How to use Inno Setup preprocessor directive in [Code] section?

I feel like this has got to be something simple I'm missing - in Inno Setup, if I have passed in a directive variable, how do I use it inside the [Code] section?

Say I pass in /DMYVAR=1 to the Inno Setup engine.

In my .iss file I can have something like:

[Setup]
AppName=MyApp v{#MYVAR}

Down in my [Code] section I'd like to be able to use it like this:

function IsVersionOne(param: String): boolean;
begin
    Result := {#MYVAR} == "1";
end;

This doesn't work :(

Preprocessor does not care, where its directives are expanded.

So {#name} syntax (inline preprocessor directive call) works everywhere, including Pascal Script.

Though, if you want to compare a define value as a string, you have to, of course, enclose it to quotes to make it a string. Also note that your == "1" syntax is wrong, Pascal uses single = and single quotes.

function IsVersionOne(param: String): boolean;
begin
    Result := '{#MYVAR}' = '1';
end;

#expr SaveToFile(AddBackslash(SourcePath) + "Preprocessed.iss")

Run the compiler with /DMYVAR=1, and check the generated Preprocessed.iss. It will show:

function IsVersionOne(param: String): boolean;
begin
    Result := '1' = '1';
end;

As the value is a number, you can, of course, use numerical comparison too:

function IsVersionOne(param: String): boolean;
begin
    Result := {#MYVAR} = 1;
end;

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For a related question with more detailed information, see Evaluate preprocessor macro on run time in Inno Setup Pascal Script.

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While the above answered your literal question, your function signature actually suggests that you want to implement a Check function to test a value of a compile-time directive, like:

[Files]
Source: "MYPROG.EXE"; DestDir: "{app}"; Check: IsVersionOne

That's an inefficient overkill.

Use a preprocessor #if directive instead:

[Files]
#if MYVAR == "1"
Source: "MYPROG.EXE"; DestDir: "{app}"
#endif

#expr SaveToFile(AddBackslash(SourcePath) + "Preprocessed.iss")

If you run the compiler with /DMYVAR=1, Preprocessed.iss will show:

[Files]
Source: "MYPROG.EXE"; DestDir: "{app}"

If you run the compiler with a different value of DMYVAR, Preprocessed.iss will show:

[Files]

If you need the script to compile even without /DMYVAR= switch, define a default value at the top of the script, like:

#ifndef MYVAR
#define MYVAR "0"
#endif