine<T>(Expression<Func<T, bool>> leftExpression, Expression<Func<T, bool>> rightExpression, Func<Expression, Expression, BinaryExpression> combineOperator)
{
var leftParameter = leftExpression.Parameters[0];
var rightParameter = rightExpression.Parameters[0];
var visitor = new ReplaceParameterVisitor(rightParameter, leftParameter);
var leftBody = leftExpression.Body;
var rightBody = visitor.Visit(rightExpression.Body);
return Expression.Lambda<Func<T, bool>>(combineOperator(leftBody, rightBody), leftParameter);
}
private class ReplaceParameterVisitor : ExpressionVisitor
{
private readonly ParameterExpression _oldParameter;
private readonly ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
return ReferenceEquals(node, _oldParameter) ? _newParameter : base.VisitParameter(node);
}
}
}
Usage is not changed and still like this:
Expression<Func<Result, bool>> noFilterExpression = item => filters == null;
Expression<Func<Result, bool>> laptopFilterExpression = item => item.x == ...
Expression<Func<Result, bool>> dateFilterExpression = item => item.y == ...
var combinedFilterExpression = noFilterExpression.Or(laptopFilterExpression.AndAlso(dateFilterExpression));
efQuery.Where(combinedFilterExpression);
(This is an example based on my actual code, but read it as pseudo-code)

I needed to achieve the same results, but using something more generic (as the type was not known). Thanks to marc's answer I finally figured out what I was trying to achieve:
public static LambdaExpression CombineOr(Type sourceType, LambdaExpression exp, LambdaExpression newExp)
{
var parameter = Expression.Parameter(sourceType);
var leftVisitor = new ReplaceExpressionVisitor(exp.Parameters[0], parameter);
var left = leftVisitor.Visit(exp.Body);
var rightVisitor = new ReplaceExpressionVisitor(newExp.Parameters[0], parameter);
var right = rightVisitor.Visit(newExp.Body);
var delegateType = typeof(Func<,>).MakeGenericType(sourceType, typeof(bool));
return Expression.Lambda(delegateType, Expression.Or(left, right), parameter);
}

I suggest one more improvement to PredicateBuilder and ExpressionVisitor solutions. I called it UnifyParametersByName and you can find it in MIT library of mine: LinqExprHelper. It allows for combining arbitary lambda expressions. Usually the questions are asked about predicate expression, but this idea extends to projection expressions as well.
The following code employs a method ExprAdres which creates a complicated parametrized expression, using inline lambda. This complicated expression is coded only once, and then reused, thanks to the LinqExprHelper mini-library.
public IQueryable<UbezpExt> UbezpFull
{
get
{
System.Linq.Expressions.Expression<
Func<UBEZPIECZONY, UBEZP_ADRES, UBEZP_ADRES, UbezpExt>> expr =
(u, parAdrM, parAdrZ) => new UbezpExt
{
Ub = u,
AdrM = parAdrM,
AdrZ = parAdrZ,
};
// From here an expression builder ExprAdres is called.
var expr2 = expr
.ReplacePar("parAdrM", ExprAdres("M").Body)
.ReplacePar("parAdrZ", ExprAdres("Z").Body);
return UBEZPIECZONY.Select((Expression<Func<UBEZPIECZONY, UbezpExt>>)expr2);
}
}
And this is the subexpression building code:
public static Expression<Func<UBEZPIECZONY, UBEZP_ADRES>> ExprAdres(string sTyp)
{
return u => u.UBEZP_ADRES.Where(a => a.TYP_ADRESU == sTyp)
.OrderByDescending(a => a.DATAOD).FirstOrDefault();
}
What I tried to achieve was to perform parametrized queries without need to copy-paste and with ability to use inline lambdas, which are so pretty. Without all these helper-expression stuff, I would be forced to create whole query in one go.

