I'm building a generic interface to expose selected string properties out of a class, and then I want to search for a text inside every one of those fields, to check if it's a match.
Here's my IFieldExposer interface:
using System;
using System.Collections.Generic;
public interface IFieldExposer<T>
{
IEnumerable<Func<T, string>> GetFields();
}
Now, I implement it like this in my DataClass to expose the properties I would like to iterate. Note that I'm also exposing a property from my ChildClass:
using System;
using System.Collections.Generic;
class DataClass : IFieldExposer<DataClass>
{
public string PropertyOne { get; set; }
public string PropertyTwo { get; set; }
public ChildClass Child { get; set; }
public IEnumerable<Func<DataClass, string>> GetFields()
{
return new List<Func<DataClass, string>>
{
a => a.PropertyOne,
b => b.Child.PropertyThree
};
}
}
class ChildClass
{
public string PropertyThree { get; set; }
}
I've also created extension methods for IFieldExposer<T> because I want to keep it simple and be able to simply call obj.Match(text, ignoreCase) everywhere else in my code. This method should tell me if my object is a match for my text. Here's the code for the ExtensionClass, which isn't working as expected:
using System;
using System.Linq.Expressions;
using System.Reflection;
public static class ExtensionClass
{
public static bool Match<T>(this IFieldExposer<T> obj, string text, bool ignoreCase)
{
Func<bool> expression = Expression.Lambda<Func<bool>>(obj.CreateExpressionTree(text, ignoreCase)).Compile();
return expression();
}
private static Expression CreateExpressionTree<T>(this IFieldExposer<T> obj, string text, bool ignoreCase)
{
MethodInfo containsMethod = typeof(string).GetMethod("Contains", new Type[] { typeof(string) });
var exposedFields = obj.GetFields();
if (ignoreCase)
{
// How should I do convert these to lower too?
// exposedFields = exposedFields.Select(e => e.???.ToLower());
text = text.ToLower();
}
Expression textExp = Expression.Constant(text);
Expression orExpressions = Expression.Constant(false);
foreach (var field in exposedFields)
{
//How should I call the contains method on the string field?
Expression fieldExpression = Expression.Lambda<Func<string>>(Expression.Call(Expression.Constant(obj), field.Method)); //this doesn't work
Expression contains = Expression.Call(fieldExpression, containsMethod, textExp);
orExpressions = Expression.Or(orExpressions, contains);
}
return orExpressions;
}
}
Please check the comments in the code above. I would like to know how to convert all my string properties to lowercase (if desired) and how to call string.Contains in each one of them. I get this error when I create my fieldExpression:
Method 'System.String <GetFields>b__12_0(DataClass)' declared on type 'DataClass+<>c' cannot be called with instance of type 'DataClass'
I don't have experience working with Expression Trees. I've spent hours reading docs and other answers for similar issues but I still can't understand how to achieve what I want... I have no clue what to do now.
I'm testing this in a console app so here's the main class if you want to build it yourself:
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main(string[] args)
{
var data = new DataClass
{
PropertyOne = "Lorem",
PropertyTwo = "Ipsum",
Child = new ChildClass
{
PropertyThree = "Dolor"
}
};
var dataList = new List<DataClass> { data };
var results = dataList.Where(d => d.Match("dolor", true));
}
}
I forgot to mention that my dataList should be IQueryable and I want to execute my code in SQL, that's why I'm trying to build the expression trees myself. So it appears my example code should be:
var dataList = new List<DataClass> { data };
var query = dataList.AsQueryable();
var results = query.Where(ExtensionClass.Match<DataClass>("lorem dolor"));
while my method becomes: (I'm following @sjb-sjb's answer and changed the GetFields() method in IFieldExposer<T> to a SelectedFields property)
public static Expression<Func<T, bool>> Match<T>(string text, bool ignoreCase) where T : IFieldExposer<T>
{
ParameterExpression parameter = Expression.Parameter(typeof(T), "obj");
MemberExpression selectedFieldsExp = Expression.Property(parameter, "SelectedFields");
LambdaExpression lambda = Expression.Lambda(selectedFieldsExp, parameter).Compile();
[...]
