C#•3mo ago

Passing Lambda as a method parameter: Some issues

I am building an IQueryable extension that can perform queries on nested object properties (that is the hope). I would like to apply this extension to a class, then pass the desired nested object to the extension to further build a where clause Here is an example of how I would call the extension: _context.ClassA.CustomExtension(a => a.ClassB) (ClassB is an object instance on Class A) All examples online show some expression that returns a value, however in this case I don't have one. I just want the extension to know it should query this nested Class on _context.ClassA. Not sure how to set up the parameter. What is the correct way to go about this?

7 Replies

mldisibio•3mo ago

If I understand you correctly, you want to pass in a Function that knows how to query your nested property, but you probably need a generic parameter to specify the expected outcome:

void Main()
{
    Bar b = new Bar("abc", new Foo("Joe", 37));
    Console.WriteLine(b.QueryProperty(b => b.Person.Age));
    Console.WriteLine(b.QueryBarProperty(b => b.Person.Name));
}

public record Foo(string Name, int Age);
public record Bar(string Tag, Foo Person);
public static class Ext
{
    // specific to Bar
    public static TOut QueryBarProperty<TOut>(this Bar src, Func<Bar, TOut> query) => query(src);
    // any class
    public static TOut QueryProperty<TIn, TOut>(this TIn src, Func<TIn, TOut> query) => query(src);
}

void Main()
{
    Bar b = new Bar("abc", new Foo("Joe", 37));
    Console.WriteLine(b.QueryProperty(b => b.Person.Age));
    Console.WriteLine(b.QueryBarProperty(b => b.Person.Name));
}

public record Foo(string Name, int Age);
public record Bar(string Tag, Foo Person);
public static class Ext
{
    // specific to Bar
    public static TOut QueryBarProperty<TOut>(this Bar src, Func<Bar, TOut> query) => query(src);
    // any class
    public static TOut QueryProperty<TIn, TOut>(this TIn src, Func<TIn, TOut> query) => query(src);
}

sideburns_01•3mo ago

Hi. thanks for getting back to me. What I'm trying to do is create a dynamic extension query that can encapsulate the where statement in_context.ClassA.Where(a => a.ClassB.someVal == someReferenceVal). I was working with Expressions trees, and the problem there is getting the nested object and its propery to build that where clause. Not sure how to go about it. Though I am open to other ideas if my initial idea is wrong. The key idea being what options exist to encapsulate the .Where(a => a.ClassB.someVal == someReferenceVal)

mldisibio•3mo ago

Ah, so you are really building the IQueryable! I got frustrated in my attempts years ago, but have a look at Joe Albahari's approach here: https://www.albahari.com/nutshell/linqkit.aspx

sideburns_01•3mo ago

ok, thanks for the reference. I was trying to build something like this using expressions rather than sql syntax

private sealed class referenceValue<T>
{
    public int Value {get; set;}
}

public static IQueryable<T> WithIdExtension<T>(this IQueryable<T> query) where T : class
{    
    //entity we are applying filter to
    var memberEntity = Expression.Parameter(typeof(T))

    //the property we want to access
    var accessEntityProperty = Expression.PropertyOrField(memberEntity, "SpecificId')
    
    var objectValue = new referenceValue<T> {Value = someInternalMethod.GetValue()}
    
    //this will parameterize value when converted to sql
    var value = Expression.PropertyOrField(Expression.Constant(objectValue, "Value");

    var equalExpression = Expression.Equal(accessEntityParameter, value)
    
    Expression<Func<T, bool>> whereLambda = Expression.Lambda<Func<T, bool>>(equalExpression, memberEntity)

    return query.Where(whereLambda)
}

dunno if reflection could be used to search for the nested Object class once I got the type T is?

mldisibio•3mo ago

I'm familiar enough with what you are doing to re-iterate that Albahari should be a gold-standard reference. But I gave up on this approach about ten years ago and went with explicit SQL and some extensions that wrapped simple Ado.Net operations with a fluent api. I won't be able to help you much with the Visitor pattern and expression tree builder needed here. 😕

sideburns_01•3mo ago

ok. Thanks again for the reference, that is very helpful. I will take a deep dive into that.

mldisibio•3mo ago

I'm curious though. Is this going against an RDBMS table, or is it Linq-to-Entity, or is it against a document database? If ClassB is a nested property of ClassA, does your query presume a join clause?

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Passing Lambda as a method parameter: Some issues