Create Custom Expression Builders in ASP.NET - More ASP.NET Extensibility

Create Custom Expression Builders in ASP.NET - More ASP.NET Extensibility

by David Hayden ( Florida .NET Developer ), Filed: ASP.NET 2.0

 

I need the concept of Expression Builders in an application, which led me to thinking about custom Expression Builders that were introduced in ASP.NET 2.0. Per the documentation:

“ExpressionBuilders in ASP.NET 2.0 are a parse-time feature that allows the page developer to add declarative syntax to assign to a control's properties. ASP.NET 2.0 ships with expression builders for:

• Connection strings. Used to access connection-strings in configuration. <%$ connectionstrings: MyConnStr %>;
• Application settings. Used to access the application setting in configuration. <%$ appsettings: MyAppValue %>, and;
• Resources. Used as the foundation for building a multi-lingual site. <%$ resources: mykey %>.“

You have probably used the ConnectionStringsExpressionBuilder in an SqlDataSource control to avoid hardcoding the connection string in your ASP.NET Page:

 

<asp:SqlDataSource ID="SqlDataSource1" runat="server" 
  ConnectionString="<%$ ConnectionStrings:Northwind %>"
  SelectCommand="SELECT * FROM [products]">
</asp:SqlDataSource>

 

You can build custom Expression Builders quite easily and use them in your applications.

First things first, you need to create a custom Expression Builder. Here is one I created quickly just now that gets information from HttpContext.Items, which is a key-value collection that can be used to organize and share data during an HTTP request. A lot of stuff in there that is basically boiler plate code that can be used over and over.

 

namespace PnPGuidance.CustomerExpressionBuilders
{
    [ExpressionPrefix("ContextItem")]
    public class ContextItemExpressionBuilder
                                  : ExpressionBuilder
    {
        public override CodeExpression GetCodeExpression
            (BoundPropertyEntry entry, object parsedData,
            ExpressionBuilderContext context)
        {
            return new CodeMethodInvokeExpression(
                new CodeTypeReferenceExpression(
                    base.GetType()),
                    "GetContextItem",
                    new CodeExpression[]
                    { new CodePrimitiveExpression(parsedData) });
        }

        public static object GetContextItem(string expression)
        {
            if (HttpContext.Current == null)
                return string.Empty;

            return HttpContext.Current.Items[expression];
        }

        public override object ParseExpression(string expression,
            Type propertyType, ExpressionBuilderContext context)
        {
            return expression;
        }
    }
}

 

The key is the GetContextItem method which returns the value in HttpContext.Items based on a particular key ( expression ) that is provided by the ParseExpression method.

You can use the ContextItemExpressionBuilder in your page as follows:

 

<asp:Label
    ID="MyLabel"
    Text="<%$ ContextItem: Key %>"
    runat="server" />

 

and the value of HttpContext.Current.Items[“Key“] in this case will be displayed as the text value of the label.

You have to register your Expression Builder in the web.config file so the compiler can find it:

 

<compilation debug="false">

    <expressionBuilders>
        <add expressionPrefix="ContextItem"
          type="....ContextItemExpressionBuilder,
             PnPGuidance.CustomerExpressionBuilders"/>
    </expressionBuilders>

</compilation>

 

 

Cool stuff!

Author: David Hayden ( Florida .NET Developer ), Filed: ASP.NET 2.0

 

posted on Friday, September 28, 2007 1:10 PM