Use the collection editor to set a property
In the Properties panel, you can set properties in many ways. The simplest way is to type a value, select a button, or select a value from a drop-down list. More complex properties use the subproperty and collection editors. You can also use advanced property options to data-bind, template-bind, or apply a resource to a property.
The following procedure shows you how to use a collection editor to add columns to a Grid layout panel.
Select a Grid object on the artboard, such as the LayoutRoot object.
Under Layout in the Properties panel, click Show advanced properties to expand the Layout category.
Next to the ColumnDefinitions property, click Edit items in this collection .
The collection editor opens.
To add a column to the collection, click Add another item.
A ColumnDefinition item is added to the collection.
You can add more columns to the collection, and you can set the properties for each column on the right side of the collection editor, under Properties. For example, you can configure how the column will resize in your running application by setting the Width property.
For information about the sizing options, see Change the sizing option for a row or column.
When you finish, click OK.