asp.net-coreasp.net-core-mvcc++tag-helpersurl-parameters
I have a lot of experience with ASP.NET MVC 1-5. Now I learn ASP.NET Core MVC and have to pass a parameter to link in page. For example I have the following Action
[HttpGet]
public ActionResult GetProduct(string id)
{
ViewBag.CaseId = id;
return View();
}
How can I implement the link for this action using tag helpers?
<a asp-controller="Product" asp-action="GetProduct">ProductName</a>
The approach recommended by the ASP.Net Core team is to use the new policy design which is fully documented here. The basic idea behind the new approach is to use the new [Authorize] attribute to designate a "policy" (e.g. [Authorize( Policy = "YouNeedToBe18ToDoThis")] where the policy is registered in the application's Startup.cs to execute some block of code (i.e. ensure the user has an age claim where the age is 18 or older).
The policy design is a great addition to the framework and the ASP.Net Security Core team should be commended for its introduction. That said, it isn't well-suited for all cases. The shortcoming of this approach is that it fails to provide a convenient solution for the most common need of simply asserting that a given controller or action requires a given claim type. In the case where an application may have hundreds of discrete permissions governing CRUD operations on individual REST resources ("CanCreateOrder", "CanReadOrder", "CanUpdateOrder", "CanDeleteOrder", etc.), the new approach either requires repetitive one-to-one mappings between a policy name and a claim name (e.g. options.AddPolicy("CanUpdateOrder", policy => policy.RequireClaim(MyClaimTypes.Permission, "CanUpdateOrder));), or writing some code to perform these registrations at run time (e.g. read all claim types from a database and perform the aforementioned call in a loop). The problem with this approach for the majority of cases is that it's unnecessary overhead.
While the ASP.Net Core Security team recommends never creating your own solution, in some cases this may be the most prudent option with which to start.
The following is an implementation which uses the IAuthorizationFilter to provide a simple way to express a claim requirement for a given controller or action:
public class ClaimRequirementAttribute : TypeFilterAttribute
{
public ClaimRequirementAttribute(string claimType, string claimValue) : base(typeof(ClaimRequirementFilter))
{
Arguments = new object[] {new Claim(claimType, claimValue) };
}
}
public class ClaimRequirementFilter : IAuthorizationFilter
{
readonly Claim _claim;
public ClaimRequirementFilter(Claim claim)
{
_claim = claim;
}
public void OnAuthorization(AuthorizationFilterContext context)
{
var hasClaim = context.HttpContext.User.Claims.Any(c => c.Type == _claim.Type && c.Value == _claim.Value);
if (!hasClaim)
{
context.Result = new ForbidResult();
}
}
}
[Route("api/resource")]
public class MyController : Controller
{
[ClaimRequirement(MyClaimTypes.Permission, "CanReadResource")]
[HttpGet]
public IActionResult GetResource()
{
return Ok();
}
}
In your GET action, create an object of your view model, load the EmployeeList collection property and send that to the view.
public IActionResult Create()
{
var vm = new MyViewModel();
vm.EmployeesList = new List<Employee>
{
new Employee { Id = 1, FullName = "Shyju" },
new Employee { Id = 2, FullName = "Bryan" }
};
return View(vm);
}
And in your create view, create a new SelectList object from the EmployeeList property and pass that as value for the asp-items property.
@model MyViewModel
<form asp-controller="Home" asp-action="Create">
<select asp-for="EmployeeId"
asp-items="@(new SelectList(Model.EmployeesList, nameof(Employee.Id), nameof(Employee.FullName)))">
<option>Please select one</option>
</select>
<input type="submit"/>
</form>
And your HttpPost action method to accept the submitted form data.
[HttpPost]
public IActionResult Create(MyViewModel model)
{
// check model.EmployeeId
// to do : Save and redirect
}
Or
If your view model has a List<SelectListItem> as the property for your dropdown items.
public class MyViewModel
{
public int EmployeeId { get; set; }
public string Comments { get; set; }
public List<SelectListItem> Employees { set; get; }
}
And in your get action,
public IActionResult Create()
{
var vm = new MyViewModel();
vm.Employees = new List<SelectListItem>
{
new SelectListItem {Text = "Shyju", Value = "1"},
new SelectListItem {Text = "Sean", Value = "2"}
};
return View(vm);
}
And in the view, you can directly use the Employees property for the asp-items.
@model MyViewModel
<form asp-controller="Home" asp-action="Create">
<label>Comments</label>
<input type="text" asp-for="Comments"/>
<label>Lucky Employee</label>
<select asp-for="EmployeeId" asp-items="@Model.Employees" >
<option>Please select one</option>
</select>
<input type="submit"/>
</form>
The class SelectListItem belongs to Microsoft.AspNet.Mvc.Rendering namespace.
Make sure you are using an explicit closing tag for the select element. If you use the self closing tag approach, the tag helper will render an empty SELECT element!
The below approach will not work
<select asp-for="EmployeeId" asp-items="@Model.Employees" />
But this will work.
<select asp-for="EmployeeId" asp-items="@Model.Employees"></select>
The above examples are using hard coded items for the options. So i thought i will add some sample code to get data using Entity framework as a lot of people use that.
Let's assume your DbContext object has a property called Employees, which is of type DbSet<Employee> where the Employee entity class has an Id and Name property like this
public class Employee
{
public int Id { set; get; }
public string Name { set; get; }
}
You can use a LINQ query to get the employees and use the Select method in your LINQ expression to create a list of SelectListItem objects for each employee.
public IActionResult Create()
{
var vm = new MyViewModel();
vm.Employees = context.Employees
.Select(a => new SelectListItem() {
Value = a.Id.ToString(),
Text = a.Name
})
.ToList();
return View(vm);
}
Assuming context is your db context object. The view code is same as above.
