C# – WPF ListBox using ItemsSource and ItemTemplate

bindingclistboxwpf

I'm confused as to how bindings are resolved when I have both an ItemsSource and an ItemTemplate in a WPF ListBox.

I have an ObservableCollection<int> called ListOfIndexes. For each index, I want to look up its record in a database table. I hope to do that in the IndexToObjectDescriptionConverter.

<ListBox ItemsSource="{Binding ListOfIndexes}" 
         ItemTemplate="{Binding Converter={StaticResource IndexToObjectDescriptionConverter}}" />

But a breakpoint in the converter is telling me that the value being read in by the ItemTemplate binding is of the window itself — i.e., the DataContext of the ItemsSource and ItemsTemplate is the same.

Pardon a bit of candidness, but this seems DUMB. The entire point of the ItemTemplate is to render each element within the ItemsSource, so I guess I figured that the DataContext of the ItemTemplate would be the individual element being rendered.

So, that said, how do I tell the ItemTemplate that it should worry about the individual elements represented by the ItemsSource and not use the entire window's DataContext?

Best Answer

You need to use a data template for the ItemTemplate. This is then applied to each item in the list

MSDN docs are here: http://msdn.microsoft.com/en-us/library/system.windows.controls.itemscontrol.itemtemplate(v=vs.110).aspx

The issue her is about data context scope. When you bind any property on the ListBox, it will use the data context of the ListBox - hence why that data context is being passed to the converter. If you set a data template inside the ItemTemplate, it will apply that template to each item in the list. I guess based on the simple code you've provided you would need to have the converter inside the data template:

<ListBox ItemsSource="{Binding ListOfIndexes}">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <ContentControl Content="{Binding Converter={StaticResource IndexToObjectDescriptionConverter}}"/>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

In this case, the ContentControl will be rendered for each item, with that item as it's data context.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – Should ‘using’ directives be inside or outside the namespace

There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:

// File1.cs
using System;
namespace Outer.Inner
{
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now imagine that someone adds another file (File2.cs) to the project that looks like this:

// File2.cs
namespace Outer
{
    class Math
    {
    }
}

The compiler searches Outer before looking at those using directives outside the namespace, so it finds Outer.Math instead of System.Math. Unfortunately (or perhaps fortunately?), Outer.Math has no PI member, so File1 is now broken.

This changes if you put the using inside your namespace declaration, as follows:

// File1b.cs
namespace Outer.Inner
{
    using System;
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now the compiler searches System before searching Outer, finds System.Math, and all is well.

Some would argue that Math might be a bad name for a user-defined class, since there's already one in System; the point here is just that there is a difference, and it affects the maintainability of your code.

It's also interesting to note what happens if Foo is in namespace Outer, rather than Outer.Inner. In that case, adding Outer.Math in File2 breaks File1 regardless of where the using goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using directive.

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – Should ‘using’ directives be inside or outside the namespace

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