using System;
using System.Linq.Expressions;
namespace Extensions
{
public class Example
{
//How to use it
public static void Main()
{
Expression<Func<string, bool>> expression1 = exp => true;
Expression<Func<string, bool>> expression2 = exp => false;
Expression<Func<string, bool>> expression3 = ExpressionExtensions.AndAlso(expression1, expression2);
Expression<Func<string, bool>> expression4 = ExpressionExtensions.OrElse(expression1, expression2);
Expression<Func<string, bool>> expression = ExpressionExtensions.AndAlso(expression3, expression4);
}
}
public static class ExpressionExtensions
{
public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
ParameterExpression parameter1 = expr1.Parameters[0];
var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
var body2WithParam1 = visitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, body2WithParam1), parameter1);
}
public static Expression<Func<T, bool>> OrElse<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
ParameterExpression parameter1 = expr1.Parameters[0];
var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
var body2WithParam1 = visitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(Expression.OrElse(expr1.Body, body2WithParam1), parameter1);
}
private class ReplaceParameterVisitor : ExpressionVisitor
{
private readonly ParameterExpression _oldParameter;
private readonly ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
}
}

One more solution (with no ExpressionVisitor) via one method and small enum to identity expression operation type
private static Expression<Func<T, bool>> BindExpressions<T>(ExpressionOperationType operationType, Expression<Func<T, bool>>[] expressionPredicates)
{
var filterExpressionPredicate = expressionPredicates.FirstOrDefault() ?? (x => false);
if (expressionPredicates.Length > 1)
for (int i = 1; i < expressionPredicates.Length; i++)
{
var expressionBody = Expression.Invoke(expressionPredicates[i], filterExpressionPredicate?.Parameters);
var handledExpressionUnits = operationType switch
{
ExpressionOperationType.AndAlso => Expression.AndAlso(filterExpressionPredicate.Body, expressionBody),
_ => Expression.OrElse(filterExpressionPredicate.Body, expressionBody),
};
filterExpressionPredicate = Expression.Lambda<Func<T, bool>>(handledExpressionUnits, filterExpressionPredicate.Parameters);
}
return filterExpressionPredicate;
}
enum ExpressionOperationType
{
AndAlso = 0,
OrElse = 1
}
For instance:
We have a model AuditLog
public class AuditLog
{
public Guid Id { get; set; }
public string OldValues { get; set; }
public string NewValues { get; set; }
public DateTime Timestamp { get; set; }
}
And we want to build specific query: search all audit records with key words "oranges", "cars", "birds" within the boundaries of date (Timestamp)
public IQueryable<AuditLog> BuildQuery()
{
var query = _context.AuditLogs.AsNoTracking();
var commonFilterList = new List<Expression<Func<AuditLog, bool>>>();
commonFilterList.Add(x => x.Timestamp >= DateTime.Now);
commonFilterList.Add(x => x.Timestamp <= DateTime.Now.AddDays(1));
//real world such simple filter case I would use way like:
//query = query
// .Where(x => x.Timestamp >= DateTime.Now)
// .Where(x => x.Timestamp <= DateTime.Now.AddDays(1));
//but this point we keep the example
//using AndAlso
query = query.Where(BindExpressions(ExpressionOperationType.AndAlso, commonFilterList.ToArray()));
//at this point we look at more useful example of using BindExpressions implementation via OrElse expression operation type
var specificFilterList = new List<Expression<Func<AuditLog, bool>>>();
var keyWordsToSearch = new List<string>() { "oranges", "cars", "birds" };
foreach (var keyWord in keyWordsToSearch)
{
//real world we would to use EF.Functions.Contains / EF.Functions.FreeText statements <- but it is question another scope
specificFilterList.Add(x => EF.Functions.Like(x.NewValues, $"%{keyWord}%"));
specificFilterList.Add(x => EF.Functions.Like(x.OldValues, $"%{keyWord}%"));
}
//using OrElse
query = query.Where(BindExpressions(ExpressionOperationType.OrElse, specificFilterList.ToArray()));
//as result we get commonFilterList AND specificFilterList
return query;
}

I think this works fine, isn't it ?
Func<T, bool> expr1 = (x => x.Att1 == "a");
Func<T, bool> expr2 = (x => x.Att2 == "b");
Func<T, bool> expr1ANDexpr2 = (x => expr1(x) && expr2(x));
Func<T, bool> expr1ORexpr2 = (x => expr1(x) || expr2(x));
Func<T, bool> NOTexpr1 = (x => !expr1(x));

We Keep Coding

C# (C-Sharp) is a programming language developed by Microsoft that runs on the .NET Framework.

Building Dynamic Expressions in C# - c#

Related

Concatenating lambda expressions [duplicate]

'The parameter 'f' was not bound in the specified LINQ to Entities query expression'

Merge two linq expressions

LINQ to SQL complex query

Combining two expressions (Expression<Func<T, bool>>)

Categories

Resources