OrderBy Expression Tree in Net Core Linq for Extension Method

Question

I want to create an Extension method which mimics this, https://dejanstojanovic.net/aspnet/2019/january/filtering-and-paging-in-aspnet-core-web-api/

However, I want to add an OrderBy (for ColumnName) after StartsWith, how would I conduct this?

tried adding following and did not work .OrderBy(parameter)

Example:

return persons.Where(p => p.Name.StartsWith(filterModel.Term ?? String.Empty, StringComparison.InvariantCultureIgnoreCase))
   .OrderBy(c=>c.Name)  
   .Skip((filterModel.Page-1) * filter.Limit)  
   .Take(filterModel.Limit);  


public static class PaginateClass
{
    static readonly MethodInfo startsWith = typeof(string).GetMethod("StartsWith", new[] { typeof(string), typeof(System.StringComparison) });

    public static IEnumerable<T> Paginate<T>(this IEnumerable<T> input, PageModel pageModel, string columnName) where T : class
    {
        var type = typeof(T);
        var propertyInfo = type.GetProperty(columnName);
        //T p =>
        var parameter = Expression.Parameter(type, "p");
        //T p => p.ColumnName
        var name = Expression.Property(parameter, propertyInfo);
        // filterModel.Term ?? String.Empty
        var term = Expression.Constant(pageModel.Term ?? String.Empty);
        //StringComparison.InvariantCultureIgnoreCase
        var comparison = Expression.Constant(StringComparison.InvariantCultureIgnoreCase);
        //T p => p.ColumnName.StartsWith(filterModel.Term ?? String.Empty, StringComparison.InvariantCultureIgnoreCase)
        var methodCall = Expression.Call(name, startsWith, term, comparison);

        var lambda = Expression.Lambda<Func<T, bool>>(methodCall, parameter);


            return input.Where(lambda.Compile()) //tried adding this and did not work .OrderBy(parameter)  
            .Skip((pageModel.Page - 1) * pageModel.Limit)
            .Take(pageModel.Limit);

    }

Other items PageModel:

public class PageModel
{

    public int Page { get; set; }
    public int Limit { get; set; }
    public string Term { get; set; }

    public PageModel()
    {
        this.Page = 1;
        this.Limit = 3;
    }

    public object Clone()
    {
        var jsonString = JsonConvert.SerializeObject(this);
        return JsonConvert.DeserializeObject(jsonString, this.GetType());
    }
}

Dynamic Linq to Entities Orderby with Pagination

Answer 1

Check the sample code for the solution:

void Main()
{
    var queryableRecords = Product.FetchQueryableProducts();

    Expression expression = queryableRecords.OrderBy("Name");

    var func = Expression.Lambda<Func<IQueryable<Product>>>(expression)
                         .Compile();

    func().Dump();
}

public class Product
{
    public int Id { get; set; }

    public string Name { get; set; }

    public static IQueryable<Product> FetchQueryableProducts()
    {
        List<Product> productList = new List<Product>()
        {
          new Product {Id=1, Name = "A"},
          new Product {Id=1, Name = "B"},
          new Product {Id=1, Name = "A"},
          new Product {Id=2, Name = "C"},
          new Product {Id=2, Name = "B"},
          new Product {Id=2, Name = "C"},
        };

        return productList.AsQueryable();
    }
}

public static class ExpressionTreesExtesion
{

    public static Expression OrderBy(this IQueryable queryable, string propertyName)
    {
        var propInfo = queryable.ElementType.GetProperty(propertyName);

        var collectionType = queryable.ElementType;

        var parameterExpression = Expression.Parameter(collectionType, "g");
        var propertyAccess = Expression.MakeMemberAccess(parameterExpression, propInfo);
        var orderLambda = Expression.Lambda(propertyAccess, parameterExpression);
        return Expression.Call(typeof(Queryable),
                               "OrderBy",
                               new Type[] { collectionType, propInfo.PropertyType },
                               queryable.Expression,
                               Expression.Quote(orderLambda));

    }


}

Result

How it Works:

Created an expression using extension method on the Queryable type, which internally calls OrderBy method of the Queryable type, expecting IQueryable to be the Input, along with the field name and thus runs the ordering function and Ordered collection is the final Output

Option 2:

This may fit your use case better, here instead of calling OrderBy method, we are creating the Expression<Func<T,string>> as an extension method to the IEnumerable<T>, which can then be compiled and supplied to the OrderBy Call, as shown in the example and is thus much more intuitive and simple solution:

Creating Expression:

public static class ExpressionTreesExtesion
{
    public static Expression<Func<T,string>> OrderByExpression<T>(this IEnumerable<T> enumerable, string propertyName)
    {
        var propInfo = typeof(T).GetProperty(propertyName);

        var collectionType = typeof(T);

        var parameterExpression = Expression.Parameter(collectionType, "x");
        var propertyAccess = Expression.MakeMemberAccess(parameterExpression, propInfo);
        var orderExpression = Expression.Lambda<Func<T,string>>(propertyAccess, parameterExpression);
        return orderExpression;
    }
}

How to Call:

var ProductExpression = records.OrderByExpression("Name");

var result  = records.OrderBy(ProductExpression.Compile());

ProductExpression.Compile() above will compile into x => x.Name, where column name is supplied at the run-time

Please note in case the ordering field can be other types beside string data type, then make that also generic and supply it while calling extension method, only condition being property being called shall have the same type as supplied value, else it will be a run-time exception, while creating Expression

Edit 1, how to make the OrderType field also generic

public static Expression<Func<T, TField>> OrderByFunc<T,TField>(this IEnumerable<T> enumerable, string propertyName)
    {
        var propInfo = typeof(T).GetProperty(propertyName);

        var collectionType = typeof(T);

        var parameterExpression = Expression.Parameter(collectionType, "x");
        var propertyAccess = Expression.MakeMemberAccess(parameterExpression, propInfo);
        var orderExpression = Expression.Lambda<Func<T, TField>>(propertyAccess, parameterExpression);
        return orderExpression;
    }

How to call:

Now both the types need to be explicitly supplied, earlier were using generic type inference from IEnumerable<T>:

// For Integer Id field

var ProductExpression = records.OrderByFunc<Product,int>("Id");

// For string name field

var ProductExpression = records.OrderByFunc<Product,string>("Name");