[javascript] How to call a C# function from JavaScript?

A modern approach is to use ASP.NET Web API 2 (server-side) with jQuery Ajax (client-side).

Like page methods and ASMX web methods, Web API allows you to write C# code in ASP.NET which can be called from a browser or from anywhere, really!

Here is an example Web API controller, which exposes API methods allowing clients to retrieve details about 1 or all products (in the real world, products would likely be loaded from a database):

public class ProductsController : ApiController
{
    Product[] products = new Product[] 
    { 
        new Product { Id = 1, Name = "Tomato Soup", Category = "Groceries", Price = 1 }, 
        new Product { Id = 2, Name = "Yo-yo", Category = "Toys", Price = 3.75M }, 
        new Product { Id = 3, Name = "Hammer", Category = "Hardware", Price = 16.99M } 
    };

    [Route("api/products")]
    [HttpGet]
    public IEnumerable<Product> GetAllProducts()
    {
        return products;
    }

    [Route("api/product/{id}")]
    [HttpGet]
    public IHttpActionResult GetProduct(int id)
    {
        var product = products.FirstOrDefault((p) => p.Id == id);
        if (product == null)
        {
            return NotFound();
        }
        return Ok(product);
    }
}

The controller uses this example model class:

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Category { get; set; }
    public decimal Price { get; set; }
}

Example jQuery Ajax call to get and iterate over a list of products:

$(document).ready(function () {
    // Send an AJAX request
    $.getJSON("/api/products")
        .done(function (data) {
            // On success, 'data' contains a list of products.
            $.each(data, function (key, item) {
                // Add a list item for the product.
                $('<li>', { text: formatItem(item) }).appendTo($('#products'));
            });
        });
});

Not only does this allow you to easily create a modern Web API, you can if you need to get really professional and document it too, using ASP.NET Web API Help Pages and/or Swashbuckle.

Web API can be retro-fitted (added) to an existing ASP.NET Web Forms project. In that case you will need to add routing instructions into the Application_Start method in the file Global.asax:

RouteTable.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = System.Web.Http.RouteParameter.Optional }
);

Documentation

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