To fix that error just list the fields you want to insert. This means, all of the fields but ID.
Solution 1:
Replace
--Insert into [Input] table; get an auto-incremented PK ID assigned to new row
INSERT INTO Input
SELECT * FROM inserted
With:
INSERT INTO Input (TS_Code, column2, column3,..., column25)
SELECT TS_Code, column2, column3,..., column25 FROM inserted;
Notice that in this case your trigger will delete records and you will have holes in your table:
ID TS_Code Column2 ....
----------------------------------
1 1001 valueX
3 1003 valueY
4 1002 valueZ <--- your trigger found a repeated 1002, ID = 2 record is deleted.
If this is what you want, go ahead with Solution 1. Also notice, that your trigger will work only for record by record insert. A bulk insert will call the trigger once and you will need to handle each record using cursors.
Solution2:
If you want to keep the records already found, you probably want to use the MERGE option:
first of all put all the records from C# into a Working table. Lets say table C_Sharp that should look like this:
CREATE TABLE C_SHARP(
TS_Code INT,
Column2,
...
Column25
)
With that C_SHARP table having all the records, perform a MERGE sentence:
MERGE INTO Input I
USING (
SELECT TS_Code, Column2, ... , Column25
FROM C_SHARP
) C
ON I.TS_Code = C.TS_Code
WHEN MATCHED THEN
UPDATE
SET Column2 = C.Column2,
...,
Column25 = C.Column25
WHEN NOT MATCHED THEN
INSERT (TS_Code, Column2, ..., Column25)
VALUES (C.TS_Code, C.Column2, ..., C.Column25)
END;
Good luck!