Published:
09 Oct 2003
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Abstract
Easily one of the coolest features of .NET! Learn to create collections for your custom objects that can be bound to a data list. |
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by J. Ambrose Little
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| "Hey, Old Woman!" |
Published: 30 May 2003
This has got to be one of the coolest things I've done with ASP.NET in quite some time. I'm binding a collection of my own custom business objects to a DataGrid. No data views, no data readers, only the objects that I made, the way I like them. Even Burger King can't beat this!
What's funny is that it's really quite simple, and had I taken the time to figure it out sooner, I could have been eating Whoppers for a long time now. All you need to do is create a collection class that implements IEnumerable, create a private field as an ArrayList, provide accessors to at least add your custom object to the collection, and of course implement the IEnumerable interface by returning the enumerator from your ArrayList's GetEnumerator method. Then you can use the new collection class as the DataSource for your data list controls.
My Custom Object
1:
2:
3:
4: public class KnightOfRoundTable
5: {
6: private string name = string.Empty;
7: private System.Drawing.Color color = System.Drawing.Color.Black;
8: private string quest = string.Empty;
9:
10:
11:
12:
13: public string Name
14: {
15: get
16: {
17: return this.name;
18: }
19: set
20: {
21: this.name = value;
22: }
23: }
24:
25:
26:
27:
28: public System.Drawing.Color Color
29: {
30: get
31: {
32: return this.color;
33: }
34: set
35: {
36: this.color = value;
37: }
38: }
39:
40:
41:
42:
43: public string Quest
44: {
45: get
46: {
47: return this.quest;
48: }
49: set
50: {
51: this.quest = value;
52: }
53: }
54: }
55:
As you can see, it's quite a simple object. Just three members, two string and one Color.
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| My Collection Object |
1:
2:
3:
4: public class KnightsOfTheRoundTable : System.Collections.IEnumerable
5: {
6:
7:
8:
9: private System.Collections.ArrayList knights =
new System.Collections.ArrayList();
10:
11: #region IEnumerable Members
12:
13:
14:
15:
16: public IEnumerator GetEnumerator()
17: {
18: return this.knights.GetEnumerator();
19: }
20: #endregion
21:
22:
23:
24:
25: public KnightOfRoundTable this[int index]
26: {
27: get
28: {
29: if (this.knights.Count > index)
30: return (KnightOfRoundTable)this.knights[index];
31: else
32: throw new ArgumentException(
"That position at the table has not been filled yet!");
33: }
34: set
35: {
36: if (this.knights.Count > index)
37: this.knights[index] = value;
38: else
39: throw new ArgumentException(
"That position at the table has not been filled yet!");
40: }
41: }
42:
43:
44:
45:
46: public KnightOfRoundTable this[string name]
47: {
48: get
49: {
50:
51: for (int i = 0; i < this.knights.Count; i++)
52: {
53:
54: if (((KnightOfRoundTable)this.knights[i]).Name.ToUpper()
== name.ToUpper())
55: return (KnightOfRoundTable)this.knights[i];
56: }
57:
58: throw new ArgumentException(
"No such knight is honored at our table!");
59: }
60: set
61: {
62:
63: for (int i = 0; i < this.knights.Count; i++)
64: {
65: if (((KnightOfRoundTable)this.knights[i]).Name.ToUpper()
== name.ToUpper())
66: {
67:
68: this.knights[i] = value;
69: return;
70: }
71: }
72:
73: throw new ArgumentException(
"No such knight is honored at our table!");
74: }
75: }
76:
77:
78:
79:
80:
81: public void Add(KnightOfRoundTable knight)
82: {
83: this.knights.Add(knight);
84: }
85:
86:
87:
88:
89:
90: public void Remove(KnightOfRoundTable knight)
91: {
92: this.knights.Remove(knight);
93: }
94: }
Notice on Line 4 that I'm specifying this class implements the IEnumerable interface--this is key for data binding. You can see on Line 9 that I declare and initialize my private ArrayList. On Lines 16-19, I implement the IEnumerable interface by simply returning the enumerator from my ArrayList. As mentioned, this is the critical part for data binding; the rest of the code is more or less arbitrary, although you will want to at least provide a way to add new objects to the collection as I have in Lines 81-84.Everything else is pretty much up to you. I chose to add two indexers, one by index (Lines 25-41) and one by name (Lines 46-75). I also provided a Remove method on Lines 90-93. You may notice that in the Add/Remove methods, I'm simply calling the corresponding methods on the ArrayList. In the integer indexer, I guard against an ugly out of bounds exception and instead provide a more thematic exception if the position is not available. In the name indexer, I cycle through the current knights and again throw a thematic exception if the desired object is not found.
