Select Tag Helper in ASP.NET Core MVC

问题:

I need some help with the select tag helper in ASP.NET Core.

I have a list of employees that I\'m trying to bind to a select tag helper. My employees are in a List<Employee> EmployeesList and selected value will go into EmployeeId property. My view model looks like this:

public class MyViewModel
{
   public int EmployeeId { get; set; }
   public string Comments { get; set; }
   public List<Employee> EmployeesList {get; set; }
}

My employee class looks like this:

public class Employee
{
   public int Id { get; set; }
   public string FullName { get; set; }
}

My question is how do I tell my select tag helper to use the Id as the value while displaying FullName in the drop down list?

<select asp-for=\"EmployeeId\" asp-items=\"???\" />

I\'d appreciate some help with this. Thanks.

回答1:

Using the Select Tag helpers to render a SELECT element in your view

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,\"Id\",\"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>

---

Getting data from your database table using entity framework

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.

Using SelectList

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.

public IActionResult Create()
{
    var vm = new MyViewModel();
    vm.Employees = new SelectList(GetEmployees(),\"Id\",\"FirstName\");
    return View(vm);
}
public IEnumerable<Employee> GetEmployees()
{
    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>

Render a SELECT element from a list of strings.

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.

Setting selected options

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.

Multi select dropdown

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>

Setting selected options in multi 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.

Using ViewBag to transfer the list of items

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 uncatched 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>

Grouping items

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.

回答2:

I created an Interface and a <options> tag helper for this. So I didn\'t have to convert the IEnumerable<T> items into IEnumerable<SelectListItem> every time I have to populate the <select> control.

And I think it works beautifully...

The usage is something like:

<select asp-for=\"EmployeeId\">
    <option value=\"\">Please select...</option>
    <options asp-items=\"@Model.EmployeesList\" />
</select>

And to make it work with the tag helper you have to implement that interface in your class:

public class Employee : IIntegerListItem
{
   public int Id { get; set; }
   public string FullName { get; set; }

   public int Value { return Id; }
   public string Text{ return FullName ; }
}

---

These are the needed codes:

The interface:

public interface IIntegerListItem
{
    int Value { get; }
    string Text { get; }
}

The <options> tag helper:

[HtmlTargetElement(\"options\", Attributes = \"asp-items\")]
public class OptionsTagHelper : TagHelper
{
    public OptionsTagHelper(IHtmlGenerator generator)
    {
        Generator = generator;
    }

    [HtmlAttributeNotBound]
    public IHtmlGenerator Generator { get; set; }

    [HtmlAttributeName(\"asp-items\")]
    public object Items { get; set; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        output.SuppressOutput();
        // Is this <options /> element a child of a <select/> element the SelectTagHelper targeted?
        object formDataEntry;
        context.Items.TryGetValue(typeof(SelectTagHelper), out formDataEntry);

        var selectedValues = formDataEntry as ICollection<string>;
        var encodedValues = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
        if (selectedValues != null && selectedValues.Count != 0)
        {
            foreach (var selectedValue in selectedValues)
            {
                encodedValues.Add(Generator.Encode(selectedValue));
            }
        }

        IEnumerable<SelectListItem> items = null;
        if (Items != null)
        {
            if (Items is IEnumerable)
            {
                var enumerable = Items as IEnumerable;
                if (Items is IEnumerable<SelectListItem>)
                    items = Items as IEnumerable<SelectListItem>;
                else if (Items is IEnumerable<IIntegerListItem>)
                    items = ((IEnumerable<IIntegerListItem>)Items).Select(x => new SelectListItem() { Selected = false, Value = ((IIntegerListItem)x).Value.ToString(), Text = ((IIntegerListItem)x).Text });
                else
                    throw new InvalidOperationException(string.Format(\"The {2} was unable to provide metadata about \'{1}\' expression value \'{3}\' for <options>.\",
                        \"<options>\",
                        \"ForAttributeName\",
                        nameof(IModelMetadataProvider),
                        \"For.Name\"));
            }
            else
            {
                throw new InvalidOperationException(\"Invalid items for <options>\");
            }

            foreach (var item in items)
            {
                bool selected = (selectedValues != null && selectedValues.Contains(item.Value)) || encodedValues.Contains(item.Value);
                var selectedAttr = selected ? \"selected=\'selected\'\" : \"\";

                if (item.Value != null)
                    output.Content.AppendHtml($\"<option value=\'{item.Value}\' {selectedAttr}>{item.Text}</option>\");
                else
                    output.Content.AppendHtml($\"<option>{item.Text}</option>\");
            }
        }
    }
}

There may be some typo but the aim is clear I think. I had to edit a little bit.

回答3:

You can also use IHtmlHelper.GetEnumSelectList.

    // Summary:
    //     Returns a select list for the given TEnum.
    //
    // Type parameters:
    //   TEnum:
    //     Type to generate a select list for.
    //
    // Returns:
    //     An System.Collections.Generic.IEnumerable`1 containing the select list for the
    //     given TEnum.
    //
    // Exceptions:
    //   T:System.ArgumentException:
    //     Thrown if TEnum is not an System.Enum or if it has a System.FlagsAttribute.
    IEnumerable<SelectListItem> GetEnumSelectList<TEnum>() where TEnum : struct;

回答4:

you can use below code for multiple select:

<select asp-for=\"EmployeeId\"  multiple=\"multiple\" asp-items=\"@ViewBag.Employees\">
    <option>Please select</option>
</select>

you can also use:

<select id=\"EmployeeId\" name=\"EmployeeId\"  multiple=\"multiple\" asp-items=\"@ViewBag.Employees\">
        <option>Please select</option>
    </select>

回答5:

In Get:

public IActionResult Create()
{
    ViewData[\"Tags\"] = new SelectList(_context.Tags, \"Id\", \"Name\");
    return View();
}

In Post:

var selectedIds= Request.Form[\"Tags\"];

In View :

<label>Tags</label>
<select  asp-for=\"Tags\"  id=\"Tags\" name=\"Tags\" class=\"form-control\" asp-items=\"ViewBag.Tags\" multiple></select>

回答6:

My answer below doesn\'t solve the question but it relates to.

If someone is using enum instead of a class model, like this example:

public enum Counter
{
    [Display(Name = \"Number 1\")]
    No1 = 1,
    [Display(Name = \"Number 2\")]
    No2 = 2,
    [Display(Name = \"Number 3\")]
    No3 = 3
}

And a property to get the value when submiting:

public int No { get; set; }

In the razor page, you can use Html.GetEnumSelectList<Counter>() to get the enum properties.

<select asp-for=\"No\" asp-items=\"@Html.GetEnumSelectList<Counter>()\"></select>

It generates the following HTML:

<select id=\"No\" name=\"No\">
    <option value=\"1\">Number 1</option>
    <option value=\"2\">Number 2</option>
    <option value=\"3\">Number 3</option>
</select>