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LINQ OrderBy with index ; getting error using Generic TKey

I am trying to order an IEnumerable<MyClass> based on the index of the selected property:
class MyClass
{
public string Name;
public string Value;
}
I have a helper method to get an Expression<> of the criteria.
It is compiled to create the Func<> to help with the ordering.
I'm having trouble trying to make the Func<> return a Generic TKey to be ordered.
public Expression<Func<T, TKey>> Helper<T>(int propIdx)
{
var param = Expression.Parameter(typeof(T), "record");
var props = typeof(T).GetProperties();
if(propIdx > props.Count())
return Expression.Lambda<Func<T, TKey>>)(Expression.Constant(true), param);
Expression propExp = Expression.Property(param, props[propIdx].Name);
return lambda = Expression.Lambda<Func<T, TKey>>(propExp, param);
}
This is used to help with dynamic order criteria.
OrderByIdx(IEnumerable<MyClass> input, int propIdx)
{
Expression<Func<T, TKey>> exp = Helper(propIdx);
var funct = exp.Compile();
return input.OrderBy(funct);
}
I am getting an error The type or namespace TKey could not be found
How can I use a Generic<TKey> to help with the ordering ?

First of all: Type.GetProperties doesn't have a defined order.
var props = typeof(T).GetProperties();
Which property will be in props[0]? And are all properties readable?
So if you want to get a defined order, my advice would be to do some sorting. For instance get the public readable properties ordered by name.
Furthermore, isn't it a bit weird if users of your code (= software, not operators) want to use your generic code based on an index?
They have a class MyClass with a property Date
They have a sequence of objects of this class.
They want to order by property Date
Instead of ordering by property Date, or by property named "Date", They have to determine the index of property Date: Date seems to be the fourth property
Then they have to call your method using this index: "OrderBy index 4"
Wouldn't it be easier if they could just say: "order by property Date", or "order by the property that is named 'Date'?
IEnumerable<MyClass> source = ...
var orderedSource = source.OrderBy(t => t.Date);
Sometimes your users don't have access to the property, they only know the name of the property. In that case you could create an extension method where you provide the name of a property, and that returns the ordered source.
If you are not familiar with extension methods. See extension methods demystified
Usage would be something like:
var orderedSource = source.OrderBy(nameof(MyClass.Date));
or for instance: "order by the selected column in my DataGridView".
string propertyName = this.DataGridView.Columns.Cast<DataGridViewColumn>()
.Where(column => column.IsSelected)
.Select(column => column.DataPropertyName)
.FirstOrDefault();
var orderedSource = source.OrderBy(propertyName);
Such a procedure would be easy to make, easy to use and reuse, easy to test, easy to maintain. And above all: they are one-liners:
public static IOrderedEnumerable<T> OrderBy<T>(
this IEnumerable<T> source,
string propertyName)
{
// TODO: handle invalid input
PropertyInfo propertyInfo = typeof(T).GetProperty(propertyName);
// TODO: handle invalid propertyname
return source.OrderBy(propertyInfo);
}
public static IOrderedEnumerable<T> OrderBy<T>(
this IEnumerable<T> source,
PropertyInfo propertyInfo)
{
// TODO: handle invalid parameters
return source.OrderBy(t => propertyInfo.GetValue(t) as propertyInfo.PropertyType);
}
If desired you could create overloads for ThenBy(this IEnumerable<T>, ...).
But I want to order by Index, not by Name!
If you really want to sort by index, it is easier to create extension methods with Lambda expressions, then to fiddle with Expressions.
Consider the following extension methods:
public static IOrderedEnumerable<T> OrderBy<T>(
IEnumerable<T> source,
int propertyIndex)
{
return source.OrderBy(propertyIndex, GetDefaultPropertyOrder(typeof(T));
}
public static IOrderedEnumerable<T> OrderBy<T>(
IEnumerable<T> source,
int propertyIndex,
IReadOnlyList<PropertyInfo> properties)
{
// TODO: handle null and out-of-range parameters
PropertyInfo sortProperty = properties[propertyIndex];
return source.OrderBy(sortProperty);
}
public static IList<PropertyInfo> GetDefaultPropertyOrder(Type t)
{
return t.GetProperties()
.Where(property => property.CanRead)
.OrderBy(property => property.Name)
.ToList();
}
Usage:
BindingList<MyClass> myObjects = ...
// display myObjects in a dataGridView
this.DataGridView.DataSource = myObjects;
// event if operator clicks on column header:
public void OnColumnHeaderClicked(object sender, ...)
{
DataGridViewColumn clickedColumn = GetClickedColumn();
// sort by column index, as you prefer:
var sortedObjects = myObjects.SortBy(clickedColumn.DisplayIndex);
ProcessSortedObjects(sortedObjects);
// but of course, you skip some intermediate methods if you sort by property name
var sortedObject = myObject.SortBy(clickedColumn.DataPropertyName);
ProcessSortedObjects(sortedObjects);
}

