Adding a DataDefaultField to a DropDownList

At times you want to return the default to show in an asp:DropDownList from the DataSource. Normally you have only two items:

• DataTextField
• DataValueField.

I will illustrate how you can add and use a DataDefaultField (boolean) to select a specific item without needing to do any C# in the page.

 

The steps are:

1. Create a class that inherits from DropDownList
2. Add in two private variable, defaultValue, isPostBack
   
Code Snippet
1. public class DropDownList : System.Web.UI.WebControls.DropDownList
2. {
3.   private string defaultValue;
4.   private bool isPostback ;
3. Do an override as shown below to detect PostBacks
   
   Code Snippet

   1. protected override void RaisePostDataChangedEvent()
   2.  {
   3.    isPostback = true;
   4.    base.RaisePostDataChangedEvent();
   5.  }
4. Add a new Property
   
   Code Snippet

   1. public virtual string DataDefaultField { get; set; }
5. Add another override (this picks up the (last) default value from the data.
   
   Code Snippet

   1. protected override void PerformDataBinding(IEnumerable data)
   2. {           
   3.   if (string.IsNullOrWhiteSpace(DataDefaultField))
   4.   {
   5.     DataDefaultField = "default";
   6.   }
   7.  
   8.   foreach (var dataItem in data)
   9.   {
   10.     bool isDefault;
   11.     if (bool.TryParse(dataItem.GetProperty(DataDefaultField), out isDefault) && isDefault)
   12.     {
   13.       defaultValue = dataItem.GetProperty(DataValueField);
   14.     }
   15.   }
   16.   base.PerformDataBinding(data);
   17. }
6. Add the last override which sets the default value IF it is not a postback
   
Code Snippet
1. public override void RenderControl(HtmlTextWriter writer)
2. {
3.   if (! isPostback && !string.IsNullOrWhiteSpace(defaultValue))
4.   {
5.     var defaultItem = Items.FindByValue(defaultValue);
6.     if (defaultItem != null)
7.     {
8.       SelectedIndex = -1;
9.       defaultItem.Selected = true;
10.     }
11.   }
12.   base.RenderControl(writer);
13. }

That’s it. You can now set the default value in the datasource.

 

Oops.. Bill just pointed out that I used extensions that I did not include.. Here there are:

 

Code Snippet

1. public static string GetProperty(this object dataItem, string field, string defaultValue)
2.   {
3.   if (string.IsNullOrWhiteSpace(field))
4.     {
5.     return defaultValue;
6.     }
7.  
8.   var props = TypeDescriptor.GetProperties(dataItem);
9.   if (props.Count > 0)
10.     {
11.     if (props[field] != null && null != props[field].GetValue(dataItem))
12.       {
13.       return props[field].GetValue(dataItem).ToString();
14.       }
15.  
16.     for (var i = 0; i < props.Count; i++)
17.       {
18.       if (string.Compare(props[i].Name, field, StringComparison.OrdinalIgnoreCase) == 0)
19.         {
20.         return props[i].GetValue(dataItem).ToString();
21.         }
22.       }
23.     }
24.  
25.   return null;
26.   }

and

Code Snippet

1. public static string GetProperty(this object dataItem, string field)
2.   {
3.   return GetProperty(dataItem, field, null);
4.   }