Webapi interface Access exception issue. An error occurred while trying to create a controller of type "TestController". Make sure the controller has a parameterless public constructor

The good WEBAPI interface that was running was suddenly reported: "An error occurred while trying to create a controller of type TestController". Make sure that the controller has a parameterless public constructor "error.
Spent half a night in the final settlement,

Cause: The API controller references a key value that is not in the config configuration file

Private Static string Watchswitch = configurationmanager.appsettings["watchswitch"]. ToString (). Trim ();  // Minimum withdrawal amount

Config file is missing the configuration of the Watchswitch attribute, plus it's good.

< appSettings >         <!-- whether the hand ring end verifies the order code (on= verification off= not verified) when the order is submitted  -    <  key= "Watchswitch"  value= "Off"/>   </appSettings>

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Webapi interface Access exception issue. An error occurred while trying to create a controller of type "TestController". Make sure the controller has a parameterless public constructor