To be able to order on a property, it must be of a type implementing IComparable. You can use this fact to replace TKey by IComparable in your code:
class MyClass
{
public string Name { get; set; }
public string Value { get; set; }
}
public Expression<Func<T, IComparable>> Helper<T>(int propIdx)
{
var param = Expression.Parameter(typeof(T), "record");
var props = typeof(T).GetProperties();
Expression propExp = Expression.Property(param, props[propIdx].Name);
return Expression.Lambda<Func<T, IComparable>>(propExp, param);
}
IEnumerable<MyClass> OrderByIdx(IEnumerable<MyClass> input, int propIdx)
{
Expression<Func<MyClass, IComparable>> exp = Helper<MyClass>(propIdx);
var funct = exp.Compile();
return input.OrderBy(funct);
}

Entity Framework execute query with not mapped properties. Expression tree

I want to execute linq method on iqueryable with an expression tree from function where I'm passing name of linq method and name of property. But my sample method works only with mapped properties. It throws an exception when I try to for example to find max of calculated property.
My classes:
public partial class Something
{
public int a { get; set; }
public int b { get; set; }
}
public partial class Something
{
public int calculated { get { return a * b; } }
}
Sample method:
public static object ExecuteLinqMethod(IQueryable<T> q, string Field, string Method)
{
var param = Expression.Parameter(typeof(T), "p");
Expression prop = Expression.Property(param, Field);
var exp = Expression.Lambda(prop, param);
Type[] types = new Type[] { q.ElementType, exp.Body.Type };
var mce = Expression.Call(typeof(Queryable),Method,types,q.Expression,exp);
return q.Provider.Execute(mce);
}

To be able to query on calculated properties, you have at least 2 options:
1) you store the calculated values in the db with the rows (or in a different table), and use them in your queries of course this requires datamodel change, and redundancy in data, but is the most performant way. But is not that exciting, so lets move on to
2) you need to be able to express the way you "calculate" the properties in a way that sql will understand, meaning the property needs to be replaced with a linq expression in the final query. I found in 2009 an amazing article from Eric Lippert on registering inline such properties, but I cannot find it anymore. As such here is a link to another, that has the same idea. Basically you define your calculation as an expression tree, and use the compiled version in your code.
To make it more convenient, you would attribute your property with a
[AttributeUsage(AttributeTargets.Property)]
class CalculatedByAttribute: Attribute
{
public string StaticMethodName {get; private set;}
public CalculatedByAttribute(string staticMethodName)
{
StaticMethodName = staticMethodName;
}
}
Like:
public partial class Something
{
[CalculatedBy("calculatedExpression")]
public int calculated { get { return calculatedExpression.Compile()(this); } }
public static Expression<Func<Something, int>> calculatedExpression = s => s.a * s.b;
}
(of course you can cache the compilation) :)
Then in your method, if the property has your attribute, you get the static property value, and use that in your queries. Something along:
public static object ExecuteLinqMethod<T>(IQueryable<T> q, string Field, string Method)
{
var propInfo = typeof(T).GetProperty(Field);
LambdaExpression exp;
var myAttr = propInfo.GetCustomAttributes(typeof(CalculatedByAttribute), true).OfType<CalculatedByAttribute>().FirstOrDefault();
if (myAttr != null)
exp = (LambdaExpression)typeof(T).GetField(myAttr.StaticMethodName, BindingFlags.Static | BindingFlags.Public).GetValue(null);
else
{
var param = Expression.Parameter(typeof(T), "p");
Expression prop = Expression.Property(param, Field);
exp = Expression.Lambda(prop, param);
}
Type[] types = new Type[] { q.ElementType, exp.Body.Type };
var mce = Expression.Call(typeof(Queryable),Method,types,q.Expression,exp);
return q.Provider.Execute(mce);
}

