Get set property for WPF combobox user control

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C#

WPF

I created my own user control provided some get set properties like SelectedIndex,SelectedValue,SelectedItem but I need get set property for MyCombobox.Items.IndexOf(_myString), so that i can get Index proerty for my User Control

What I have tried:

public int Items[string str]
{
get
{
return myCombobox.Items.IndexOf(_myString);
}
set
{
this.myCombobox.Items.IndexOf(_myString) = value;
}
}