Get dynamic OrderBy in LINQ

Question

I am using datatables in quite a few pages in my ASP.NET MVC 3 site. They use server side paging and now I want to implement sorting based on the column headers. Datatables comes with iSortCol_0 which is an int value of what column is clicked.

I did not like this approach as the query would end up something like:

if(iSortCol_0 == 0)
{    
    // query
    // OderBy(x => x.CarName)
}

Then this would be repeated for each column (plus an else clause on each to order by descending). So I have changed my approach and now pass the column name to the server.

I have come up with the following:

Expression<Func<vw_Car, string>> sortExpression1 = null;
Expression<Func<vw_Car, int>> sortExpression2 = null;

switch(columnToSort) 
{
    case "InvoiceNo": sortExpression1 = x => x.CarNo; break;
    case "ClientNumber": sortExpression1 = x => x.ForeName; break;
    case "ClientName": sortExpression1 = x => x.SurName; break;
    default: sortExpression2 =  x => x.Age.Value; break;
}

// start of query
.OrderByDir(sortDirection, sortExpression1 , sortExpression2)

Now the OrderByDir looks like below:

public static IOrderedQueryable<T> OrderByDir<T>(this IQueryable<T> source, string dir, Expression<Func<T, string>> column1, Expression<Func<T, int>> column2)
{
    if (column1 != null)
    {
        return dir == "asc" ? source.OrderBy(column1) : source.OrderByDescending(column1);
    }         
    if (column2 != null)
    {
        return dir == "asc" ? source.OrderBy(column2) : source.OrderByDescending(column2);
    }    
    return null;
}

This works in that it sorts columns that are of type string or int. I have another column of DateTime that I want to sort, so I would need to write another sortExpression3 and then add that to my OrderByDir.

However I don't really like the implementation - I had attempted to write a generic sort expression that took an object as second parameter instead of string, int, Datetime, but when had this code in place, I was getting Unable to cast the type 'System.DateTime' to type 'System.Object'. LINQ to Entities only supports casting Entity Data Model primitive types.

Any one any ideas as to a possible better approach to this?

Answer 1

Such extension should fits your needs:

public static class LinqExtensions
{
    public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder(source, property, "OrderBy");
    }

    public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder(source, property, "OrderByDescending");
    }

    private static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, string methodName)
    {
        var props = property.Split('.');
        var type = typeof(T);
        var arg = Expression.Parameter(type, "x");
        Expression expr = arg;
        foreach (var prop in props)
        {
            // use reflection (not ComponentModel) to mirror LINQ
            var pi = type.GetProperty(prop);
            expr = Expression.Property(expr, pi);
            type = pi.PropertyType;
        }
        var delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
        var lambda = Expression.Lambda(delegateType, expr, arg);

        var result = typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { source, lambda });
        return (IOrderedQueryable<T>)result;
    }
}

Invoke example:

query.OrderBy("Age.Value");