Convert String[] to List<SelectList>

I'm assuming you need either a SelectList or a List<SelectListTiem>, not a List<SelectList>. SelectList has a constructor that takes a collection:

string[] strings = new [] { .. strings .. };
SelectList sl = new SelectList(strings);

or you can project to a List<SelectListItem>:

string[] strings = new [] { .. strings .. };
var sl = strings.Select(s => new SelectListItem {Value = s})
                .ToList();

Note that SelectList implements IEnumerable<SelectListItem>, so if you have a model property of type IEnumerable<SelectListItem> you can create a SelectList and assign it to that property rather than projecting to a List<SelectListItem>. It's functionally the same but the code will be a little cleaner.

This is all assuming we're talking about MVC, not Web Forms

Second to D Stanley's answer, another solution:

string[] strings = new [] { ... strings ... };
var selectListItems = strings.Select(x => new SelectListItem() { Text = x, Value = x, Selected = x == "item 1" });

A list of SelectListItem can also be used to populate an MVC drop down list.

With this method, you can also set other properties on a SelectListItem such as, display value.

We can't call Select on a IQueryable using the SelectListItem constructor because LINQ will try and convert that to SQL. Which unless there is a provider for it, is impossible, and also not what we want to achieve.

In order to always assure we can enumerate like I have shown above, we need to force EF or other ORMs to get all our data back. We can do this by calling ToList() BEFORE we enumerate with Select:

var selectListItems = strings.ToList().Select(x => new SelectListItem() { Text = x, Value = x, Selected = x == "item 1" });

As @BCdotWEB has pointed out:

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

Is the constructor that this list will inevitably get put into. If I can remember correctly, the razor view should look like this:

@Html.DropDownListFor(x => x.SelectedString, new SelectList(x.Strings, "Value", "Text"))

Where x.SelectedString is where you want the chosen value from the drop down to be put. x.Strings is that selectListItems we created in the Controller/Service