MemberExpression as object of its type

I'm building Linq Extension methods.
Shortly, I've built an extension method in order to create a MemberExpression looks like:
public static Expression Field<T>(this object entity, string field)
{
Type entityType = entity.GetType();
PropertyInfo propertyInfo = entityType.GetProperty(field);
if (propertyInfo == null)
throw new ArgumentException(string.Format("{0} doesn't exist on {1}", field, entityType.Name));
ParameterExpression parameterExpression = Expression.Parameter(entityType, "e");
return Expression.Property(parameterExpression, propertyInfo);
}
So, I'm able to do that:
IEnumerable<C> classes = this.backend.cs.Where(
c => c.Field<C>("Matter").EndsWith(string.Empty)<<<<<<<< Compilation error.
);
Since MemberExpression have not EndsWith method, I'm not able to extend this MemberExpression like a String property access like:
IEnumerable<C> classes = this.backend.cs.Where(
c => c.Matter.EndsWith(string.Empty)
);
Is there some way to do that.
As you are able to figure out I'm trying to get something a bit more complex, Nevertheless, this example is for explaining the situation.
I hope it's enought clear.
Scope
My UI is using a backend.
This backend have three implementations. Each one of them provides a Linq implementation (Linq collections, NHibernate, custom-made Linq provider).
So, my UI is able to work on collections, a database or getting data from our server.
I'd like to provide util extension methods like AnyField().
So, after digging a bit I'm thinking on two approaches:
AnyField() generates an expression tree which is able to be translated by every Linq provider (first answer of this post).
Provide a default implementation of Anyfield() for Linq Collections, and then use each Linq provider extension mechanism for handle it. Or, if you are building a Linq Provider, support it on implementation.

