问题
I need to create a dynamic linq expression for a dynamic search.The basic search is working but it fails to work with collection. I am able to get the book's title and author but fails to get the required page heading. I get the exception in line "left11 = Expression.Property(page1, "Heading");". I think the expression that i built is unable to recognise the List. How could this be possible? Please see the below code and stacktrace exception.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;
namespace XMLStorageAndFilter
{
public class Books
{
public Books()
{
Page = new List<Page>();
}
public string Title { get; set; }
public Author Author { get; set; }
public List<Page> Page { get; set; }
}
public class Author
{
public string FirstName { get; set; }
}
public class Page
{
public string Heading { get; set; }
}
public class Program2
{
static void Main()
{
Page page = new Page();
page.Heading = "Heading";
Books bok = new Books();
bok.Title = "Title";
bok.Author = new Author() { FirstName = "FirstName" };
bok.Page.Add(page);
List<Books> testList = new List<Books>();
testList.Add(bok);
IQueryable<Books> queryableTestData = testList.AsQueryable<Books>();
ParameterExpression pe11 = Expression.Parameter(typeof(Books), "p");
Expression left11 = Expression.Property(pe11, "Title");
Expression right11 = Expression.Constant("Title");
Expression e11 = Expression.Equal(left11, right11);
var author = Expression.Property(pe11, "Author");
left11 = Expression.Property(author, "FirstName");
right11 = Expression.Constant("FirstName");
Expression e21 = Expression.Equal(left11, right11);
Expression predicateBody11 = Expression.And(e11, e21);
Expression<Func<Books, bool>> condition = Expression.Lambda
<Func<Books, bool>>(predicateBody11, new ParameterExpression[] { pe11 });
var q = queryableTestData.Where(condition);
var page1 = Expression.Property(pe11, "Page");
left11 = Expression.Property(page1, "Heading");
right11 = Expression.Constant("Heading");
Expression e22 = Expression.Equal(left11, right11);
Expression predicateBody12 = Expression.And(e11, e22);
Expression<Func<Books, bool>> condition2 = Expression.Lambda
<Func<Books, bool>>(predicateBody12, new ParameterExpression[] { pe11 });
var qq1 = queryableTestData.Where(condition2);
}
}
}
Exception Message:- {"Instance property 'Heading' is not defined for type >'System.Collections.Generic.List`1[XMLStorageAndFilter.Page]'"}
StackTrace:-
at System.Linq.Expressions.Expression.Property(Expression expression, String propertyName)
at XMLStorageAndFilter.Program2.Main() in c:\Users\Administrator\Documents\Visual Studio 2013\Projects\XMLStorageAndFilter\NavProperty.cs:line 61
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
回答1:
You can use the method described here.
You would need to cast the result of the method to Expression<Func<T,bool>>. T being your type.
I will provide an complete example when i get home.
