Dynamically build select list from linq to entities query

Question

I\'m looking for a way to dynamically create a select list from a iQueryable object.

Concrete example, i want to do something like the following:

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Answer 1

Dynamic select expression to a compile time known type can easily be build using Expression.MemberInit method with MemberBindings created using the Expression.Bind method.

Here is a custom extension method that does that:

public static class QueryableExtensions
{
    public static IQueryable Select(this IQueryable source, string[] columns)
    {
        var sourceType = source.ElementType;
        var resultType = typeof(TResult);
        var parameter = Expression.Parameter(sourceType, "e");
        var bindings = columns.Select(column => Expression.Bind(
            resultType.GetProperty(column), Expression.PropertyOrField(parameter, column)));
        var body = Expression.MemberInit(Expression.New(resultType), bindings);
        var selector = Expression.Lambda(body, parameter);
        return source.Provider.CreateQuery(
            Expression.Call(typeof(Queryable), "Select", new Type[] { sourceType, resultType },
                source.Expression, Expression.Quote(selector)));
    }
}

The only problem is what is the TResult type. In EF Core you can pass the entity type (like EntityModel.Core.User in your example) and it will work. In EF 6 and earlier, you need a separate non entity type because otherwise you'll get NotSupportedException - The entity or complex type cannot be constructed in a LINQ to Entities query.

UPDATE: If you want a to get rid of the string columns, I can suggest you replacing the extension method with the following class:

public class SelectList
{
    private List members = new List();
    public SelectList Add(Expression> selector)
    {
        var member = ((MemberExpression)selector.Body).Member;
        members.Add(member);
        return this;
    }
    public IQueryable Select(IQueryable source)
    {
        var sourceType = typeof(TSource);
        var resultType = typeof(TResult);
        var parameter = Expression.Parameter(sourceType, "e");
        var bindings = members.Select(member => Expression.Bind(
            resultType.GetProperty(member.Name), Expression.MakeMemberAccess(parameter, member)));
        var body = Expression.MemberInit(Expression.New(resultType), bindings);
        var selector = Expression.Lambda>(body, parameter);
        return source.Select(selector);
    }
}

with sample usage:

var selectList = new SelectList();
selectList.Add(e => e.UserType);
selectList.Add(e => e.Name);

var selectResult = selectList.Select(entities);