Okay, so you're getting tripped up on the syntactic sugar that C# provides for you when building ExpressionTrees
Where expects Expression<Func<TObjectType, TReturnType>> or a compiled lambda; Func<TObjectType, TReturnType>.
Your method Field currently only returns an untyped Expression. That means your query is actually returning Expression<Func<TObjectType, Expression>>. That's not right! It should be returning a Expression<Func<TObjectType, string>>! But how do we do that? That would mean our method would have to return a string, but we want to build an expression tree.
To get it working as you're expecting, it's quite a bit more difficult than you would imagine, but that's only because we're so spoiled with the syntactic sugar.
What we actually need to do is write methods which accept lambda methods, and return lambda methods, each one re-writing the body a little bit.
So... what does that look like?
public static Expression<Func<TElementType, object>> Field<TElementType, TReturnType>(this Expression<Func<TElementType, TReturnType>> expr, string field)
{
Type entityType = expr.Body.Type;
PropertyInfo propertyInfo = entityType.GetProperty(field);
if (propertyInfo == null)
throw new ArgumentException(string.Format("{0} doesn't exist on {1}", field, entityType.Name));
ParameterExpression parameterExpression = Expression.Parameter(entityType, "e");
return Expression.Lambda<Func<TElementType, object>>(
Expression.Property(parameterExpression, propertyInfo),
parameterExpression
);
}
Notice that it's almost the exact same as what you wrote, but we wrap it with Lambda<Func<TElementType, TReturnType>>. And the signature is a bit different too.
Instead of operating on an object, we want to operate on a lambda expression. We also return a lambda expression.
So how do we use it?
var classes = objects.Where(
ExpressionExtensions.Field<Test, Test>(q => q, "Matter")
);
Great! Now we're passing Expression<Func<Test, string>> to Where, rather than Expression<Func<Test, MemberExpression>>. Making progress.
But that won't compile, and rightly so. We're returning a string, but we're using a filtering method, which requires a bool.
So let's now write EndsWith:
public static Expression<Func<T, bool>> EndsWith<T, TReturnType>(this Expression<Func<T, TReturnType>> expr, string str)
{
var endsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
var newBody = Expression.Call(expr.Body, endsWithMethod, Expression.Constant(str));
var result = Expression.Lambda<Func<T, bool>>(newBody, expr.Parameters);
return result;
}
And using it:
var classes = objects.Where(
ExpressionExtensions.Field<Test, Test>(q => q, "Matter")
.EndsWith("A")
);
Which is now compiling! And the expression tree looks like this:
UserQuery+Test[].Where(e => e.Matter.EndsWith("A"))
That's not too pretty, having Field take a redundant lambda, though. Let's add a helper method to make it look prettier:
public static Expression<Func<TElementType, TElementType>> Query<TElementType>(this Expression<Func<TElementType, TElementType>> expr)
{
return expr;
}
Putting it all together:
void Main()
{
var objects = new[] { new Test { Matter = "A" } }.AsQueryable();
var classes = objects.Where(
ExpressionExtensions.Query<Test>(q => q)
.Field("Matter")
.EndsWith("A")
);
classes.Expression.Dump();
}
public class Test
{
public string Matter { get; set;}
}
public static class ExpressionExtensions
{
public static Expression<Func<TElementType, TElementType>> Query<TElementType>(this Expression<Func<TElementType, TElementType>> expr)
{
return expr;
}
public static Expression<Func<TElementType, object>> Field<TElementType, TReturnType>(this Expression<Func<TElementType, TReturnType>> expr, string field)
{
Type entityType = expr.Body.Type;
PropertyInfo propertyInfo = entityType.GetProperty(field);
if (propertyInfo == null)
throw new ArgumentException(string.Format("{0} doesn't exist on {1}", field, entityType.Name));
ParameterExpression parameterExpression = Expression.Parameter(entityType, "e");
return Expression.Lambda<Func<TElementType, object>>(
Expression.Property(parameterExpression, propertyInfo),
parameterExpression
);
}
public static Expression<Func<T, bool>> EndsWith<T, TReturnType>(this Expression<Func<T, TReturnType>> expr, string str)
{
var endsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
var newBody = Expression.Call(expr.Body, endsWithMethod, Expression.Constant(str));
var result = Expression.Lambda<Func<T, bool>>(newBody, expr.Parameters);
return result;
}
}

I don't know if you mess the code to show a piece of code or if it was intended, but you have a generic extension that wants A T but you don't use it
Anyway if what you want is a method that returns you the value of a property, why don't you do a static exception that return T ?
public static class EntityExtension {
public static T Field<T>(this object entity, string field) {
Type entityType = entity.GetType();
PropertyInfo propertyInfo = entityType.GetProperty(field);
if (propertyInfo == null) {
throw new ArgumentException(string.Format("{0} doesn't exist on {1}", field, entityType.Name));
}
return (T)propertyInfo.GetValue(entity);
}
}
this is a fiddle i've done to show you the usage, pretty simple
https://dotnetfiddle.net/PoSfli
posting the code too in case fiddle get lost:
using System;
using System.Reflection;
using System.Linq.Expressions;
public class Program
{
public static void Main()
{
YourClass c = new YourClass() {
PropA = 1,
PropB = 2,
PropC = "ciao"
};
var propBValue = c.Field<int>("PropB");
Console.WriteLine("PropB value: {0}", propBValue);
var propCValue = c.Field<string>("PropC");
Console.WriteLine("PropC value: {0}", propCValue);
}
}
public static class EntityExtension {
public static T Field<T>(this object entity, string field) {
Type entityType = entity.GetType();
PropertyInfo propertyInfo = entityType.GetProperty(field);
if (propertyInfo == null) {
throw new ArgumentException(string.Format("{0} doesn't exist on {1}", field, entityType.Name));
}
return (T)propertyInfo.GetValue(entity);
}
}
public class YourClass {
public int PropA { get; set; }
public int PropB { get; set; }
public string PropC { get; set; }
}
nota that you can improve a lot, using a typed extension and a property expression as argument instead of a string

