DataBinder.Eval Method, Uses reflection to parse and evaluate a data-binding expression against an object at run time.
Eval, Data-binding expressions are contained within <%# and %> delimiters and use the Eval and Bind functions. The Eval function is used to define one-way (read-only) binding. The Bind function is used for two-way (updatable) binding.
Actually, by Reflector, we found:
protected internal object Eval(string expression) { this.CheckPageExists(); return DataBinder.Eval(this.Page.GetDataItem(), expression); } |
Inside Eval method, it will call DataBinder.Eval method.
Circumspective reader may find that we can't locate the definition for the Bind method. This following article tell you why in some way.
there isn’t a bind method in ASP.NET
A recent question on one of our internal mailing lists asked where the definition for the Bind() method is located in ASP.NET. The asker had located the definition of the Eval() method on the TemplateControl class, but the Bind() method was nowhere to be found, even using Reflector.
If you're familiar with databinding in ASP.NET 2.0, you probably know that for read-only values such as Labels you can use the Eval() statement, and for read-write values such as TextBoxes (also known as "two-way databinding") you can use the Bind() statement. Where does that Bind() statement come from?
To the surprise of many readers, there isn’t a bind method in ASP.NET! When ASP.NET parses your file and sees you're using a databinding expression (in the angle-bracket-percent-pound format, "<%# %>") it has special-case code to parse for the Bind syntax and generates some special code for it. When you use <%# Bind("Name") %> it's not a real function call. If ASP.NET parses the code and detects a Bind() statement, it splits the statement into two parts. The first part is the one-way databinding portion, which ends up being just a regular Eval() call. The second part is the reverse portion, which is typically some code along the lines of "string name = TextBox1.Text" that grabs the value back out from where it was bound.
Non-Bind() databinding statements are literal code (we use CodeSnippetExpressions in CodeDom), so arbitrary code in the language of your choice is allowed. However, because ASP.NET has to parse Bind() statements, two-way databinding doesn’t support anything other than Bind(). For example, the following syntax is invalid because it tries to invoke arbitrary code and use Bind() at the same time:
<%# FormatNameHelper(Bind("Name")) %>
The only formats supported in two-way databinding are Bind("field") and Bind("field", "format string {0}"). There are some very minor variations of these syntax examples, such as allowing use of single quotes and not just double quotes. Since some languages supported by ASP.NET favor one format over the other, we have to support both formats, even though the language you're using may support only one.
The moral of the story is: There are some things that Reflector won't tell you.
If you're familiar with databinding in ASP.NET 2.0, you probably know that for read-only values such as Labels you can use the Eval() statement, and for read-write values such as TextBoxes (also known as "two-way databinding") you can use the Bind() statement. Where does that Bind() statement come from?
To the surprise of many readers, there isn’t a bind method in ASP.NET! When ASP.NET parses your file and sees you're using a databinding expression (in the angle-bracket-percent-pound format, "<%# %>") it has special-case code to parse for the Bind syntax and generates some special code for it. When you use <%# Bind("Name") %> it's not a real function call. If ASP.NET parses the code and detects a Bind() statement, it splits the statement into two parts. The first part is the one-way databinding portion, which ends up being just a regular Eval() call. The second part is the reverse portion, which is typically some code along the lines of "string name = TextBox1.Text" that grabs the value back out from where it was bound.
Non-Bind() databinding statements are literal code (we use CodeSnippetExpressions in CodeDom), so arbitrary code in the language of your choice is allowed. However, because ASP.NET has to parse Bind() statements, two-way databinding doesn’t support anything other than Bind(). For example, the following syntax is invalid because it tries to invoke arbitrary code and use Bind() at the same time:
<%# FormatNameHelper(Bind("Name")) %>
The only formats supported in two-way databinding are Bind("field") and Bind("field", "format string {0}"). There are some very minor variations of these syntax examples, such as allowing use of single quotes and not just double quotes. Since some languages supported by ASP.NET favor one format over the other, we have to support both formats, even though the language you're using may support only one.
The moral of the story is: There are some things that Reflector won't tell you.
-