The XAP files are plain ZIP files, hence they cannot be obfuscated. However the DLL files in the XAP file could be.
Opening an obfuscated assembly in JustDecompile is not a supported scenario. If JustDecompile successfully decompiles the non-obfuscated version of your XAP file but fails on the obfuscated one then you can safely assume obfuscation worked for you.
However, keep in mind that some of the deobfuscators out there could possibly deobfuscate it successfully. In this case there is a chance JustDecompile to open it and show the source code. E.g. JustDecompile offers the de4dot plugin that does just that.
If you need further assistance, please, do not hesitate to contact us.
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