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To obtain the complete error message for a NullReferenceException in C#, follow these steps:

1. Run your code in a debugger. This can be done in your IDE, such as Visual Studio or Rider.
2. When the exception is thrown, the debugger will stop at the line of code where it occurred.
3. Take note of the error message that appears in the debugger console. This will include a stack trace showing the exact line of code where the exception occurred, as well as any inner exceptions that may have been thrown.
4. Use the error message to identify the object or variable that is null and causing the exception. This may involve inspecting the code where the exception occurred, looking at the values of different variables or members, or checking for null values in any input or output parameters.
5. Once you have identified the cause of the exception, adjust your code as necessary to prevent the null reference from occurring. This may involve adding null checks, initializing objects or variables, or rewriting code to avoid null values altogether.