Windows Forms 2.0 programming
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You can handle the CellPainting event and draw directly into the cell itself, and you can achieve pretty much whatever you want with the built-in cell types and some possibly complex drawing code. But if you want to be able to just plug your column or cell type in a reusable way with the same ease as using the built-in types, then you can derive your own column and cell types instead. The model you should follow for plugging in custom column types matches what you have already seen for the built-in types: You need to create a column type and a corresponding cell type that the column will contain.
You do this by simply inheriting from the base DataGridViewColumn and DataGridViewCell classes, either directly or indirectly, through one of the built-in types. The best way to explain this in detail is with an example. I want to be able to set a status using a custom-enumerated value, and cells in the column will display a graphic indicating that status based on the enumerated value set on the cell. To do this, I define a StatusColumn class and a StatusCell class I disposed of the built-in type naming convention here of prefixing DataGridView on all the types because the type names get sooooooooooo long.
I want these types to let me simply set the value of a cell, either programmatically or through data binding, to one of the values of a custom-enumerated type that I call StatusImage. StatusImage can take the values Green , Yellow , or Red , and I want the cell to display a custom graphic for each of those based on the value of the cell. To achieve this, the first step is to define the custom cell type. If you are going to do your own drawing, you can override the protected virtual Paint method from the DataGridViewCell base class.
However, if the cell content you want to present is just a variation on one of the built-in cell types, you should consider inheriting from one of them instead. That is what I did in this case.
Because my custom cells are still going to be presenting images, the DataGridViewImageCell type makes a natural base class. I also want the cell value to be able to handle integers as long as they are within the corresponding numeric values of the enumeration, so that I can support the common situation where enumerated types are stored in a database as their corresponding integer values.
The first declaration in this code is the enumeration StatusImage. That is the value type expected by this cell type as its Value property.
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There is a default status field and corresponding property that lets the default value surface directly. The constructor also sets the ImageLayout property of the base class to Zoom , so the images are resized to fit the cell with no distortion. The key thing a custom cell type needs to do is either override the Paint method, as mentioned earlier, or override the GetFormattedValue method as the StatusCell class does. This method will be called whenever the cell is rendered and lets you handle transformations from other types to the expected type of the cell.
The way I have chosen to code GetFormattedValue for this example is to first set the value to a default value that will be used if all else fails.
The code then checks to see if the current Value property is a StatusImage enumerated type or an integer, and if it is an integer, it casts the value to the enumerated type. Once the status value to be rendered is determined, the GetFormattedValue method uses a switch-case statement to select the appropriate resource name corresponding to the image for that status value. You embed bitmap resources in the assembly by adding them to the Visual Studio project and setting the Build Action property on the file to Embedded Resource.
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The code then uses the GetManifestResourceStream method on the Assembly class to extract the bitmap resource out of the assembly, sets the alignment on the CellStyle argument passed into the method, and then returns the constructed image as the object from the method. The object that you return from this method will be the one that is passed downstream to the Paint method as the formatted value to be rendered. So now you have a custom cell class that could be used in the grid, but you also want to have a custom column class that contains StatusCell s and can be used for setting up the grid and data binding.
If you were going to use the custom cell type completely programmatically, you could just construct an instance of the DataGridViewColumn base class and pass in an instance of a StatusCell to the constructor, which sets that as the CellTemplate for the column. To support that, you need to implement a custom column type that the designer can recognize. The implementation of the StatusColumn class is shown in Listing 6.
You implement a default constructor that passes an instance of your custom cell class to the base class constructor. This sets the CellTemplate property on the base class to that cell type, making it the cell type for any rows that are added to a grid containing your column type. The next thing the class does is define a public property named DefaultStatus.
This lets anyone using this column type to set which of the three StatusImage values should be displayed by default if no value is explicitly set on the grid through data binding or programmatic value setting on a cell. The setter for this property changes the member variable that keeps track of the current default. The DefaultStatus property on the column is accessed from the StatusCell. GetFormattedValue method, as described earlier.
Another important thing for you to do in your custom column type is to override the Clone method from the base class, and in your override, return a new copy of your column with all of its properties set to the same values as the current column instance. This method is used by the design column editors to add and edit columns in a grid through the dialogs discussed in Figures 6. The last thing the custom column class does is to override the CellTemplate property. If someone tries to access the CellTemplate , the code gets it from the base class. But if someone tries to change the CellTemplate , the setter checks to see if the type of the cell being set is a StatusCell.
If not, it raises an exception, preventing anyone from programmatically setting an inappropriate cell type for this column. Now that you have defined the custom cell and column types, how can you use them? Well, you can define them as part of any Windows application project type in Visual Studio, but generally when you create something like this, you are doing it so you can reuse it in a variety of applications. Whenever you want reuse code, you need to put that code into a class library.
So if you define a class library project, add the classes just discussed to the class library, along with the images you want to use for displaying status as embedded resources in the project. This creates an assembly that you can then reference from any Windows application that you want to use the column and cell type within.
