ClassLibrary1.Class1 obj = new ClassLibrary1.Class1();

Set a breakpoint on that line.
Start debugging.
The following message appears:

Debugging Not Enabled
The page cannot be run in debug mode because debugging is not enabled in the
Web.config file.
What would you like to do?
  [checked] Add a new Web.config file with debugging enabled.
            ! Debugging should be disabled in the Web.config file before
deploying the Web site to a production environment.
[unchecked] Run without debugging. (Equivalent to Ctrl+F5)

I clicked OK (add Web.config).
Web site started running under the ASP.NET Development Server - Port 1129.
(Note that in my real situation I am running under IIS.)
Break point hit.
Step into obj's constructor.
Step to foo assignment.
Attempt to change string.
I get the following error:

Edit and Continue
Changes are not allowed when the debugger has been attached to an already
running process or the code being debugged is optimized.

Note that the default Build properties of ClassLibary1 do not specify
optimized code.

Microsoft Windows XP Professional Version 2002 SP2

Microsoft Visual Studio 2005 Standard Edition Version 8.0

Microsoft .NET Framework 2.0

Microsoft Visual C# 2005

File->New->Web Site

ASP.NET Web Site

OK

File->Add->New Project

Class Library

OK

Add the following code to Class1.cs:

public Class1()

{

            string foo = "bar";

ClassLibrary1.Class1 obj = new ClassLibrary1.Class1();

Set a breakpoint on that line.

Start debugging.

The following message appears:

Debugging Not Enabled

The page cannot be run in debug mode because debugging is not enabled in the
Web.config file.

What would you like to do?

  [checked] Add a new Web.config file with debugging enabled.

            ! Debugging should be disabled in the Web.config file before
deploying the Web site to a production environment.

[unchecked] Run without debugging. (Equivalent to Ctrl+F5)

I clicked OK (add Web.config).

Web site started running under the ASP.NET Development Server - Port 1129.
(Note that in my real situation I am running under IIS.)

Break point hit.

Step into obj's constructor.

Step to foo assignment.

Attempt to change string.

I get the following error:

Edit and Continue

Changes are not allowed when the debugger has been attached to an already
running process or the code being debugged is optimized.

Note that the default Build properties of ClassLibary1 do not specify
optimized code.