How to infer TResult from the actual Type?

Question

I am using the Entity Framework, ASP.NET and C#3.5 I borrowed the following code to make sorting possible using a sortExpression from a GridView instead of the property of an entity:

    public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, string sortExpression)
    {
        string[] sortParts = sortExpression.Split(' ');
        var param = Expression.Parameter(typeof(T), string.Empty);

        var property = Expression.Property(param, sortParts[0]);

        var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), param);           

        if (sortParts.Length > 1 && sortParts[1].Equals("desc", StringComparison.OrdinalIgnoreCase))
        {
            return source.AsQueryable<T>().OrderByDescending(sortLambda);
        }

        return source.AsQueryable<T>().OrderBy(sortLambda);
    }

The problem is that LINQ to Entities does not support casting to object. Instead of Func, I need the actual return type instead of object. I worked out how to do it:

    public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, string sortExpression)
    {
        string[] sortParts = sortExpression.Split(' ');
        var param = Expression.Parameter(typeof(T), string.Empty);

        var property = Expression.Property(param, sortParts[0]);

        // NEW CODE HERE
        Type propertyType = property.Type;
        Type lambdaType = typeof(Func<,>).MakeGenericType(typeof(T), propertyType);
        var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, propertyType), param);
        //var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), param);

        if (sortParts.Length > 1 && sortParts[1].Equals("desc", StringComparison.OrdinalIgnoreCase))
        {
            return source.AsQueryable<T>().OrderByDescending(sortLambda);
        }

        return source.AsQueryable<T>().OrderBy(sortLambda);
    }

The problem now is that if I have an Int32, it will not cast to object, which is still the return type. I worked around it like this:

    public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, string sortExpression)
    {
        string[] sortParts = sortExpression.Split(' ');
        var param = Expression.Parameter(typeof(T), string.Empty);

        var property = Expression.Property(param, sortParts[0]);

        // New code here
        Type propertyType = property.Type;
        Type lambdaType = typeof(Func<,>).MakeGenericType(typeof(T), propertyType);

        // NEWEST CODE HERE
        var sortLambda = Expression.Lambda(lambdaType, Expression.Convert(property, propertyType), param);
        //var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, propertyType), param);
        //var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), param);


        if (sortParts.Length > 1 && sortParts[1].Equals("desc", StringComparison.OrdinalIgnoreCase))
        {
            return source.AsQueryable<T>().OrderByDescending(sortLambda);
        }

        return source.AsQueryable<T>().OrderBy(sortLambda);
    }

This however does not compile any longer. The error is:

The type arguments for method 'System.Linq.Enumerable.OrderByDescending(System.Collections.Generic.IEnumerable, System.Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

The problem is that I do not want to specify the type arguments explicitly.

Does anyone know how to work around this or anyhow infer the TResult type from "propertyType"?

Answer 1

The approach used here, with the refinement here, should do what you need. By necessity it uses reflection to do the guts in the middle, but it works. Note also that Expression.GetFuncType and Expression.GetActionType can avoid some of the work in your approach.