All you need to do is set a reference to that assembly from the Windows Forms project in which you want to use them, and the custom column types will display in the Add Column dialog, as shown in Figure 6. Within your Windows Forms application, you can programmatically add StatusColumns to a grid, or use the designer to do so.
If you add the column through the designer and then look at it in the Edit Columns dialog, you will see that DefaultStatus appears in the property list and is settable as an enumerated property with its allowable values see Figure 6. With a column of this type added to the grid, you can either populate the grid programmatically with either of the types that the cell is able to handle for values either StatusImage values or integers within the value range of StatusImage , or you can data bind to it with a collection of data that contains those values.
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Here is a simple example of setting the values programmatically on a grid containing two columns: a text box column,. Note that you can set the values with either the enumerated value or with an appropriate integer value. The CustomColumnAndCell sample application in the download code also demonstrates creating a data set and data binding against the status column.
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You have probably noticed that the DataGridView is much more focused at the cell level that its DataGrid predecessor was. Specifically, users want spread-sheet-like functionality that mimics the interaction model millions of people have become accustomed to with programs like Microsoft Excel and other spreadsheet applications. Once again, the DataGridView comes to the rescue and makes supporting that model fairly easy. You have already seen some of the cell-level events that let you control what is displayed at the cell level CellFormatting event and that tell you when users interact with a cell by editing the contents EditControlShowing event or simply click on it CellClick event.
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There are actually over 30 events raised by the DataGridView that expose interactions and modifications at the cell level that you can subscribe to for providing cell-oriented features. Additionally, there are different selection modes that you can use to change the way the grid highlights cells, columns, or rows when the user clicks in different places in the grid. The DataGridView control supports the selection modes described in Table 6. Regardless of which of these modes you select, clicking on the upper left header cell the one that is above the row header cells and to the left of the column header cells selects all the cells in the grid.
As an example of a more cell-oriented application, the download code includes an application called SimpleSpread. This application mimics a simple spreadsheet and lets you do summations of the numeric values in a cell. The SimpleSpread sample application is shown in Figure 6. As you can see, the application lets you enter numbers into the cells; then you can select a sequence of cells and press the Sum button in the tool strip control at the top to get it to calculate the sum and place that in the next cell to the right or below the sequence of selections.
As Figure 6. The logic is nowhere near complete to handle all combinations of selections and cell contents, but it gives you a good idea of how to set something like this up.
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To code this up as shown in Listing 6. As I mentioned, I wanted to support a spreadsheet-like selection model, where you can select individual cells, but that selecting a column or row header would select the entire column or row, respectively. To do this, I set the SelectionMode for the grid to RowHeaderSelect , turned off sorting for all the columns as I created them and added them to the grid, and then handled the ColumnHeaderMouseClick event to manually select all the cells in a column when the user clicks on a column header.
In this case, I just programmatically added some rows and columns to the grid, set the column headers to be the letters of the alphabet, and turned off sorting on the column by setting the SortMode property to NotSortable. If you were going to support very large spreadsheets, you might need to maintain an in-memory sparse array, and only render the cells as you need them which you could do with virtual mode to avoid the overhead of maintaining a large number of cells, their contents, and their selections if the grid will be sparsely populated.
To get the row numbers to display in the row headers, I handled the RowAdded event and set the header cell value in that handler:. Another selection mode you might want to support is to have hot cells, meaning that the selection of cells changes as you move the mouse around the grid without having to click.
To do this, you could just handle the CellMouseEnter and CellMouseLeave events, selecting and deselecting the cell under the mouse in those handlers, respectively. The last topic I want to cover about the DataGridView is how to handle custom formatting of cells. As mentioned earlier, the grid supports a rich formatting model.
The styles in the grid work in a layered model, which lets you set styles at a more macro level, then refine it at a more micro level. For example, you might set a default cell style that applies to all cells in the grid, but then have one entire column that has a different cell formatting, and have selected cells within that column have yet a different cell formatting.
You do this by setting a series of default cell style properties that are exposed on the grid, which you can then refine by setting cell styles at the individual cell level. As can be seen in Figure 6. Above that layer sits the DataGridViewColumn. DefaultCellStyle property, representing the default styles on a column-by-column or row-by-row basis.
The grid also supports an alternating row cell style that is set through the AlternatingRowsDefaultCellStyle property on the grid. Finally, the top-level layer that will override the settings of any of the lower layers if set is the DataGridViewCell. CellStyle property. You can set these properties programmatically by accessing the appropriate property member on the instance of the grid, column, row, or cell. All of these properties are of type DataGridViewCellStyle , which exposes properties for setting fonts, colors, alignment, padding, and formatting of values.
You can also configure the cell styles through the designer. Whenever you access one of the cell style properties on the grid or a column through the Properties window or Smart Tag property editors in the designer, you will see the CellStyle Builder dialog shown in Figure 6. Using the property fields in this dialog, you can set fine-grained options for how the cell will display its content, and you can even see what it is going to look like in the Preview pane at the bottom of the dialog.