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| Usage the First Part: The Page Class |
1:
2:
3:
4: public class TheBridge : System.Web.UI.Page
5: {
6: protected System.Web.UI.WebControls.DataGrid BridgeMaster;
7:
8: private void Page_Load(object sender, System.EventArgs e)
9: {
10: if (!this.IsPostBack)
11: this.BindGrid();
12: }
13:
14: private void BindGrid()
15: {
16: KnightOfRoundTable arthur, galahad, bedevere;
17: KnightsOfTheRoundTable roundTable =
new KnightsOfTheRoundTable();
18: arthur = new KnightOfRoundTable();
19: arthur.Name = "King Arthur";
20: arthur.Color = System.Drawing.Color.Gold;
21: arthur.Quest = "To seek the Holy Grail.";
22: roundTable.Add(arthur);
23:
24: galahad = new KnightOfRoundTable();
25: galahad.Name = "Sir Galahad";
26: galahad.Color = System.Drawing.Color.Yellow;
27: galahad.Quest = "Oooaaaaaah!";
28: roundTable.Add(galahad);
29:
30: bedevere = new KnightOfRoundTable();
31: bedevere.Name = "Sir Bedevere";
32: bedevere.Color = System.Drawing.Color.Blue;
33: bedevere.Quest = "Determine the flight velocity
of an unladen African (or European) swallow.";
34: roundTable.Add(bedevere);
35:
36: this.BridgeMaster.DataSource = roundTable;
37: this.BridgeMaster.DataBind();
38: }
39:
40: #region Web Form Designer generated code
41: override protected void OnInit(EventArgs e)
42: {
43:
44:
45:
46: InitializeComponent();
47: base.OnInit(e);
48: }
49:
50:
51:
52:
53:
54: private void InitializeComponent()
55: {
56: this.Load += new System.EventHandler(this.Page_Load);
57:
58: }
59: #endregion
60: }
Clearly, the workhorse on this page is the BindGrid method (Lines 14-38). In this, you can see that I create three KnightOfRoundTable objects and name them Arthur, Galahad, and Bedevere respectively, assigning each attributes proper to their characters. I also create my KnightsOfTheRoundTable collection and add each of these knights to it. And lastly, on Lines 36-37, I set my KnightsOfTheRoundTable instance to be the DataSource for my BridgeMaster DataGrid and call DataBind on that grid. And, not entirely insignificantly, I make a call in the Page.Load handler to BindGrid if this is not a post back, on Lines 10-11.
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| Usage The Second Part: Nifty Databinding |
1: <asp:TemplateColumn HeaderText="Favorite Color">
2: <ItemTemplate>
3: <asp:Label runat="server"
Text='<%# ((BizDataBinding.KnightOfRoundTable)
Container.DataItem).Color.Name %>'
ID="Label1">
4: </asp:Label>
5: </ItemTemplate>
6: </asp:TemplateColumn>
7: With this section, I just wanted to demonstrate that you can access the members of each instance in the collection in a very real and binding way by casting Container.DataItem as your object type. As you can see, I've done this to access the Color member of my object in order to display its Name property (ignore the line breaks--they're for printability only). If you just use a BoundColumn or DataBinder.Eval, the ToString() method will be called on the particular member. In the case of Color, this is something like "Color [ColorName]" where ColorName is the actual name of the Color. So if you do not have control over the ToString method on your class members' types, you may need to use this syntax to specify how you want the bound item to render that member.
