C# conditional compilation if assembly exists

Question

I have a project with a reference that may or may not exist. I have code that uses that reference and I'd like to compile it only if the assembly exists. I'm thinking something along the lines of:

#if ASSEMBLY_EXISTS
    AssemblyClass.DoSomething();
#endif

I could put a #define at the top and comment/uncomment as needed, but I'd prefer if it could just somehow know if it's there without my manual intervention, which leads me to believe that #if won't work for this situation. Is there another way to conditionally compile based on whether an assembly exists?

Gallia

Try, Catch, Finally? I assume you would get an object ref. not set to an instance of an object exception so have a specific catch block to handle that etc. Not a very efficient approach but I don't know that this code is required to be fast. – Automatize

@stuartd, isn't that link checking at runtime?? – Gallia

There is no precompilation symbol that you can use for the existence of a referenced assembly. – Jaquiss

@Gallia oops, yes it is.. – Relique

@Automatize - the code won't compile if the assembly isn't there. – Gallia

@RonBeyer - that's what I figured. So is there another way? – Gallia

You could define your own symbol and use it, there just isn't anything built-in. There isn't a way to conditionally compile code based on references without doing a little bit of leg-work. You can also use ConditionalAttribute but it still requires you to define the symbol yourself, personally I'd use the #if/#endif because I'm not sure all compilers are required to abide by the attribute. – Jaquiss

By the way, symbols #defined in a file are only valid for that file, you'd have to define them in every file in the project that uses the symbol. For this you can define global symbols in the properties which is a lot easier to maintain. – Jaquiss

As an alternative you could load assembly type dynamically using Type.GetType( "TypeName,DllName" ) and if exists call its methods using reflection. – Tenorrhaphy

@GuidC0DE - that would functionally work, however, this question is specifically about compilation time checking rather than runtime. – Gallia

Answer 1

Maybe do it with a condition inside MSBUILD;

It would look something like it

<PropertyGroup>
     <DefineConstants Condition="Exists('my.dll') ">$(DefineConstants);DLLEXISTS</DefineConstants>
</PropertyGroup>

and should go quite far down in your .csproj file.

This reads, roughly, as "redefine the constants by appending DLLEXISTS, if my.dll exists"

Now you should be able to do

#if DLLEXISTS
    // your stuff here
#endif

You might need to fiddle around with the EXISTS expression to find an appropriate relative path.

Manx

This worked perfectly. For the Exists directory, since the reference is included in the project I just copied its path exactly. – Gallia

Works good! MsBuild/.csproj file can be accessed by unloading project and right clicking it then click edit... doc.postsharp.net/configuration-msbuild – Neuritis

Answer 2

No you cannot do this. You cannot define the result of a conditional compilation symbol at compile time.

If you want to get fancy you could write a new program which detects the missing assembly and modifies your source. You can then execute this program in the Pre-build event of your project.

The modification of the source could simply be the addition or removal of your suggested #define at the top of the source files.

Harri

I like your "get fancy" idea. This would obviously work and seems like a good fallback if there isn't an easier way. – Gallia