mjhjmtu

I have a document class test.cls, in which I have defined a custom environment myenvironment using the environ package as follows:

NeedsTeXFormatLaTeX2e
ProvidesClasstest
LoadClassarticle
RequirePackagechangepage
RequirePackageenviron
NewEnvironmyenvironment
 beginadjustwidth10pt10pt
 BODY
 endadjustwidth

Now if I try to use tikz-cd inside myenvironment, it fails, e.g. this gives an error:

beginmyenvironment
 [begintikzcd
 1 ar[r] ar[d] & 2 ar[d] \
 3 ar[r] & 4 \
 endtikzcd]
endmyenvironment

On the other hand, if I simply write it like this—that is, I substitute what the environment would do—, everything is fine:

beginadjustwidth10pt10pt
 [begintikzcd
 1 ar[r] ar[d] & 2 ar[d] \
 3 ar[r] & 4 \
 endtikzcd]
endadjustwidth

What could be the problem?

You have to be very careful when collecting an environment with environ, as this usually brings problems. When environ collects its the environment it "freezes" the tokens of the environment. That is, everything will mean what it was when the outer environment was called. This will break every environment that somewhere redefines the category codes of any character (verbatim environments, for example), or that redefine some character.

In this particular case, the meaning of the & changes when you enter tikzcd. See:

documentclassarticle
usepackageamsmath
usepackagetikz-cd
begindocument

When textttenviron collects the environment:

ttfamilymeaning&

bigskip

What texttttikzcd expects texttt& to be:

begintikzcd
vbox to textwidthttfamilymeaning&
endtikzcd
enddocument

So basically, inside tikzcd, & means one thing, but when environ reads it, it freezes the alignment tab character & meaning, so it won't work.

So use environ only when you are sure that you need it and you are sure that it won't break anything.

In your case you don't need environ at all because you aren't applying any operation to the collected contents, so this:

newenvironmentmyenvironment
 %
 beginadjustwidth10pt10pt%
 %
 %
 endadjustwidth
 

will suffice.

It doesn't work because myenvironment absorbs the body of the environment as a macro argument; in these cases you have to pass ampersand replacement=& to tikzcd and use & instead of &, so

beginmyenvironment
 [begintikzcd[ampersand replacement=&
 1 ar[r] ar[d] & 2 ar[d] \
 3 ar[r] & 4 \
 endtikzcd]
endmyenvironment

However, there's no need for environ here:

newenvironmentmyenvironment
 beginadjustwidth10pt10pt
 endadjustwidth

will do as well. And better, actually.

The cargo-cult version would be

newenvironmentmyenvironment
 adjustwidth10pt10pt
 endadjustwidth