Edit:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Collections;
using System.Reflection;
namespace ExpressionPredicateBuilder
{
public enum OperatorComparer
{
Contains,
StartsWith,
EndsWith,
Equals = ExpressionType.Equal,
GreaterThan = ExpressionType.GreaterThan,
GreaterThanOrEqual = ExpressionType.GreaterThanOrEqual,
LessThan = ExpressionType.LessThan,
LessThanOrEqual = ExpressionType.LessThan,
NotEqual = ExpressionType.NotEqual
}
public class ExpressionBuilder
{
public static Expression<Func<T,bool>> BuildPredicate<T>(object value, OperatorComparer comparer, params string[] properties)
{
var parameterExpression = Expression.Parameter(typeof(T), typeof(T).Name);
return (Expression<Func<T, bool>>)BuildNavigationExpression(parameterExpression, comparer, value, properties);
}
private static Expression BuildNavigationExpression(Expression parameter, OperatorComparer comparer, object value, params string[] properties)
{
Expression resultExpression = null;
Expression childParameter, predicate;
Type childType = null;
if (properties.Count() > 1)
{
//build path
parameter = Expression.Property(parameter, properties[0]);
var isCollection = typeof(IEnumerable).IsAssignableFrom(parameter.Type);
//if it´s a collection we later need to use the predicate in the methodexpressioncall
if (isCollection)
{
childType = parameter.Type.GetGenericArguments()[0];
childParameter = Expression.Parameter(childType, childType.Name);
}
else
{
childParameter = parameter;
}
//skip current property and get navigation property expression recursivly
var innerProperties = properties.Skip(1).ToArray();
predicate = BuildNavigationExpression(childParameter, comparer, value, innerProperties);
if (isCollection)
{
//build subquery
resultExpression = BuildSubQuery(parameter, childType, predicate);
}
else
{
resultExpression = predicate;
}
}
else
{
//build final predicate
resultExpression = BuildCondition(parameter, properties[0], comparer, value);
}
return resultExpression;
}
private static Expression BuildSubQuery(Expression parameter, Type childType, Expression predicate)
{
var anyMethod = typeof(Enumerable).GetMethods().Single(m => m.Name == "Any" && m.GetParameters().Length == 2);
anyMethod = anyMethod.MakeGenericMethod(childType);
predicate = Expression.Call(anyMethod, parameter, predicate);
return MakeLambda(parameter, predicate);
}
private static Expression BuildCondition(Expression parameter, string property, OperatorComparer comparer, object value)
{
var childProperty = parameter.Type.GetProperty(property);
var left = Expression.Property(parameter, childProperty);
var right = Expression.Constant(value);
var predicate = BuildComparsion(left, comparer, right);
return MakeLambda(parameter, predicate);
}
private static Expression BuildComparsion(Expression left, OperatorComparer comparer, Expression right)
{
var mask = new List<OperatorComparer>{
OperatorComparer.Contains,
OperatorComparer.StartsWith,
OperatorComparer.EndsWith
};
if(mask.Contains(comparer) && left.Type != typeof(string))
{
comparer = OperatorComparer.Equals;
}
if(!mask.Contains(comparer))
{
return Expression.MakeBinary((ExpressionType)comparer, left, Expression.Convert(right,left.Type));
}
return BuildStringCondition(left, comparer, right);
}
private static Expression BuildStringCondition(Expression left, OperatorComparer comparer, Expression right)
{
var compareMethod = typeof(string).GetMethods().Single(m => m.Name.Equals(Enum.GetName(typeof(OperatorComparer), comparer)) && m.GetParameters().Count() == 1);
//we assume ignoreCase, so call ToLower on paramter and memberexpression
var toLowerMethod = typeof(string).GetMethods().Single(m => m.Name.Equals("ToLower") && m.GetParameters().Count() == 0);
left = Expression.Call(left, toLowerMethod);
right = Expression.Call(right, toLowerMethod);
return Expression.Call(left, compareMethod, right);
}
private static Expression MakeLambda(Expression parameter, Expression predicate)
{
var resultParameterVisitor = new ParameterVisitor();
resultParameterVisitor.Visit(parameter);
var resultParameter = resultParameterVisitor.Parameter;
return Expression.Lambda(predicate, (ParameterExpression)resultParameter);
}
private class ParameterVisitor : ExpressionVisitor
{
public Expression Parameter
{
get;
private set;
}
protected override Expression VisitParameter(ParameterExpression node)
{
Parameter = node;
return node;
}
}
}
}
This can be used like
var predicate = ExpressionBuilder.BuildPredicate<Books>("Heading",OperatorComparer.Equals,"Page","Heading");
query = query.Where(predicate);
回答2:
Based on your description I'm not sure that you need Expression. Creating an Expression with a complex object model is quite difficult. Do you really need to create dynamic expression or you simply need to create a dynamic query? If the object model is fixed then you don't need Expression.