Some people prefer to use SelectList class to hold the items needed to render the options.
public class MyViewModel
{
public int EmployeeId { get; set; }
public SelectList Employees { set; get; }
}
Now in your GET action, you can use the SelectList constructor to populate the Employees property of the view model. Make sure you are specifying the dataValueField and dataTextField parameters. You can use a nameof expression to link the field names statically.
public IActionResult Create()
{
var vm = new MyViewModel();
vm.Employees = new SelectList(GetEmployees(), nameof(Employee.Id), nameof(Employee.FirstName));
return View(vm);
}
public IEnumerable<Employee> GetEmployees()
{
// hard coded list for demo.
// You may replace with real data from database to create Employee objects
return new List<Employee>
{
new Employee { Id = 1, FirstName = "Shyju" },
new Employee { Id = 2, FirstName = "Bryan" }
};
}
Here I am calling the GetEmployees method to get a list of Employee objects, each with an Id and FirstName property and I use those properties as DataValueField and DataTextField of the SelectList object we created. You can change the hardcoded list to a code which reads data from a database table.
The view code will be same.
<select asp-for="EmployeeId" asp-items="@Model.Employees" >
<option>Please select one</option>
</select>
Sometimes you might want to render a select element from a list of strings. In that case, you can use the SelectList constructor which only takes IEnumerable<T>
var vm = new MyViewModel();
var items = new List<string> {"Monday", "Tuesday", "Wednesday"};
vm.Employees = new SelectList(items);
return View(vm);
The view code will be same.
Some times,you might want to set one option as the default option in the SELECT element (For example, in an edit screen, you want to load the previously saved option value). To do that, you may simply set the EmployeeId property value to the value of the option you want to be selected.
public IActionResult Create()
{
var vm = new MyViewModel();
vm.Employees = new List<SelectListItem>
{
new SelectListItem {Text = "Shyju", Value = "11"},
new SelectListItem {Text = "Tom", Value = "12"},
new SelectListItem {Text = "Jerry", Value = "13"}
};
vm.EmployeeId = 12; // Here you set the value
return View(vm);
}
This will select the option Tom in the select element when the page is rendered.
If you want to render a multi select dropdown, you can simply change your view model property which you use for asp-for attribute in your view to an array type.
public class MyViewModel
{
public int[] EmployeeIds { get; set; }
public List<SelectListItem> Employees { set; get; }
}
This will render the HTML markup for the select element with the multiple attribute which will allow the user to select multiple options.
@model MyViewModel
<select id="EmployeeIds" multiple="multiple" name="EmployeeIds">
<option>Please select one</option>
<option value="1">Shyju</option>
<option value="2">Sean</option>
</select>
Similar to single select, set the EmployeeIds property value to the an array of values you want.
public IActionResult Create()
{
var vm = new MyViewModel();
vm.Employees = new List<SelectListItem>
{
new SelectListItem {Text = "Shyju", Value = "11"},
new SelectListItem {Text = "Tom", Value = "12"},
new SelectListItem {Text = "Jerry", Value = "13"}
};
vm.EmployeeIds= new int[] { 12,13} ;
return View(vm);
}
This will select the option Tom and Jerry in the multi select element when the page is rendered.
If you do not prefer to keep a collection type property to pass the list of options to the view, you can use the dynamic ViewBag to do so.(This is not my personally recommended approach as viewbag is dynamic and your code is prone to uncaught typo errors)
public IActionResult Create()
{
ViewBag.Employees = new List<SelectListItem>
{
new SelectListItem {Text = "Shyju", Value = "1"},
new SelectListItem {Text = "Sean", Value = "2"}
};
return View(new MyViewModel());
}
and in the view
<select asp-for="EmployeeId" asp-items="@ViewBag.Employees">
<option>Please select one</option>
</select>
It is same as above. All you have to do is, set the property (for which you are binding the dropdown for) value to the value of the option you want to be selected.
public IActionResult Create()
{
ViewBag.Employees = new List<SelectListItem>
{
new SelectListItem {Text = "Shyju", Value = "1"},
new SelectListItem {Text = "Bryan", Value = "2"},
new SelectListItem {Text = "Sean", Value = "3"}
};
vm.EmployeeId = 2; // This will set Bryan as selected
return View(new MyViewModel());
}
and in the view
<select asp-for="EmployeeId" asp-items="@ViewBag.Employees">
<option>Please select one</option>
</select>
The select tag helper method supports grouping options in a dropdown. All you have to do is, specify the Group property value of each SelectListItem in your action method.
public IActionResult Create()
{
var vm = new MyViewModel();
var group1 = new SelectListGroup { Name = "Dev Team" };
var group2 = new SelectListGroup { Name = "QA Team" };
var employeeList = new List<SelectListItem>()
{
new SelectListItem() { Value = "1", Text = "Shyju", Group = group1 },
new SelectListItem() { Value = "2", Text = "Bryan", Group = group1 },
new SelectListItem() { Value = "3", Text = "Kevin", Group = group2 },
new SelectListItem() { Value = "4", Text = "Alex", Group = group2 }
};
vm.Employees = employeeList;
return View(vm);
}
There is no change in the view code. the select tag helper will now render the options inside 2 optgroup items.
Best Answer
You can use the attribute prefix
asp-route-to prefix your route variable names.Example: