How do make modal dialog in WPF?

A message box is a dialog box that can be used to display textual information and to allow users to make decisions with buttons. The following figure shows a message box that displays textual information, asks a question, and provides the user with three buttons to answer the question.

To create a message box, you use the MessageBox class. MessageBox lets you configure the message box text, title, icon, and buttons, using code like the following.

// Configure the message box to be displayed
string messageBoxText = "Do you want to save changes?";
string caption = "Word Processor";
MessageBoxButton button = MessageBoxButton.YesNoCancel;
MessageBoxImage icon = MessageBoxImage.Warning;

To show a message box, you call the static Show method, as demonstrated in the following code.

// Display message box
MessageBox.Show(messageBoxText, caption, button, icon);
When code that shows a message box needs to detect and process the user's decision (which button was pressed), the code can inspect the message box result, as shown in the following code.

// Display message box
MessageBoxResult result = MessageBox.Show(messageBoxText, caption, button, icon);

// Process message box results
switch (result)
{
    case MessageBoxResult.Yes:
        // User pressed Yes button
        // ...
        break;
    case MessageBoxResult.No:
        // User pressed No button
        // ...
        break;
    case MessageBoxResult.Cancel:
        // User pressed Cancel button
        // ...
        break;
}


For more information on using message boxes, see MessageBox, MessageBox Sample, and Dialog Box Sample. 
Source: http://msdn.microsoft.com/en-us/library/aa969773.aspx