I suggest first of all to clean your object model:
- Rename the
Booksclass toBook(this class represents a Book not a list of books) - Rename property
PagetoPages(this property returns a list of pages)
Now you can write a dynamic where using just LINQ and one or more helper functions one for each property that you need to search. For example to search for Heading you can write:
private static bool SearchByHeading(Book b, string heading)
{
if (string.IsNullOrEmpty(heading))
return true;
else
return b.Pages.Any(p => p.Heading == heading);
}
Here you can also see why your previous code didn't work. The expression to search for a given Heading is book.Pages.Any(p => p.Heading == x) and not book.Pages.Heading == x.
In any case given one or more functions like this you can rewrite your code with something like:
using System.Collections.Generic;
using System.Linq;
namespace XMLStorageAndFilter
{
public class Book
{
public Book()
{
Pages = new List<Page>();
}
public string Title { get; set; }
public Author Author { get; set; }
public List<Page> Pages { get; set; }
}
public class Author
{
public string FirstName { get; set; }
}
public class Page
{
public string Heading { get; set; }
}
public class Program2
{
static void Main()
{
Page page = new Page();
page.Heading = "Heading1";
Book bok = new Book();
bok.Title = "Title1";
bok.Author = new Author() { FirstName = "FirstName1" };
bok.Pages.Add(page);
List<Book> testList = new List<Book>();
testList.Add(bok);
var searchResult = Search(testList,
title: "Title1",
author: "FirstName1",
heading: "Heading1");
}
private static IEnumerable<Book> Search(IEnumerable<Book> books, string author = null, string title = null, string heading = null)
{
return books
.Where((b) => SearchByAuthor(b, author))
.Where((b) => SearchByHeading(b, heading))
.Where((b) => SearchByTitle(b, title))
.ToList();
}
private static bool SearchByAuthor(Book b, string author)
{
if (string.IsNullOrEmpty(author))
return true;
else
return b.Author.FirstName == author;
}
private static bool SearchByTitle(Book b, string title)
{
if (string.IsNullOrEmpty(title))
return true;
else
return b.Title == title;
}
private static bool SearchByHeading(Book b, string heading)
{
if (string.IsNullOrEmpty(heading))
return true;
else
return b.Pages.Any(p => p.Heading == heading);
}
}
}
I have skipped search values when null or empty, just an example. This code has also the advantage that is verified at compile time.
回答3:
Update
The way to query a collection has been answered in the following:
Building a dynamic expression tree to filter on a collection property
Original Response
I believe Davide Lcardi is correct with his statement:
Heading is book.Pages.Any(p => p.Heading == x) and not book.Pages.Heading == x.
if you want to query the list you need to use Any() method to do so. I could not get it exactly right but it should look something like the following using Expression.Call:
ParameterExpression pe41 = Expression.Parameter(typeof (Page), "pg");
Expression left41 = Expression.Property(pe41, "Heading");
Expression right41 = Expression.Constant("Heading");
Expression e41 = Expression.Equal(left41, right41);
var methodCall = Expression.Call( Expression.Property(pe11, "Pages"), "Any", new Type[] {typeof(Page), typeof(Boolean)}, e41 );
I am getting this error: No method 'Any' exists on type 'System.Collections.Generic.List`1[SO.Page]'. SO is my NameSpace where class Page exists.
I think I am not sending the correct Type or the whole call may be incorrect. I think this is the correct direction thought to help you find your solution.
Here are some examples I was looking at:
http://msdn.microsoft.com/en-us/library/bb349020(v=vs.110).aspx
http://msdn.microsoft.com/en-us/library/dd402755(v=vs.110).aspx
http://community.bartdesmet.net/blogs/bart/archive/2009/08/10/expression-trees-take-two-introducing-system-linq-expressions-v4-0.aspx
http://blogs.msdn.com/b/csharpfaq/archive/2009/09/14/generating-dynamic-methods-with-expression-trees-in-visual-studio-2010.aspx
回答4:
On the whole, it is not bad to consider DynamicLinq when you are dealing with dynamic matters
回答5:
You should use Contains - you are looking inside a list - not Equals.
来源：https://stackoverflow.com/questions/23718054/dynamic-linq-building-expression