To sum up, I have demonstrated how to create a collection of custom objects and bind them to a DataGrid. It's really quite simple once you get down to it--it comes down to just creating a class that implements IEnumerable. Slick, simple, and cool, what more could you ask for from a development framework? |
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User Comments
Title:
help
Name:
Francisco Cruz
Date:
2009-12-17 3:11:43 PM
Comment:
Object Persistance between postbacks
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Title:
Very helpful
Name:
Ben
Date:
2008-09-15 2:22:35 PM
Comment:
just what I needed thanks
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Title:
Mr
Name:
Phillip Knezevich
Date:
2007-02-06 3:45:56 PM
Comment:
It's a geeky example, but I found it useful.
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Title:
Serialization
Name:
Ambrose
Date:
2006-09-21 7:49:08 PM
Comment:
Russ B,
I guess you're trying to use XML serialization like in a Web Service or directly? If so, yeah, it needs an Add method for when it deserializes so that it can add the deserialized instances to the new collection. You just need to implement an Add method, as far as I recall.
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Title:
Serialization
Name:
Russ B
Date:
2006-09-20 4:25:10 PM
Comment:
This is exactly what I've been looking for too.
I tried to serialize your KnightsOfTheRoundTable class and got this error: "System.InvalidOperationException: You must implement the Add(System.Object) method on BizDataBinding.KnightsOfTheRoundTable because it inherits from IEnumerable."
But you did implement Add. Any idea what else needs to be implemented?
...I'm still in 2003
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Title:
Binding to Complex Properties
Name:
J. Ambrose Little
Date:
2005-03-08 9:36:56 AM
Comment:
Hi Mark,
What I was saying is that if you just bind to the Color property, it will print out something like "Color [Black]" instead of the color name, which is really what we're after in this example. So in order to print out the value you want, just access the property of the property that you want to display, e.g., Color.Name. I hope this helps.
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Title:
Thanks!
Name:
Mark Miller
Date:
2005-03-08 12:27:37 AM
Comment:
I was looking around the Internet for an article like this. I'm a bit surprised it was not easier to find.
After finishing my first big web project, I wish I had used a technique like this. Unfortunately too many databinding examples online and in the MSDN docs just show you how to bind to a dataset.
I am a bit confused about the last part, where you say, "So if you do not have control over the ToString method on your class members' types, you may need to use this syntax to specify how you want the bound item to render that member." It seems you're referring to the string "Color [ColorName]", "ColorName" being the name of the color, but I don't know what this syntax would mean in relation to databinding. Aren't you trying to extract the name of the color so that it can display in whatever you are databinding to? What does the "Color [ColorName]" syntax accomplish?
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Title:
Using CollectionBase
Name:
Scott M
Date:
2004-12-12 4:19:12 PM
Comment:
Hey J,
Thanks for the article. I found it really useful. I read (in hardcopy) and article about inheriting from CollectionBase the other day and thought I'd let you know, turns out by your comment that you found it too!
So anyway, cheers and roll on generics,
Scott.
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Title:
Using CollectionBase
Name:
J. Ambrose Little
Date:
2004-08-31 12:53:30 PM
Comment:
Glad the article helped. I've actually found (since this article was published) that inheriting from CollectionBase is even easier than just implementing IEnumerable for my strongly-typed collections. Now I can't hardly wait for 2.0 where we won't even need to bother declaring our own collections and can use the generic ones instead!
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Title:
Thanks
Name:
Scott
Date:
2004-08-31 1:37:07 AM
Comment:
Hey Mr Little, great article. I was a little confused about implementing IEnumerable but once I dissected your example I managed to get my productset (using a dataset stored internally) going to bind it to a asp:repeater... I implemented ienumerator too but your article got me started, so cheers for putting it online.
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