.net – Why doesn’t Edit and Continue work on the x64 CLR

64-bitnet

Microsoft have explained that they won't be supporting Edit and Continue under the x64 CLR in Visual Studio 2010:

When creating a new Visual C# Console Application in VS2010 for .NET 4.0, the default target settings for the project is to target the x86 platform instead of Any CPU (MSIL) like Visual Studio 2008 does

[…]

Adding true support for EnC to the 64-bit CLR is unfortunately a large work item and other features were prioritized over this given the work around of changing the platform target to x86.

(from http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=455103)

The description on Microsoft Connect makes it appear as though 64-bit Edit and Continue is a major architectural change. My question is: what is different about x64 that makes EnC difficult?

I haven't been able to find much in the way of technical detail on the web, other than "64-bit EnC doesn't work".

Best Answer

Edit and Continue requires that the compiler patches a running executable. This is typically done by replacing all altered functions. Obviously, the JITted versions thereof have to be discarded as well, and callers adjusted to possibly new locations.

This isn't especially difficult for x64, probably about as hard as on x86. But unlike x86, this simply hasn't been done for x64 yet. And the differences between x86 and x64 are big enough that you can't simply take the x86 EnC code and change every 4 in an 8.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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