C# – What’s the difference setting Embed Interop Types true and false in Visual Studio

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In Visual Studio, when adding one reference to the project, the properties window has an option Embed Inteop Types, should we set it to True or False? What's the difference?

Since we have a lot of projects, among some of them, reference was set to False, others were set to True, it is totally mess up. And the bulid server also have the same warnings:

What does “reference was created to embedded interop assembly” mean?

So we plan to change all the Embed Inteop Types to False, what risk would we get?

Best Answer

This option was introduced in order to remove the need to deploy very large PIAs (Primary Interop Assemblies) for interop.

It simply embeds the managed bridging code used that allows you to talk to unmanaged assemblies, but instead of embedding it all it only creates the stuff you actually use in code.

Read more in Scott Hanselman's blog post about it and other VS improvements here.

As for whether it is advised or not, I'm not sure as I don't need to use this feature. A quick web search yields a few leads:

The only risk of turning them all to false is more deployment concerns with PIA files and a larger deployment if some of those files are large.

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

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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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