}
And then it seems that I have to dinamically call selectedFieldsExp with Expression.Lambda. I came up with:
Expression.Lambda(selectedFieldsExp, parameter).Compile();
and that works, but I don't know how to properly call DynamicInvoke() for the lambda expression.
It throws Parameter count mismatch. if I call it without parameters and Object of type 'System.Linq.Expressions.TypedParameterExpression' cannot be converted to type 'DataClass'. if I do DynamicInvoke(parameter).
Any ideas?
Before getting to the implementation, there are some design flaws that needs to be fixed.
First, almost all query providers (except LINQ to Object which simply compiles the lambda expressions to delegates and executes them) don't support invocation expressions and custom (unknown) methods. That's because they do not execute the expressions, but translate them to something else (SQL for instance), and translation is based on pre knowledge.
One example of invocation expression are Func<...> delegates. So the first thing you should do is to use Expression<Func<...>> wherever you currently have Func<...>.
Second, the query expression trees are built statically, i.e. there is no real object instance you can use to obtain the metadata, so the idea of IFieldExposer<T> won't work. You'd need a statically exposed list of expressions like this:
class DataClass //: IFieldExposer<DataClass>
{
// ...
public static IEnumerable<Expression<Func<DataClass, string>>> GetFields()
{
return new List<Expression<Func<DataClass, string>>>
{
a => a.PropertyOne,
b => b.Child.PropertyThree
};
}
}
Then the signature of the method in question could be like this
public static Expression<Func<T, bool>> Match<T>(
this IEnumerable<Expression<Func<T, string>>> fields, string text, bool ignoreCase)
with usage like this
var dataList = new List<DataClass> { data };
var query = dataList.AsQueryable()
.Where(DataClass.GetFields().Match("lorem", true));
Now the implementation. The desired expression could be built purely with Expression class methods, but I'll show you an easier (IMHO) method, which composes expression from compile time expression by replacing the parameter(s) with other expression(s).
All you need is a small helper utility method for replacing lambda expression parameter with another expression:
public static partial 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)
=> node == Source ? Target : base.VisitParameter(node);
}
}
Internally it uses ExpressionVistor to find each instance of the passed ParameterExpression and replace it with the passed Expression.
With this helper method, the implementation could be like this:
public static partial class ExpressionUtils
{
public static Expression<Func<T, bool>> Match<T>(this IEnumerable<Expression<Func<T, string>>> fields, string text, bool ignoreCase)
{
Expression<Func<string, bool>> match;
if (ignoreCase)
{
text = text.ToLower();
match = input => input.ToLower().Contains(text);
}
else
{
match = input => input.Contains(text);
}
// T source =>
var parameter = Expression.Parameter(typeof(T), "source");
Expression anyMatch = null;
foreach (var field in fields)
{
// a.PropertyOne --> source.PropertyOne
// b.Child.PropertyThree --> source.Child.PropertyThree
var fieldAccess = field.Body.ReplaceParameter(field.Parameters[0], parameter);
// input --> source.PropertyOne
// input --> source.Child.PropertyThree
var fieldMatch = match.Body.ReplaceParameter(match.Parameters[0], fieldAccess);
// matchA || matchB
anyMatch = anyMatch == null ? fieldMatch : Expression.OrElse(anyMatch, fieldMatch);
}
if (anyMatch == null) anyMatch = Expression.Constant(false);
return Expression.Lambda<Func<T, bool>>(anyMatch, parameter);
}
}
The input => input.ToLower().Contains(text) or input => input.Contains(text) is our compile time match expression, which we then replace the input parameter with the body of the passed Expression<Func<T, string>> lambda expressions, with their parameter replaced with a common parameter used in the final expression. The resulting bool expressions are combined with Expression.OrElse which is the equivalent of the C# || operator (while Expression.Or is for bitwise | operator and in general should not be used with logical operations). Same btw for && - use Expression.AndAlso and not Expression.And which is for bitwise &.
This process is pretty much the expression equivalent of the string.Replace. In case the explanations and code comments are not enough, you can step through the code and see the exact expression transformations and expression building process.