Linq: Queryable.OrderBy() using a collection of Expressions

Question

I wonder how I can store orderby expressions in a list. This is what I wanted to write:

List<Expression<Func<Products,Object>>> list = new List<Expression<Func<Products,Object>>>()
{
  p => p.Name,
  p => p.Id
};

Then:

var expr = list[0];
myProducts.OrderBy( expr );

which works for p.Name, but does not work for p.Id (list[1]) as it drops follwing exception

An unhandled exception of type 'System.NotSupportedException' occurred in EntityFramework.SqlServer.dll Additional information: Unable to cast the type 'System.Int32' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types.

What type of list do I have to use?

Answer 1

Here's my solution (using Reflection and based on DynamicLinq ideas):

Defining a ConvertableExpression class so we can intercept calls to our custom OrderBy():

public class ConvertableExpression<T>
{
    public ConvertableExpression(Expression<Func<T, object>> expr)
    {
        this.Expression = expr;
    }

    public Expression<Func<T, object>> Expression { get; private set; }
}

Introducing an Extension-Method for easier casting from normal Expression:

public static class ExpressionExtensions
{
    public static ConvertableExpression<T> AsConvertable<T>(this Expression<Func<T, object>> expr)
    {
        return new ConvertableExpression<T>(expr);
    }
}

Extending IQueryable with Reflection-based implementation of OrderBy():

public static class QueryableExtensions
{
    public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, ConvertableExpression<T> expr)
    {
        Expression queryExpr = source.Expression;

        var exprBody = SkipConverts(expr.Expression.Body);
        var lambda = Expression.Lambda(exprBody, expr.Expression.Parameters);
        var quote = Expression.Quote(lambda);

        queryExpr = Expression.Call(typeof(Queryable), "OrderBy", new[] { source.ElementType, exprBody.Type }, queryExpr, quote);

        return (IOrderedQueryable<T>)source.Provider.CreateQuery(queryExpr);
    }

    private static Expression SkipConverts(Expression expression)
    {
        Expression result = expression;

        while (result.NodeType == ExpressionType.Convert || result.NodeType == ExpressionType.ConvertChecked)
            result = ((UnaryExpression)result).Operand;

        return result;
    }
}

Usage:

myProducts.OrderBy(expr.AsConvertable());