FunctionStartup.cs

using System.Web.Http;
using Microsoft.AspNetCore.OData.Batch;
using Microsoft.OData.Edm;
using WebApiOData.V4.Samples.Controllers;
using WebApiOData.V4.Samples.Models;

namespace WebApiOData.V4.Samples.Startups;

public class FunctionStartup : Startup
{
	public FunctionStartup()
		: base(typeof(ProductsController))
	{
	}

	protected override void ConfigureController(HttpConfiguration config)
	{
		config.MapODataServiceRoute(
			routeName: "OData functions",
			routePrefix: "functions",
			model: GetEdmModel(config),
			batchHandler: new DefaultODataBatchHandler(new HttpServer(config)));
	}

	private static IEdmModel GetEdmModel(HttpConfiguration config)
	{
		ODataModelBuilder builder = new ODataConventionModelBuilder(config);

		builder.EntitySet<Product>("Products");

		var productType = builder.EntityType<Product>();

		// Function bound to a collection
		// Returns the most expensive product, a single entity
		productType.Collection
			.Function("MostExpensive")
			.Returns<double>();

		// Function bound to a collection
		// Returns top 3 product, a collection
		productType.Collection
			.Function("MostExpensives")
			.ReturnsCollectionFromEntitySet<Product>("Products");

		// Function bound to a collection
		// Returns the top 10 product, a collection
		productType.Collection
			.Function("Top10")
			.ReturnsCollectionFromEntitySet<Product>("Products");

		// Function bound to a single entity
		// Returns the instance's price rank among all products
		productType
			.Function("GetPriceRank")
			.Returns<int>();

		// Function bound to a single entity
		// Accept a string as parameter and return a double
		// This function calculate the general sales tax base on the 
		// state
		productType
			.Function("CalculateGeneralSalesTax")
			.Returns<double>()
			.Parameter<string>("state");

		// Function bound to a single entity
		// Returns the list of movies with a product placement for this product
		productType
			.Function("Placements")
			.ReturnsCollectionFromEntitySet<Movie>("Movies");

		// Unbound Function
		builder.Function("GetSalesTaxRate")
			.Returns<double>()
			.Parameter<string>("state");

		return builder.GetEdmModel();
	}
}