Select some properties based on $select query

Extending IQueryable.md

I wanted to have a replacement for the OData package in dotnet core, since I was only using the $select part of it.

I've asekd the question on StackOverflow https://stackoverflow.com/questions/58836295/create-a-dynamic-linq-statement and got a great answer.

Below is the Generic version of the code provided in the answer.

IQueryablyExtensions.cs

// Generic version created from this sample https://stackoverflow.com/a/58836648/639153
public static class IQueryableExtensions {
        public static IQueryable<TSource> SelectProperties<TSource>(
        this IQueryable<TSource> sources,
        ISet<string> propertiesToSelect)
    {
        if (sources == null) throw new ArgumentNullException(nameof(sources));
        if (propertiesToSelect == null) throw new ArgumentNullException(nameof(propertiesToSelect));

        // Check if properties exist in TSource
        var properties = typeof(TSource).GetProperties().Where(p => propertiesToSelect.Contains(p.Name)).Select(p => p.Name);
        if (!properties.Any()) throw new ArgumentException("Specified properties not found", nameof(propertiesToSelect));

        var type = typeof(TSource);
        // p =>
        var parameter = Expression.Parameter(type, "p");
        // create bindings for initialization
        var bindings = properties
            .Select(s => {
                // property
                var property = type.GetProperty(s);
                // original value 
                var propertyExpression = Expression.Property(parameter, property);
                // set value "Property = p.Property"
                return Expression.Bind(property, propertyExpression);
            }
        );
        // new TSource()
        var newViewModel = Expression.New(type);
        // new TSource { Property1 = p.Property1, ... }
        var body = Expression.MemberInit(newViewModel, bindings);
        // p => new TSource { Property1 = p.Property1, ... }
        var selector = Expression.Lambda<Func<TSource, TSource>>(body, parameter);

        return sources.Select(selector);
    }

    public static IQueryable<TSource> SelectProperties<TSource>(this IQueryable<TSource> sources, string propertiesToSelect)
    {
        if (sources == null) throw new ArgumentNullException(nameof(sources));
        if (propertiesToSelect == null) throw new ArgumentNullException(nameof(propertiesToSelect));

        HashSet<string> names = new HashSet<string>();
        var properties = propertiesToSelect.Split(',');
        foreach (var p in properties)
        {
            names.Add(p);
        }
        return sources.SelectProperties(names);
    }
}

Usage.cs

// Use the IQueryableExtensions on some IQueryable

someDbContext.People.AsQueryable().SelectProperties("Id,DisplayName,...")