How can I do something like this:
var result = db.MyTable.Where(x => x."MyProperty" == "Test" );
As you can see I want to access "MyProperty" but give the property name as a sting.
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3see Dynamic Linq: weblogs.asp.net/scottgu/archive/2008/01/07/…BrokenGlass– BrokenGlass2011-08-10 20:25:10 +00:00Commented Aug 10, 2011 at 20:25
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1possible duplicate of Dynamic LINQ on IEnumerable?Craig Stuntz– Craig Stuntz2011-08-10 20:30:15 +00:00Commented Aug 10, 2011 at 20:30
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4 Answers
You could use reflection
x.GetType( ).GetProperty("MyProperty").GetValue( x, null );
although this might work, i wouldn't advise on doing this, why don't pass in your where clause as an expression like:
myMethod<T>(Expression<Func<T,bool>> where)
example after comment:
consider the following type:
you see there are three properties where the name is of type string and id is of type int. now if we wrap our database context in a service like this
public class MyTypeOfXService
{
private DataDataContext Context;
public MyTypeOfXService()
{
Context = new DataDataContext("example code");
}
public IQueryable<MyTypeOfX> GetTypeOfX(Expression<Func<MyTypeOfX, bool>> where)
{
return this.Context.MyTypeOfXes.Where(where);
}
}
in our get method there is an Expression parameter that takes two generics, the first is our type x and the second is a boolean. The advantage of this approach is that we can abstract all the data context creation and only express an where clause in our code see final piece of code:
class Program
{
static void Main(string[] args)
{
var service = new MyTypeOfXService();
var queryById = service.GetTypeOfX((mytype) => mytype.Id == 1);
var queryByName = service.GetTypeOfX((mytype) => mytype.Name == "MyName");
var queryByName = service.GetTypeOfX((mytype) => mytype.Name == "MyName" && mytype.Id == 1);
}
}
as you can see, we can build a where clause on any property or combination of properties.
2 Comments
NKnusperer
I'am using Linq To SQL so GetProperty() doesn't work. How to do this with an expression ?
Didier Caron
i'll provide you with an example, give me 15 mins.
I think you can try this:
public static IQueryable<T> SortByPropertyName<T>(this IQueryable<T> queryable, string orderFieldName) where T : Entity
{
var param = Expression.Parameter(typeof(T), typeof(T).Name);
var orderExpression = Expression.Lambda<Func<T, object>>(Expression.Property(param, orderFieldName), param);
return queryable.OrderBy(orderExpression);
}
It works fine for order.
Comments
I don't know how x is implemented, but if x has an indexer, then you could formulate your query like:
var result = db.MyTable.Where(x => x["MyProperty"] == "Test" );
see http://msdn.microsoft.com/en-us/library/6x16t2tx.aspx for indexers in c#
answered Aug 10, 2011 at 20:31
Olivier Jacot-Descombes
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Comments
You can use reflection to get it unless you are querying a database directly using LINQ to SQL.
Here is a sample of how to get property info using reflection:
class Program
{
class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
public byte Age { get; set; }
public override string ToString()
{
return string.Format("{0}, {1} ({2})", LastName, FirstName, Age);
}
}
static void Main(string[] args)
{
Person p1 = new Person() { FirstName = "Bill", LastName = "Johnson", Age = 34 };
Person p2 = new Person() { FirstName = "Sally", LastName = "Jones", Age = 21 };
Person p3 = new Person() { FirstName = "Jame", LastName = "Smith", Age = 28 };
List<Person> people = new List<Person>(new Person[] { p1, p2, p3 });
IEnumerable<Person> foundPeople = people.Where(p => p.GetType().GetProperty("LastName").GetValue(p,null).ToString().StartsWith("J"));
foreach (Person person in foundPeople)
{
Console.WriteLine(person.ToString());
}
Console.ReadKey();
}
}
Just be careful because this could lead to performance issues if you are querying a large amount data.
