Comparing against child property in generic/dynamic linq predicate with reflection

Question

I am having to generically build a comparative predicate for an Entity Framework Linq query. I'm using reflection and am able to build a single level Lambda expression without any trouble. However where I am starting to get stuck is I have an Entity that has a relationship

public class Parent {
    public virtual Child child { get; set; }
    .... Other Stuff...
}

public class Child {
    public int property { get; set; }
    public virtual Parent parent { get; set; }
    .... Other Stuff.....
}

How can I can I pass in "Child.property" into Reflection to be able to create a lambda expression comparing and come up with a lambda expression similar to item => item.Child.property == value?

score 3 · Answer 1 · answered Nov 05 '12 at 17:52

I think you are looking for this:

ParameterExpression parameter = Expression.Parameter(typeof(Parent), "item");
Expression child = Expression.PropertyOrField(parameter, "child");
Expression childProperty = Expression.PropertyOrField(child, "property");
int value = 1;
Expression comparison = Expression.Equal(childProperty, Expression.Constant(value));

Expression<Func<Parent, bool>> lambda = Expression.Lambda<Func<Parent, bool>>(comparison, parameter);


var sample = new[] { new Parent() { child = new Child() { property = 1 } } };
var result = sample.Where(lambda.Compile());

score 3 · Answer 2 · answered Nov 05 '12 at 18:04

3

I assume you are wanting a generic solution supporting nested properties:

public Expression buildLambda(Type startingType, string propertyPath, object value) {

  var parameter=Expression.Parameter(startingType,"item");
  var valueExpression = Expression.Constant(value);
  var propertyExpression=propertyPath.Split('.').Aggregate(parameter,(Expression parent,string path)=>Expression.Property(parent,path));
  return Expression.Lambda(Expression.Equal(propertyExpression,valueExpression),parameter);
}

answered Nov 05 '12 at 18:04

Bob Vale

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This works. How would you handle it for a collection? – Kenci Oct 14 '15 at 12:11
@Kenci as in using property indexing? `Wibble[5].Value` – Bob Vale Oct 14 '15 at 15:42
What I meant was what if the OP's Child property was a collection, and you wanted to wrap a "contains" around that collection so that your lambda would be something like: Parent.Where(x => someList.Contains(x.property); – Kenci Oct 15 '15 at 08:17
@Kenchi then you would need to replace the Expression.Equal call. You would need to construct a method invocation of the Contains method – Bob Vale Oct 20 '15 at 11:37
@Kenci the hard part is you would need to know if you were calling contains on the collection itself or utilizing the linq Contains method. – Bob Vale Oct 20 '15 at 11:38

Comparing against child property in generic/dynamic linq predicate with reflection

2 Answers2