How do I select an item in my WPF ComboBox based on my data binding?

data-binding, wpf, xaml

Solution

You're binding too much; you only need to set `SelectedValue` and `SelectedValuePath`, or `SelectedItem`. In this case, it looks like you're actually trying to bind to a specific object. If you're trying to have the `ComboBox` set the `Category` property on your `Book` and the current `Book` object actually has a reference to a `Category` instance that's in `categoryList`, then you should use the `SelectedItem` binding and remove the bindings for `SelectedValue` and `SelectedValuePath`.

Edit

To expand a little on how this is done, `SelectedValue` is designed to be used when you have a common piece of information to link your bound item with a property on the list source. For instance, let's say I have a `Book` class with a `CategoryID` property.

public class Book
{
    public string CategoryID { get; set; }
    public string Title { get; set; }
}

public class CategoryID
{
    public string ID { get; set; }
    public string Name { get; set; }
}

In this case, you would do:

<ComboBox SelectedValue = "{Binding CategoryID}"
          SelectedValuePath = "ID"
          DisplayMemberPath = "Name" />

`SelectedItem`, on the other hand, is for when the bound instance has an actual reference to (or, more precisely, an object that is equivalent to) an item in the bound list. So, let's assume the `Book` class actually looks like this:

public class Book
{
    public Category Category { get; set; }
    public string Title { get; set; }
}

In this case, you would do:

<ComboBox SelectedItem = "{Binding Category}"
          DisplayMemberPath = "Name" />

But it's very important to note that this will not work unless the `Book` class has a reference to the same instance of `Category` as you have in the list. If the references are different (even if the values on the classes are equal) then this won't work, as the `ComboBox` won't be able to find the `Category` referenced in the `Book` in the list.

The real problem with the way you were binding it up top (by binding to `Category.ID`) is that you're mixing the schemes. You have a reference, but you're trying to bind on the key instead. All this will do is try to set the value on your reference, it will not try to change the reference on your class.

Problem

Original source