Linq Where with variable header - linq

Question

I´m trying to do something like this:
public class Person{string name;string surname;}
//...
List<Person> listExample;
//We add Person object in listExample
string variable="name";
listexample.Where(x=>x.(variable)=="John");
Is it posible to do something similar ?

Answer 1

If you need to access non-public fields then you can use reflection:
class Person
{
public Person(string name)
{
this.name = name;
}
string name;
}
List<Person> people = new List<Person>()
{
new Person("Jane"),
new Person("John")
};
string variableName = "name";
string criteria = "John";
var selectedPeople =
people
.Where(person =>
typeof(Person)
.GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
.Where(fieldInfo => String.Equals(fieldInfo.Name, variableName))
.Select(fieldInfo => fieldInfo.GetValue(person) as string)
.SingleOrDefault() == criteria
)
.ToList();
// At this point 'selectedPeople' will contain one 'Person' named "John"
If you need to access properties then you can use GetProperties(BindingFlags bindingAttr) method instead of GetFields(...) method.
class Person
{
public Person(string name)
{
this.Name = name;
}
public string Name { get; set; }
}
// in a method
var selectedPeople =
people
.Where(person =>
typeof(Person)
.GetProperties(BindingFlags.Public | BindingFlags.Instance)
.Where(propertyInfo => String.Equals(propertyInfo.Name, variableName))
.Select(propertyInfo => propertyInfo.GetValue(person) as string)
.SingleOrDefault() == criteria
)
.ToList();
You should take a look at BindingFlags enum so that you can select the appropriate enum values.
BindingFlags.Static for static members
BindingFlags.Instance for instance members
BindingFlags.Public for public members
BindingFlags.NonPublic for private / protected members
etc.

Answer 2

The easiest way, considering the class has only two possible fields to filter on, would be to use simple if-else block :
string targetProperty = "name";
string targetValue = "John";
IEnumerable<Person> query = listExample;
if(targetProperty == "name") query = query.Where(x => x.name == targetValue)
else if(targetProperty == "surname") query = query.Where(x => x.surname == targetValue)
var result = query.ToList();
If the actual class has a lot of possible properties/fields to filter on, and you want to avoid too many if-branches, it's time to use Expression as mentioned in the comment below the question :
string targetProperty = "name";
string targetValue = "John";
var param = Expression.Parameter(typeof(Person), "x");
var body = Expression.Equal(
Expression.PropertyOrField(param, targetProperty),
Expression.Constant(targetValue));
var predicate = Expression.Lambda<Func<Person, bool>>(body, param);
var result = listExample.Where(predicate.Compile()).ToList();
The above codes should build predicate expression equivalent to x => x.name == "John" (since targetProperty = "name" and targetValue = "John")

Answer 3

There's a nuget package for this that lets you write;
using System.Linq.Dynamic; //Import the Dynamic LINQ library
//The standard way, which requires compile-time knowledge
//of the data model
var result = myQuery
.Where(x => x.Field1 == "SomeValue")
.Select(x => new { x.Field1, x.Field2 });
//The Dynamic LINQ way, which lets you do the same thing
//without knowing the data model before hand
var result = myQuery
.Where("Field1=\"SomeValue\"")
.Select("new (Field1, Field2)");