Compile errors after installing an add-on in NinjaTrader 8

What is causing it

Usually duplicate or conflicting code. Two copies of the same add-on, an old version left behind, or an add-on written for a different NinjaTrader version.

Why this happens

All NinjaScript code in your installation is compiled together into a single assembly. That means one broken or duplicated file stops the whole set from compiling, which is why every custom indicator disappears at once rather than just the new one.

It also means the fix is almost always removal of a conflict rather than repair of the indicator you were trying to add.

The fix, step by step

1. Open the NinjaScript Editor and look at the error list, which names the file and line for each error. Start with the first error, because later ones are often knock-on effects.
2. If the errors name a file from an add-on you have just imported twice, remove the older copy using the platform's option for removing an installed NinjaScript assembly, then restart.
3. If the errors name a vendor's file rather than your own code, contact that vendor with the exact error text, since only they can supply a corrected build.
4. Check whether the add-on was written for NinjaTrader 8, as code for the earlier version will not compile.
5. Do not delete files by hand from the platform folders as a first move, because a half-removed add-on causes worse errors than the original problem.
6. After any change, compile again and confirm the error list is empty before opening charts.

How to confirm it is fixed

Compiling produces no errors, and your previously working indicators are back in the indicator list on a chart.

Stopping it happening again

Import one add-on at a time and compile after each. Remove the old version of a tool before importing an update, rather than layering them.