ASP.NET MVC SelectList Constructor

I was creating a drop down list in an MVC application and the list was okay, but the selected item was not being selected.

I was using the SelectList object and the constructor that takes four parameters, the last parameter being selectedValue.  The documentation on MSDN is not exactly enlightening.  All it says for the parameter is “The selected value”.  Wow, big help.

This is what I finally figured out.

Here is the constructor:

public SelectList(
    IEnumerable items,
    string dataValueField,
    string dataTextField,
    Object selectedValue
)

• items is the list of “things” you want to put in the drop down list (“select” HTML tag).
• dataValueField is the name of the property on your “things” that you want to be put into the “value” attribute of the “option” tag rendered in the HTML.
• dataTextField is the name of the property on your “things” that you want to be displayed in the drop down list (the text between the “<option>” and “</option>” in the “select”) .
• selectedValue is the value you want to end up as selected in the drop down list. It has to be the same as what will be put into the “value” attribute of the “option” tag in the rendered HTML.

Example:  I have a class called Thing that has a property “ID” and a property “Description”.  I want something like this:

<select>
<option value="1">Description One</option>
<option value="2" selected="selected">Description Two</option>
<option value="3">Description Three</option
</select>

I have a List<Thing> that I will use to populate the drop down. I have a Thing that is the selected Thing.

List<Thing> things = GetThings();
Thing selectedThing = GetSelectedThing();
var selectList = new SelectList(things, "ID", "Description", selectedThing.ID);
ViewData["things"] = selectList;

So it works if the fourth constructor parameter is the exact value you want to match.  I’m not sure if it needs to be a string or not, I’ve not had time to check that.

03/11/2013 – I just came across these two excellent articles on using the Html.DropDownList and Html.DropDownListFor helper methods.  These might also shed some light on how to work with SelectList and why you may not even need to:

Drop-down Lists and ASP.NET MVC

DropDownListFor with ASP.NET MVC