C# Continued. Learning Objectives:

Transcription

1 Learning Objectives: C# Continued Open File Dialog and Save File Dialog File Input/Output Importing Pictures from Files and saving Bitmaps Reading and Writing Text Files Try and Catch Working with Strings ListBoxes 1

2 OpenFileDialog and SaveFileDialog A Dialog Box is like a mini-form that pops up when you do something in the form. A MessageBox is one kind of Dialog Box. We ve also seen a ColorDialog box. OpenFileDialog is another. OpenFileDialog: Allows you to browse to a file you want to open. It could be a text file or an image file. You won t be able to open an MSWord file or other specialized files. To get an OpenFileDialog, look in the toolbox under dialogs and drag it to the form. Create a method that you would use to call the OpenFileDialog. You could do this by double-clicking on File Open in your menu. In that method, type:openfiledialog1.showdialog(); This will bring up the dialog box. Then go back to the design mode and double click on the OpenFileDialog1 icon. This will create a method that executes when the person clicks Open in the dialog. To 2 get the path and filename, use openfiledialog1.filename.

3 Try It! Create an OpenFileDialog box and use it to display the filename of a file that the user chooses in a MessageBox. SaveFileDialog: Allows you to browse to where you want to save a file. Use it the same way, but replace OpenFileDialog with SaveFileDialog. Try It! Create an SaveFileDialog box and use it to display the filename of a file that the user chooses in a MessageBox. 3

4 Importing Bitmaps (pictures) from Files and Saving Bitmaps You can use the actual path to the file when you create the Bitmap Example: Bitmap pic = new Bitmap( H:\\CS101\\filename ); Note that you need \\ to separate the folders and filenames (otherwise it thinks you are doing something like \n) If you are using an OpenFileDialog, you can get the picture using: Bitmap pic = new Bitmap(openFileDialog1.FileName); If you already have a global Bitmap variable, don t redeclare it, instead use: pic = new Bitmap(openFileDialog1.FileName); Saving Pics: pic.save( Drive:\\path\\filename.jpg ); //or if using a savefiledialog pic.save(savefiledialog1.filename); 4

5 Reading from a Text File Note: At the top of the file, you will need to add using System.IO; You will need a StreamReader to read from a file and a StreamReader needs a Stream. You can get the Stream by opening the file. Here is an example how to read a line from a file. Stream s = openfiledialog1.openfile(); StreamReader sr = new StreamReader(s); String firstline = sr.readline(); ReadLine() always returns a String, but you can convert that to numbers the same way you did before: Example: x = System.Convert.ToDouble(firstLine); 5

6 Reading from a File To Read all of the lines in a file, use a loop. Example that reads each line from a file and puts each line in a MessageBox: Stream st = openfiledialog1.openfile(); StreamReader sr = new StreamReader(st); String s; while (!sr.endofstream) //sr.endofstream means end of the file { s = sr.readline(); MessageBox.Show(s); } 6

7 Writing to a File Once you have created a SaveFileDialog object from the ToolBox, you can either show it and have the user select the file (using the ShowDialog() method), or you can set the filename like this: savefiledialog1.filename= highscore.txt ; //if you always want the //same filename You will need a StreamWriter to write to a file and a StreamWriter needs a Stream. You can get the Stream by opening the file. Here is an example how to write a line to a file. String highscore=system.convert.tostring(total); Stream s = savefiledialog1.openfile(); StreamWriter sw = new StreamWriter(s); sw.writeline(highscore); When you are done writing to the file, use: sw.close(); //this will write lines from the buffer to the file and close it 7

8 try and catch You have probably experienced run-time errors already this semester. They are the errors that pop up after you ve started running the program. Examples are null exceptions, index out of range, etc. You can use try and catch to stop these if you expect them to happen. One place that they are likely to occur is when you try to use File Input or Output. If it can t find the file, it will give an error. To avoid this, surround the code that will cause the error with a Try block and put a Catch below it. The Catch part will occur if the try part throws an error (aka exception). Example: try { Stream s = openfiledialog1.openfile(); StreamReader sr = new StreamReader(s); String firstline = sr.readline(); } catch { MessageBox.Show( Couldn t open File ); } Note: catch by itself catches all errors. It can also be used to just catch 8 specific errors. You could use this to catch blank textboxes.

9 ListBoxes A listbox is like a textbox, except it is more conducive to multiple lines and allows the user to select lines. It contains multiple strings and the Items in the listbox work somewhat like vectors. Get a listbox by dragging one from the ToolBox onto your form. To add a String as a new line in a listbox use: listbox1.items.add( Your string goes here ); //like push_back() To get a line from a listbox, use: listbox1.items[ i ] where i is the index of the line starting at 0 To get the number of lines in your listbox, use: listbox1.items.count To empty the list box of all its lines: listbox1.items.clear(); 9