Update on a view with NULL column

Let's set up a experiment:

CREATE TABLE T 
(
	c1 INT,
	c2 DATETIME2
);

GO

INSERT INTO T
VALUES (1, GETUTCDATE());

GO

CREATE VIEW V AS
SELECT c1, NULL as c2 FROM T;

GO

CREATE TRIGGER VT on V 
INSTEAD OF UPDATE AS

	UPDATE V SET c1 = 1
	FROM V JOIN inserted AS i ON V.c1 = i.c1

GO

Now assume you want to run an update statement through the view and for some reason you want to update in your statement column c2 (Yes I agree this isn't very sensible thing you might want to do).

UPDATE V SET c2 = GETUTCDATE()
WHERE c1 = 1

This fails with error:
Implicit conversion from data type datetime to int is not allowed. Use the CONVERT function to run this query.

How to workaround this? Well the solution is suggested by error message. It seems NULL is interpreted as integer by default. You need to cast this NULL in view to proper data type:

ALTER VIEW V AS
SELECT c1, CAST(NULL AS DATETIME2) AS c2

FROM T

Now the above update will be successful.

Normally you shouldn't have constant columns in a view, which your code tries to update or insert into. Yet I had such situation and I am sure this can happen to others. That's why I wanted to document this peculiarity.