You can also do something really simple like if you want to use the property name:
IEnumerable<C> classes = this.backend.cs.Where(
c => c.Field<C>("Matter").ToString().EndsWith(string.Empty)
Or if your are filtering by property type:
IEnumerable<C> classes = this.backend.cs.Where(
c => c.Field<C>("Matter").Type.ToString().EndsWith(string.Empty)

How about something like this? Actually, your generic approach is not of use right now.
public static bool Evaluate<TField>(this object entity, string fieldName, Predicate<TField> condition)
{
Type entityType = entity.GetType();
PropertyInfo propertyInfo = entityType.GetProperty(field);
if (propertyInfo == null)
throw new ArgumentException(string.Format("{0} doesn't exist on {1}", field, entityType.Name));
var value = (TField)propertyInfo.GetValue(entity); //read the value and cast it to designated type, will raise invalid cast exception, if wrong
return condition.Invoke(value); //invoke the predicate to check the condition
}
Usage would be then.
.Where(item => item.Evaluate<string>("Matter", prop => prop.EndsWith(string.Empty))

You can add a new extension method which returns your desired type.
public static T Compile<T>(this Expression expression)
{
return Expression.Lambda<Func<T>>(expression).Compile()();
}
In your statement you just have to add .Compile<type>()
IEnumerable<C> classes = this.backend.cs.Where(
c => c.Field<C>("Matter").Compile<string>().EndsWith(string.Empty));

How to use expressions to sort a collection based on an inherited interface property

This problem has been discussed to an extent in this question: Create Generic Expression from string property name but perhaps I'm missing the answer, or it is subtly different.
I have the following queryable extension method:
public static IQueryable<TSource> OrderByPropertyDescending<TSource>(IQueryable<TSource> source, string propertyName)
{
var sourceType = typeof (TSource);
var parameter = Expression.Parameter(sourceType, "item");
var orderByProperty = Expression.Property(parameter, propertyName);
var orderBy = Expression.Lambda(orderByProperty, new[] { parameter });
return Queryable.OrderByDescending(source, (dynamic) orderBy);
}
For the purposes of this problem let us assume that the queryable source instance has a property called 'Created' which is a type of DateTime. If we define a class that has the property 'Created' on it and then OrderByDescending then the above will work fine. e.g.
var queryable = new List<EntityClassWithCreatedProperty>().AsQueryable();
var result = queryable.OrderByPropertyDescending("Created").ToList();
If we do the same thing but with an interface such as ICreated which has the Created property on it:
public interface ICreated
{
DateTime Created { get; }
}
Then the following also works:
var queryable = new List<ICreated>().AsQueryable();
var result = queryable.OrderByPropertyDescending("Created").ToList();
If however, you have the following interface hierarchy:
public interface ITimestamped : ICreated
{
...
}
Then the following will fail:
var queryable = new List<ITimestamped>().AsQueryable();
var result = queryable.OrderByPropertyDescending("Created").ToList();
With the similar error message to that of the other question: Instance property 'Created' is not defined for type ITimestamped. I'm assuming that I need to find the property on the declaring type of interface (which I can do by crawling the source type) but then what do I do with it? Most attempts I have tried result in the incorrect original source type not being cast-able back to the IQueryable. Do I need to use a ConvertType call somewhere? Thanks.

I would use the other Expression.Property() method that takes a propertyInfo, e.g
public static IQueryable<TSource> OrderByPropertyDescending<TSource>(IQueryable<TSource> source, string propertyName)
{
var sourceType = typeof (TSource);
var parameter = Expression.Parameter(sourceType, "item");
var propertyInfo = FindMyProperty(sourceType, propertyName);
var orderByProperty = Expression.Property(parameter, propertyInfo);
var orderBy = Expression.Lambda(orderByProperty, new[] { parameter });
return Queryable.OrderByDescending(source, (dynamic) orderBy);
}
private static PropertyInfo FindMyProperty(Type type, string propertyName)
{
return type.GetProperty(propertyName);
}
At this point your question has nothing to do with expressions, it's a simple reflection question, and you have to find the correct way to get the properties you want. There are a lot of scenarios here. For your exact one, meaning get property from parent interface, you can do something like:
private static PropertyInfo FindMyProperty(Type type, string propertyName)
{
var result = type.GetProperty(propertyName);
if (result == null)
{
foreach(var iface in type.GetInterfaces())
{
var ifaceProp = FindMyProperty(iface, propertyName);
if (ifaceProp != null)
return ifaceProp;
}
}
return result;
}
DISCLAIMER!
This is by no means the best way to get a property from a type, but it should work in your case. You should google around to find an implementation for FindMyProperty that satisfies all your requirements :)

Creating an expression from the string of a property name?

