Create generic selector for Select() while using Entity Framework

Question

I would like to create a function to retrieve a List of a given property name's Type. But i dont know yet how to create a working lambda selector.

public IList<object> GetDistinctListOfProperty(string propertyName)
{
    var propInfoByName = typeof(T).GetProperty(propertyName);
    if (propInfoByName == null) return new List<object>();

    using (var db = new ApplicationDbContext())
    {
        var lambda = //TODO
        return db.Set<T>().Select(lambda).Distinct().ToList();
    }
}

Do you have to pass in the property name as a string? Could you pass in an Expression instead? — ste-fu, Jun 27 '19 at 11:08
i am working with nameof(modelClass.property) as parameter, thats why string is used. @RenéVogt yes true thanks for the hint. — eddy white, Jun 27 '19 at 11:50

score 4 · Accepted Answer · edited Jun 27 '19 at 13:08

4

You can use this method to generate lambda

public static Expression<Func<T, object>> LambdaGenerator<T>(string propertyName)
    {
        var arg = Expression.Parameter(typeof(T), "current");
        var property = Expression.Property(arg, propertyName);
        var conv = Expression.Convert(property, typeof(object));
        var exp = Expression.Lambda<Func<T, object>>(conv, new ParameterExpression[] { arg });
        return exp;
    }

and then use this method to create your expression

var lambda = LambdaGenerator<T>("Your Property Name");

edited Jun 27 '19 at 13:08

Mong Zhu

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answered Jun 27 '19 at 11:25

Mofid.Moghimi

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had to change object to string, to use it also to populate my comboboxes but it works now. Thank you. – eddy white Jun 27 '19 at 12:51

Mong Zhu · Answer 2 · 2019-06-27T13:07:59.923

You can use an expression for this:

private IList<Tout> GetDistinctListOfProperty<Ttable, Tout>(Expression<Func<Ttable, Tout>> returnField) where Ttable : class
{
    using (var db = new ApplicationDbContext())
    {
        return db.Set<Ttable>().Select(returnField).Distinct().ToList();
    }
}

You need to wrap the Func into an Expression so that entity can translate it into a valid sql. This version allows you to use intellisense when you choose your parameter. You would call it like this:

var result = GetDistinctListOfProperty<YourTableType>(x => x.YourProperty);

This version will work on every table that is known to your ApplicationDbContext

@eddywhite then you probably should use `Set`. I edited my post, please try again — Mong Zhu, Jun 27 '19 at 13:07

Create generic selector for Select() while using Entity Framework

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