Ewald Stieger Ewald Stieger - 1 month ago 13
C# Question

Generic method to get property values with Linq Expression and reflection

Dear Gods of Reflection

I would like to have a generic

GetValue<TEntity, T>
method that can return the following property values given the following
User
class:

public class User
{
public int Id { get; set; }
public int ClientId { get; set; }
public string UserName { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string MobileNumber { get; set; }
public bool IsActive { get; set; }

public Client Client { get; set; }
public List<Package> Packages { get; set; }

}


Example usage of what
GetValue<TEntity, T>
should be able to do:

var firstName = dataCollector.GetValue<User, string>(x => x.FirstName);
var client = dataCollector.GetValue<User, Client>(x => x.Client);
var packages = dataCollector.GetValue<User, List<Package>>(x => x.Packages);

var packageFirst = dataCollector.GetValue<User, Package>(x => x.Packages[0]);
var packageName = dataCollector.GetValue<User, string>(x => x.Packages[0].Name);
var clientName = dataCollector.GetValue<User, string>(x => x.Client.Name);


So far I have the following method which works for the first 3 scenarios:

public T GetValue<TEntity, T>(Expression<Func<TEntity, T>> propertyExpression) where TEntity : class
{
var response = _responses.FirstOrDefault(p => p.GetType() == typeof(TEntity)) as TEntity;
if (response != null)
{
var expr = (MemberExpression)propertyExpression.Body;
var prop = (PropertyInfo)expr.Member;
return (T)prop.GetValue(response);
}
return default(T);
}


But it does not work for the last 3 scenarios:

var packageFirst = dataCollector.GetValue<User, Package>(x => x.Packages[0]);


Throws:
Unable to cast object of type 'System.Linq.Expressions.InstanceMethodCallExpressionN' to type 'System.Linq.Expressions.MemberExpression'.


var packageName = dataCollector.GetValue<User, string>(x => x.Packages[0].Name);


Throws:
Object does not match target type.


var clientName = dataCollector.GetValue<User, string>(x => x.Client.Name);


Throws:
Object does not match target type.


What changes do I need to make to the method?

I shall now sacrifice a USB flash drive whilst awaiting your answers :)

Answer

The problem is here:

if (response != null)
{
   var expr = (MemberExpression)propertyExpression.Body;
   var prop = (PropertyInfo)expr.Member;
   return (T)prop.GetValue(response);
}

This only works if your expression references a property directly, otherwise propertyExpression.Body will not be a MemberExpression and you'll get a run-time cast error. The three that don't work do not reference a property directly - the first two reverence a method on top of a property (the indexer) and the last references a nested property.

Since all you want is the value of the expression, though, I think you can just do:

if (response != null)
{
   Func<TEntity, T> func = propertyExpression.Compile();  
   return func(response);
}

If you intended to do other things with the expression (like get the name of the property), then you'll need to decide if you want to support expressions that don't reference a property directly and add handlers for that.

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