I am trying to create a query based on some JSON, I currently have the JSON parsed into a set of rules, each rule contains the name of the field, the type of comparison (=, > etc) and the value to compare.
The issue I am having is getting it from that rule, to an IQueryable object, I am guessing I need to use reflection and somehow build the expression tree, but I'm not sure on the right approach...
Assuming I have:
public class Order : BaseEntity
{
public int OrderID{ get; set; }
}
and I have the rule which is:
public class Rule
{
public string field { get; set; }
public Operations op { get; set; }
public string data { get; set; }
}
Running it I get:
field = "OrderID"
op = "eq"
data = "123"
I have the method to parse it with the signature:
public IQueryable<T> FilterObjectSet<T>(IQueryable<T> inputQuery) where T : class
As part of this method I want to do:
inputQuery = inputQuery.Where(o => propertyInfo.Name == rule1.data);
This doesn't work because it basically just generates the sql "OrderID" = "123" which is obviously wrong, I need it to take the column name from inputQuery that has the same name as propertyInfo.Name and build the query that way...
Hope that made sense? Any suggestions?
Edit: I guess what I am asking is to convert a string (Because I can build one pretty simply from the rule) to an expression, maybe using Dynamic LINQ?

Something like this:
public static IQueryable<T> FilterObjectSet<T>(IQueryable<T> inputQuery,
Rule rule) where T : class
{
var par = Expression.Parameter(typeof(T));
var prop = Expression.PropertyOrField(par, rule.field);
var propType = prop.Member.MemberType == System.Reflection.MemberTypes.Field ?
((FieldInfo)prop.Member).FieldType :
((PropertyInfo)prop.Member).PropertyType);
// I convert the data that is a string to the "correct" type here
object data2 = Convert.ChangeType(rule.data,
propType,
CultureInfo.InvariantCulture);
var eq = Expression.Equal(prop, Expression.Constant(data2));
var lambda = Expression.Lambda<Func<T, bool>>(eq, par);
return inputQuery.Where(lambda);
}
If you need some explanation, you can ask. Note that this won't work on types that have special implicit conversions (like a MyString type that has an implicit conversion from string). This because Convert.ChangeType uses only the IConvertible interface.
Null handling for data is perhaps something else that should be handled.
Be aware that I'm not sure the Expression.PropertyOrField is handled by the various IQueryable<T> engines (LINQ-to-SQL and EF). I have only tested it with the AsQueryable() engine. If they don't "accept" it, you must split it in a Expression.Property or Expression.Field depending on what rule.field is.
A nearly equivalent version that doesn't use Expression.PropertyOrField:
public static IQueryable<T> FilterObjectSet<T>(IQueryable<T> inputQuery,
Rule rule) where T : class
{
Type type = typeof(T);
var par = Expression.Parameter(type);
Type fieldPropertyType;
Expression fieldPropertyExpression;
FieldInfo fieldInfo = type.GetField(rule.field);
if (fieldInfo == null)
{
PropertyInfo propertyInfo = type.GetProperty(rule.field);
if (propertyInfo == null)
{
throw new Exception();
}
fieldPropertyType = propertyInfo.PropertyType;
fieldPropertyExpression = Expression.Property(par, propertyInfo);
}
else
{
fieldPropertyType = fieldInfo.FieldType;
fieldPropertyExpression = Expression.Field(par, fieldInfo);
}
object data2 = Convert.ChangeType(rule.data, fieldPropertyType);
var eq = Expression.Equal(fieldPropertyExpression,
Expression.Constant(data2));
var lambda = Expression.Lambda<Func<T, bool>>(eq, par);
return inputQuery.Where(lambda);
}

In the end I used a Dynamic Linq library I found on Guthrie's Blog:
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
Using this I was able to properly parse out and use the parameters I had built into the rules