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Figure 13-15. The sign of a modal type editor To create a modal type editor, you need to create the dialog box form. To create a basic example, consider the DirectoryTree control first presented in 11. Although it isn t difficult to change the DirectoryTree.Drive property, it would be nice if you could run a little bit of extra code to find all the drives on the current computer and allow the user to choose from them. Here s a SelectDrive form that does exactly that. It gets an array of drives and shows them in a list. When the developer selects a new drive, the Select.DriveSelection property is updated. The OK button is set with a DialogResult of DialogResult.OK, so clicking it closes the window. Public Class SelectDrive ' Store the selected drive. Private _driveSelection As Char Public Property DriveSelection() As Char Get Return _driveSelection End Get Set(ByVal value As Char) _driveSelection = value ' Select the drive in the list. lstDrives.SelectedIndex = lstDrives.FindString( _ DriveSelection.ToString())

Arguments to instance methods follow the same convention as for static methods, and they must be within parentheses and separated by commas To retrieve the value of a property or field, you need only the name of member, and you set it using the left arrow (<-) The next example demonstrates how to create a SystemIOFileInfo object and then use various members of the class to manipulate it in different ways On the first line, you make the SystemIO namespace available to F# On the second, you create the FileInfo object, passing it the name of the file in which you re interested Next, you check whether the file exists using the Exists instance property If it doesn t exist, you create a new file using the CreateText() instance method and then set it to read-only using the Attributes instance property.

CHAPTER 16 WRITING FUNCTIONAL CODE IN C#

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End Set End Property Private Sub New() InitializeComponent() Dim drives() As String = System.IO.Directory.GetLogicalDrives() lstDrives.DataSource = drives End Sub Private Sub lstDrives_SelectedIndexChanged(ByVal sender As Object, _ ByVal e As EventArgs) Handles lstDrives.SelectedIndexChanged DriveSelection = lstDrives.Text(0) End Sub ' Allow quick select-and-close. Private Sub lstDrives_DoubleClick(ByVal sender As Object, _ ByVal e As EventArgs) Handles lstDrives.DoubleClick DialogResult = DialogResult.OK End Sub End Class All the type editor needs to do is create an instance of the SelectDrive dialog box, show it, and then read the DriveSelection property once the dialog box is closed. Here s the complete type editor code: Public Class DriveEditor Inherits UITypeEditor Public Overrides Function GetEditStyle( _ ByVal context As ITypeDescriptorContext) _ As System.Drawing.Design.UITypeEditorEditStyle ' Use a dialog box for property editing. Return UITypeEditorEditStyle.Modal End Function Public Overrides Function EditValue( _ ByVal context As ITypeDescriptorContext, _ ByVal provider As IServiceProvider, ByVal value As Object) _ As Object Dim frm As New SelectDrive() ' Set current drive in window. frm.DriveSelection = CType(value, Char)

' Show the dialog box. If frm.ShowDialog() = DialogResult.OK Then ' Return the new value. Return frm.DriveSelection Else ' Return the old value. Return value End If End Function Public Overrides Function GetPaintValueSupported( _ ByVal context As ITypeDescriptorContext) As Boolean ' No special thumbnail will be shown in the Properties window. Return False End Function End Class The type editor is attached to the appropriate property using an Editor attribute: <Editor(GetType(DriveEditor), GetType(UITypeEditor))> _ Public Property Drive() As Char Figure 13-16 shows the drive-selection window that appears when the user edits the Drive property.

This way of calling the curry expression is similar to using the lambda expression shown at the beginning of this section, except that a set of parentheses replaces the comma. This is because you are calling a function that returns a function that you can call again. This approach has the advantage that you can create a lambda expression that can be assigned to a variable and then called at a later time. The arrangement makes it possible to avoid dragging around the first parameter, since it is embedded in the first method call, as follows: Func<string, string> curriedFunction = curry("+++"); curriedFunction("hello"); The variable curriedFunction contains state. When it is called, the method will generate a buffer, just as was done by the original lambda expression shown at the beginning of this section.

Here, you use the use binding to clean up resources, by calling their Dispose method when they drop out of scope: open SystemIO // create a FileInfo object let file = new FileInfo("testtxt") // test if the file exists, // if not create a file if not fileExists then use stream = fileCreateText().

Figure 13-16. A custom drive-selection window One benefit to this design is that you can reuse this type editor with any drive property in any control. It s specific to the property data type, not the control. An alternative approach is to use a DirectoryInfo object instead of an underlying char to represent the drive. Because the property editing is now handled by the type editor, there s no need to choose a basic type that can be edited with the default design-time support built into the property grid.

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