Creating a Bitmap from an External File in Visual Studio .NET

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11334 Creating a Bitmap from an External File
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Another way to create a bitmap is by specifying the path to an image file This is accomplished with a constructor that accepts a single parameter, a string with the path to the candidate file This second Bitmap class constructor is defined as follows:
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public Bitmap( string filename);
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This method has two important requirements One is that there must be enough memory to accommodate the bitmap If there is not, the call fails The second requirement is that the specified file must have an image in a format that the constructor understands We have been able to create bitmaps from the following file types:
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Bitmap files (bmp) with 1, 4, 8, or 24 bits per pixel JPEG (jpg) files GIF (gif) files PNG (png) files
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Among the unsupported graphics file formats are TIFF (tif) files This constructor throws an exception if the filename provided is not a recognized format or if it encounters other problems when attempting to open the file or create the bitmap For that reason, it makes sense to wrap this constructor in a try catch block Listing 112 provides an example of calling this constructor, with a filename provided by the user in a File Open dialog box
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Listing 112: Creating a Bitmap with a Filename
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try { bmpNew = new Bitmap(strFileName); } catch { MessageBoxShow("Cannot create bitmap from " + "File: " + strFileName); }
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11335 Creating a Bitmap from a Resource
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When a program needs a bitmap for its normal operation, it makes sense to packagethebitmapasaresourceResourcesareread-onlydataobjectsthat are bound into a program s executable file22 at program build time The benefit of binding a bitmap to an executable file is that it is always available and cannot be accidentally deleted by a user Resources have been a part of Windows programming from the very first version of Windows In a native-mode program, resources are used
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22 Resources can be bound into any executable module, meaning any program (exe) or library (dll) file
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113 Raster Graphics
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for bitmaps and icons and also to hold the definitions of dialog boxes and menus In managed-code programs, resources are still used for bitmaps and icons, although some program elements including dialog boxes and menus are not defined in a resource but instead are defined using code in the InitializeComponent method by the Designer
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Because memory is scarce on a Windows CE powered device, it helps to know when and how memory gets used When a resource gets added to a module, the resource occupies space in the module s file but uses no program memory until the resource is explicitly opened and used This is true for both native resources and managed resources
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While resources are used in both native code and managed code, native resources can be used only from native-mode code, and managed resources can be used only from managed code The only exception is the program icon for a managed-code program, which is defined as a native icon In managed code, there are two types of resources: typed resources and untyped resources Typed Resources We like to use typed resources to hold literal strings, which aid in the localization of programs To access typed resources, a program creates an instance of a ResourceManager23 class and then makes calls to methods such as GetObject and GetString Typed resources are defined using XML in files that have an extension of resx In a typed resource, an XML attribute provides type information, as shown in this example:
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<data name="dlgFileOpenLocation" type="SystemDrawingPoint, SystemCFDrawing, Version=7050000, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"> <value>125, 17 </value> </data>
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23 SystemResourcesResourceManager
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The Designer makes extensive use of typed resources For each form created in the Designer, an associated file is used to store a variety of details about the form The Visual Studio NET Solution Explorer does not normally display resource files, but you can make them appear by clicking the Show All Files icon For example, bitmaps in the image collection of an ImageList control on a form are stored as typed resources in the typed resource file of the form that contains the control The bitmaps themselves are serialized into the XML and are not stored in their original, binary form While a programmer could convert bitmap files into XML resources, we prefer to avoid this extra step and use untyped resources when we add bitmap resources to a project Untyped Resources Untyped resources are also known as manifest resources because they are made available through an assembly s manifest (or table of contents) As the name implies, an untyped resource contains no type information and is made available as a raw stream of bytes It does have a name, however, that is created by combining the default project namespace with the filename that contained the original resource data You must know this name because you use the name to retrieve the resource If you have trouble figuring out the resource name, the ildasmexe utility can help Open the program file and then click on the manifest Listing 113 shows three bitmap resource names in a fragment from the manifest for the ShowBitmap sample program presented later in this chapter