Razor Pages Add Select option to a Dropdown

This code sample demonstrates how to add a select option to a dropdown/select element using EF Core and a data provider using SQL-Server NorthWind database using category and contact type tables.

GitHub Source code

For each select, data is read from a database table as follows were the DECLARE is replaced for the data provider Microsoft.Data.SqlClient with a parameter and with EF Core inserted to a list as the first item.

Note the parameter below @SelectText

DECLARE @SelectText AS NVARCHAR(50) = N'Select';

SELECT -1 AS CategoryID,
       @SelectText AS CategoryName
UNION ALL
SELECT CategoryID,
       CategoryName
FROM dbo.Categories
ORDER BY CategoryName;

SELECT -1 AS ContactTypeIdentifier,
       @SelectText AS ContactTitle
UNION ALL
SELECT ContactTypeIdentifier,
       ContactTitle
FROM dbo.ContactType
ORDER BY ContactTitle;

Then for the data provider after data is read into a list the following provides a custom order by which starts with -1 which represents the item to have a user make a selection.

var identities = Enumerable.Range(-1, count).ToList();

Data is read then ordered using the above list.

list.OrderBy(c => identities.IndexOf(c.CategoryID)).ToList();

Select text

The select text is stored in appsettings.json, in this case one for each page.

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning",
      "Microsoft.EntityFrameworkCore.Database.Command": "Warning"
    }
  },
  "AllowedHosts": "*",

  "ApplicationFeatures": {

    "IndexPage": {
      "SelectText": "Select"
    },
    "Index1Page": {
      "SelectText": "Select"
    }
  },
  "ConnectionStrings": {
    "DefaultConnection": "Data Source=.\\SQLEXPRESS;Initial Catalog=Northwind2020;Integrated Security=True;Encrypt=False"
  }
}

Model for above

public class ApplicationFeatures
{
    /// <summary>
    /// Index page section in appsettings.json
    /// </summary>
    public const string Index = "ApplicationFeatures:IndexPage";
    /// <summary>
    /// Index1 page section in appsettings.json
    /// </summary>
    public const string Index1 = "ApplicationFeatures:Index1Page";
    public string SelectText { get; set; }
}

• Two constants point to sections in appsettings.json
• SelectText property points to text used for the first item in each dropdown.

Index page

This page uses EF Core for populating the select element.

1. In the page constructor gets the selection text from appsettings
2. Reads data from the Categories table with an order by on CategoryName
3. Inserts select text into the list
4. Creates a List<SelectListItem> for the list
5. Then in the frontend create the select element set to the SelectListItem and name set to the primary key, in this case CategoryID which on post will have the selected item.

<label asp-for="SelectedCategory">
    Categories
    <select name="CategoryID"
            asp-for="SelectedCategory"
            class="form-select"
            style="width: 9.2em"
            asp-items="Model.Options">
    </select>
</label>

Index1 page

This page uses the data provider for populating the select element.

1. In the page constructor gets the selection text from appsettings
2. Reads data from the Categories table with an order by on CategoryName and add the select option from the class ReferenceTableOperations
3. Inserts select text into the list
4. Creates a List<SelectListItem> for the list
5. Then in the frontend create the select element set to the SelectListItem and name set to the primary key, in this case CategoryID which on post will have the selected item.

<label asp-for="SelectedCategory">
   Categories
   <select name="CategoryID"
      asp-for="SelectedCategory"
      class="form-select"
      style="width: 9.2em"
      asp-items="Model.Options">
   </select>
</label>

Both pages select element

Although both pages load the data differently, they both use the same HTML configuration.

Try it out

Index2 page is ready for you the reader to try out where all setup is done for EF Core. Following along with Index Page which presents Categories and finish this page with ContactType.

1. Copy code from Index.cshtml starting with the style section
2. Paste into Index2.cshtml (at this point disrerard any errors)
3. Copy code from Index.cshtml.cs, from the open class bracket to the end class bracket
4. Replace Index2.cshtml.cs with the above
5. Now alter code from categories to contact type.

Note for Index2 page and the other pages

Configuration has already been done in Program.cs for reading the select text.

private static void ApplicationConfigurations(WebApplicationBuilder builder)
{
    builder.Services.Configure<ApplicationFeatures>(ApplicationFeatures.Index,
        builder.Configuration.GetSection(ApplicationFeatures.Index));

    builder.Services.Configure<ApplicationFeatures>(ApplicationFeatures.Index1,
        builder.Configuration.GetSection(ApplicationFeatures.Index1));

    builder.Services.Configure<ApplicationFeatures>(ApplicationFeatures.Index2,
        builder.Configuration.GetSection(ApplicationFeatures.Index2));
}

To learn more about reading values from appsettings.json see Storing and reading values from appsettings.json

Source code

Clone the following GitHub repository.