Cannot implicitly convert type 'string' to 'System.Collections.Generic.List<string>'

Question

This question has probably been answered hundreds of times, but here goes.

I have this chunck of code:

private void PopulateStringDropdownList(List<ObjectInfo> listObject,
                                        object selectedType = null)
{
    List<string> listString = listObject.OrderBy(x => x.m_Type).ToString();
    for (int i = 0; i < listString .Count; i++)
    {
        for (int j = i + 1; j < listString .Count; j++)
        { 
            if (String.Equals(listString [i], listString [j]))
            {
                listString.RemoveAt(j);
            }
        }
    }
    ViewBag.property1 = new SelectList(listString );
}

So basically I'm trying to populate a dropdown List with strings coming from a property of each objects contained in the list I pass in parameter.

But the code doesn't compile because of the error you're seeing up there, and I've yet to understand why exactly. Any help anyone?

Solution 1

This is the problematic line:

List<string> listString = listObject.OrderBy(x => x.m_Type).ToString();

ToString returns a string, and you're trying to assign to a List<string> variable.

You need to do something like this:

List<string> listString = listObject.OrderBy(x => x.m_Type)
                                    .Select(x => x.ToString())
                                    .ToList();

This statement will order your listObject enumerable to the order that your want, then convert the values to strings, and finally convert the enumerable to a List<string>.

Solution 2

Look at this code:

List<string> listString = listObject.OrderBy(x => x.m_Type).ToString();

The right hand side expression is calling ToString() on the result of OrderBy - which isn't useful itself, and will result in a string.

You're then trying to assign that string expression to a new variable of type List<string>. That's not going to work.

I suspect you just want ToList instead of ToString:

List<string> listString = listObject.OrderBy(x => x.m_Type).ToList();

Solution 3

I think you want ToList instead of ToString.

Also, if all you're trying to do is remove duplicates, you can simply use LINQ and do this:

List<string> listString = listObject.OrderBy(x => x.m_Type).
    Select(x => x.ToString()).Distinct().ToList();

Solution 4

There was still a bit of correcting to be done. Here's the complete code:

private void PopulateStringDropdownList(List<ObjectInfo> listObjects, object selectedType = null)

        {
            List<string> listString = listObjects.OrderBy(x => x.m_Type)
                                             .Select(x => x.m_Type.ToString())
                                             //.Distinct()
                                             .ToList();
            ViewBag.cardType = new SelectList(listString );
        }

That way the correct value found is the string of the Type and not the type of the object.

Thanks everyone for your help!

Solution 5

Convert list of int to list of string:

List<string> listString =  listObject.OrderBy(x => x.m_Type).
Select(x =>x.ToString()).ToList();

View more solutions

Comments

[hsim]

It's not that I don't want to; rather, it doesn't work. I thought first that it was because the list that receives the data was a string list, but... nope.

[hsim]

Oh! Will try that one too.

[Jon Skeet]

@HervéSimard: What do you mean? What failed when you tried this? What's the type of the m_Type field? (Please be more specific in future.)

[AndyClaw]

listObject is a list of ObjectInfo, not string, so use listObject.OrderBy(x => x.m_Type).Select(l => l.ToString()).Distinct().ToList()

[Matthew Strawbridge]

@AndyClaw Thanks Andy. Think I was editing my answer just as you commented.